Recent discussions have made me think about the pronunciation of the scientific names of plants in California. This is a complicated and confusing subject for a great variety of reasons. These names are often of Latin or Greek derivation or are derived from other languages, or based on proper or geographic names, or arrived at by some other fashion entirely, yet these names need to be spoken by people who are used to their own lan-
guage and how it sounds to them. Regardless of where these names originated, they are all treated to some extent as if they were Latin. There are at least two basic systems of pronouncing botanical Latin names, the English system used by most gardeners and horticulturists in the English-speaking world and the more traditional reformed academic system utilized by classical scholars and in vogue among botanists in continental Europe. Other systems may exist in different parts of the world. Some pronunciations depend on whether the names are of Greek origin or not. Even within the guidelines of individual authorities, there are contradictions leading to different pronunciations. Many botanists and horticulturists still hold tight to classical rules of Latin pronunciation. And then there are personal preferences that may in many cases influence the choice of pronunciation. I here echo the sentiments of A.T. Johnson who stated in Plant Names Simplified that "the writer claims no infallibility."

Several times during the course of researching this essay, I have almost decided not to continue with it because my intention was not to say to other people, "Your pronunciation is wrong," but rather to find out what rules or guidelines there were that would help me to be comfortable in speaking these names out loud, to be as consistent as possible, and to share them with others who might be interested, yet the conflicting opinions and guidelines as expressed by other authorities can be dizzying. I have been around many enough pro-
fessional botanists to know that they all have their preferred ways of saying these names, ways that they have grown accustomed to in the course of their careers or or which they were themselves taught. No doubt in most cases they are as correct as it is possible to be. They more than anyone know how tricky an endeavor it is to try to apply consistent standards of pronunciation, and for the most part I believe they don't feel they need to. For those of us who are less knowledgeable about botanical etymology, there is an urge to develop a nice tidy set of rules that we can apply to any given name. And despite the fact that botanical Latin does its best to resist this urge, even scholars have attempted to outline systems of proper pronunciation. I have been pleased in general to find that about 90% of the time my intuitive manner of pronouncing these names is in accord with or at least does not definitely violate the guidelines as set down here. It is the remaining 10% that causes most of the difficulties.

Spelling is something that is determined by the International Botanical Congresses which are held approximately every five years, the most recent in 2005 in Vienna, Austria, but this body does not deal with pronunciation. There are two things that are involved here: first, how the names are to be divided into syllables and properly accented, and, second, how the vowels and consonants are to be sounded. I quote from Coombes' Dictionary of Plant Names: "Unlike the use of scientific names, their pronunciation is not governed by rules. The majority of people who use scientific names treat them as if they are in their own language. Where pronunciation is ambiguous by this method, it is common to en-
counter several ways of saying a word." In researching the pronunciation of a specific name, it is often the case that several authors will give contrary pronunciations, which may reflect the fact that they are using different systems, and without knowing what is their preference, it is difficult to explain why one might say 'no-LIE-na' and another 'no-LEE-
na.' Thus we are left with those guidelines mainly as outlined in works such as Stearn's Botanical Latin and Borror's Dictionary of Word Roots and Combining Forms, and of course with the pronunciations that we are familiar with hearing.

Botanical Latin is not classical Latin. That language derived from the Roman writers of the early first millenium and remained the single internationally-used language of learning throughout Europe until at least the 18th century. Were this not the case, there might well be no uniform international system of botanical nomenclature today. Herbalists of the 16th century established the tradition subsequently taken up by Linnaeus that plants should be given Latin names. If there had been no common knowledge and usage of Latin, the vernacularly-written works of local botanists would have been unknown outside their own region. Linnaeus' work reflects the fact that a huge advance had been made in the know-
ledge of the complexities of structure and relationship of plants, and language had to expand accordingly. Botanical Latin however has grown far beyond its original form with the inclusion of vast numbers of new words describing things that were essentially un-
known in the ancient world. Stearn makes the point that Pliny (the Elder) would have well understood the Latin descriptions of plants in the 15th and 16th centuries, but would have been lost by the divergent Latin of the 18th and 19th centuries. Proof of the giant leap in knowledge from the time of the ancients to the present day can be found in the fact that the early botanists like Theophrastus in the third century B.C. described about 500 plants, Pliny three centuries later described about 1000, herbalists of the 15th and 16th centuries perhaps 4000, Linnaeus in the 18th century around 7300, and modern botanists some 250,000 to 300,000 species of flowering plants.

There is therefore little need to utilize strictly-classical Latin pronunciation. Over the years, usage has resulted in certain informal rules of pronunciation, but even these may give way to a person's own preferences, and are naturally influenced by such things as where he or she grew up, and what pronunciations they were exposed to during their lifetimes. It may seem simplistic, but what sounds right is often the best standard by which to decide how to pronounce botanical names. However, consistency of pronunciation is to be strived for, and the person who finds himself speaking such names aloud usually does develop a fairly uniform style. The Jepson Manual emphasizes the following points: (1) classical scholars don't always agree on pronunciation; (2) professional botanists vary significantly in their pronunciation; (3) individual botanists rarely are completely consistent in pronunciation; and (4) people tend to pronounce names the way they first learned them regardless of any subsequently-encountered rules.

William Stearn in his Botanical Latin says "Botanical Latin is essentially a written lan-
guage, but the scientific names of plants often occur in speech. How they are pronounced really matters little provided they sound pleasant and are understood by all." And A.W. Smith in A Gardener's Handbook of Plant Names echoes this sentiment by saying: "Within reasonable limits, nobody need be too disturbed about pronunciation." Obviously however some people are disturbed by pronunciation, but I believe that the sound and flow of the words should govern or certainly at least affect how they are pronounced, and therefore awkward or cumbersome pronunciations should be avoided even when they are technically pursuant to some rule. To quote the great horticulturist and Dean of American gardeners L.H. Bailey, "There is no standard agreement on rules for the pronunciation of botanical binomials. Even in the best practice, there may be variations in pronunciation of a given word; this is unavoidable, and no more to be regretted than similar variations in pronouncing many English words. The particular sound to be given the vowels (within the categories "long" and "short") rests with the individual."

There are a few guidelines that may be helpful to those learning to pronounce the names I have included in this website. One should try to divide the names into separate syllables on the presumption that each vowel is a different syllable, and try to pronounce each and every syllable. This is not always the case because there are combinations of two vowels such as 'ae' and 'oe' (dipthongs) that form a single sound. In two-syllable words, there is no problem because the accent always falls on the first syllable (MI-nor or AS-per). The vowel of the stressed syllable is short if followed by two or more consonants (Cistus, angustifolia), and long if followed by a single consonant (Rosa). How it is stressed if the vowel is followed by another vowel is unclear. According to the traditional English system of pronunciation, for words of more than two syllables, the accent is customarily on the penultimate syllable where it contains a long vowel (vul-GAR-is or in-de-COR-um). Where that penultimate syllable consists of a short vowel the accent can fall either on that syllable or on the syllable preceding it (EL-e-gans or Di-TAX-is). Although in names of many syllables there can be a secondary stress toward the beginning, the primary stress is never on a syllable before the antepenultimate one. Some additional guidelines on classical systems of syllabification which have largely been preserved but which we need not be overly concerned with are: (1) a consonant between two vowels is united with the following vowel; (2) double consonants are separated between the syllables (stel-lata); (3) the consonant pair 'st' is divided between the syllables within a word (venus-tus); and (4) compound words are divided according to their root elements (tetra-phyllum).

Many plant names have been formed from the proper names of botanists and collectors. As with other aspects of pronunciation, there are conflicting guidelines that apply here.
The Jepson Manual suggests that the pronunciation of proper names be retained when they are incorporated into a botanical name. Sometimes this may be perfectly appropri-
ate, as with hallii or jonesii, and names like parishii and douglasii may comfortably be stressed on either the first or second syllable. However it is frequently cumbersome as in the case of a name like Krascheninnikovia or with other names where the emphasis is on the first syllable of multi-syllable names such as Pearson ( peirsoniana), Eaton (eatonii) or Johnston (johnstonii). A fairly hard and fast rule of Latin accentuation is that no syllable before the antepenultimate one should be stressed, but exceptions have to be made in cases of names such as kennedyi or lemmonii where it would be awkward to accent any syllable but the first. Nevertheless, as a convention, it would probably be preferable to to try to retain the proper name's pronunciation when it can be done in a graceful manner, but there is no absolute right or wrong. When plant names are derived from foreign proper names, it presents another set of problems. Stearn makes the point that when the names are from languages we are familiar with such as the western Euro-
pean ones, we commonly try to retain their pronunciation, but when they derive from less familiar languages we often ignore this practice and just try to say them as though they were English names. Thus Gilia should be pronounced 'JEE-lee-a' because it derives from the Italian proper name Gilii and in Italian the 'g' is soft and an 'i' following a 'g' is pronounced as 'ee.' Similarly, Pluchea should be pronounced 'PLOO-shuh' because it derives from the French proper name Pluche and in French the 'ch' is spoken as 'sh,' and Viguiera should be pronounced 'vig-wee-AY-a' since the name it derives from is the French Viguier (VIG-wee-ay).

Generic names (and those of species and subspecies) are typically made up of two, three, four, five, six or occasionally even more syllables. As a general rule, all vowels are to be spoken as separate syllables. Stearn gives the example of Cotoneaster, which should be pronounced 'co-to-ne-as-ter' and not 'cot-on-easter.' However, an excep-
tion is that certain dipthongs or other vowel combinations such as 'ae' as in Chamaesyce, Chaenactis or Suaeda, 'oe' as in Oenothera or Foeniculum, 'ei' as in Cheilanthes, 'ou' as in Acourtia, 'ai' as in Descurainia, 'eu' as in Eucnide, or 'au' as in Daucus, produce a single spoken sound and would be difficult in the extreme to pronounce individually. 'Ia' when at the end of a word is often spoken as a single sound, as in Ambrosia (am-BRO-
zhe), but may be more correctly pronounced as two syllables such as with Bowlesia (BOWL-zee-a). My dictionary gives 'am-bro-zhe' as the proper pronunciation, but that does not refer to the botanical name. Artemisia and Freesia are two other similar names that are pronounced in both ways, and there is no doubt that Fuchsia is properly pro-
nounced that way. However in the case of other such names as Aloysia, Ivesia, Jamesia, Langloisia, Simmondsia, or Venegasia, it has not seemed proper to pronounce the final 'ia' as a single vowel sound.

The general rule of Latin pronunciation as previously alluded to is that in words of more than two syllables the accent properly falls on the penultimate syllable when it contains a long vowel or dipthong, or when two or more consonants separate the last two vowels, but either on the penultimate or the antepenultimate (last syllable but two) syllable when the penultimate syllable contains a short vowel. Thus it could be either 'a-bro-NI-a' or 'a-
BRO-ni-a' because the penultimate syllable is short, but 'cor-on-O-pus' instead of 'cor-
O-no-pus' because the penultimate syllable ('o') is long. A.W. Smith, who gives phonetic spellings according to the English system, says, "There is no hard and fast rule. When at a loss, use the one that sounds best." Sometimes names are pronounced with either three or four syllables, such as Ambrosia (am-BRO-zhe or am-BRO-see-a) or Ivesia (IVES-ee-
a or i-VEES-ee-a) and this may alter where the stress is placed. Some names may be correctly pronounced according to two different aspects of the rule. A good example would be Sarcostemma, which since its penultimate syllable is short should have the stress placed on the antepenultimate syllable, thus accenting it as 'sar-COS-temma.' This also corresponds with the rule about separating the consonant pair 'st.' However, since its final two vowels are divided by two consonents, it could be equally correct to say that the stress should be placed on the penultimate syllable, thus making it 'sar-co-STEM-ma.' This corresponds with the advice to maintain the integrity of the root words that make up the name, in this case, STEMMA, a crown or garland. Another example would be Porophyllum (either por-OFF-il-um or por-oh-PHYLL-um). In both of these cases, I have heard them pronounced either way. Other names that would seem to fit under both rules of pronunciation, such as Haplopappus, Chamomilla, Keckiella, and Coreopsis, all of which have both penultimate syllables with short vowel sounds and two consonents dividing the final two vowels, are pronounced in a single fairly standard and commonly-
accepted fashion. It appears to me (and I will continue to study further examples to see if this really is the case) that the rule about two consonents dividing the final two vowels has usually but not always trumped the rule regarding the penultimate syllable with a short vowel in cases where both apply, but there will never be total consistency.

Borror's Dictionary of Word Roots and Combining Forms has the most extensive explanation of pronunciation that I've found. He includes the following rules. In names beginning with Ps (Psilocarphus), Pt (Pterostegia), Ct (Ctenium), Cn (Cnicus), Gn (Gnaphalium) or Mn (Mniopsis), the initial letter is silent, but when these letters appear together within a name, the initial letter is pronounced as in Eucnide (yook-NY-dee) or ignota (ig-NO-ta). Interestingly, he does not mention Pn (Pneumonanthe) which follows the same principle and which we are more familiar with from the word 'pneumonia.' A vowel placed at the end of a name takes the long sound (Cakile, cooperi), except for 'a' which is elided to sound like 'uh' (Clarkia). The vowel before a consonant in the final syllable of a name takes the short sound (Carex, Chloris), except for 'es' which sounds like ease (as in Fragmites). An 'x' sounds like 'z' at the beginning of a name (Xanthium), but like 'ks' within the name (Ditaxis). 'Ch' usually has the 'k' sound except in names derived from languages other than Greek. When a 'cc' is followed by an 'i' or a 'y,' it sounds like 'ks' (occidentalis, 'oks-i-dent-al-is' or flaccidus, 'flak-si-dus'). In his section on accentuation, he includes the following as cases of stress being placed on the penul-
timate syllable: words ending in '-ina' (he gives Spartina as an example), words ending in '-ica' (he gives Melica as an example although other sources put the stress on the antepenultimate syllable), and word ending in '-pogon' (Tragopogon would surely be an example but as with the previous reference, other sources stress the antepenultimate syllable). These may well be examples of the more classical pronunciation and thus may not be in common use in America, because I have seen for instance sources that pronounce 'flaccidus' as 'fla-si-dus' which is the way I've always said it and will continue to say it.

It is often difficult to know whether a particular vowel sound should be pronounced long or short, and since the rule about accentuation in penultimate or antepenultimate syllables depends in large measure on the pronunciation of those vowels, it is a matter of no little significance. Fortunately, we can usually refer back to the roots from which the word is derived, and this will help us to know whether vowels should be long or short. The following examples using the vowel 'i' will show how this can be applied. The names 'Linanthus' and 'Linaria' both derive from 'linum' or 'linon' and the first syllable should thus be said as in 'pie.' Similarly 'Limonium' from 'leimon.' However, 'Lithocarpus' comes from 'lithos,' 'Limnanthes' from 'limne' and 'ligulatus' from 'ligula,' and the first syllables in these names should be said as in 'with,' 'rim' and 'pig.' Specific and generic epithets also come from personal names and the pronunciation of those names should be retained as much as possible, as in 'lindleyi' from John Lindley and 'Lippia' from Dr. Agostino Lippi. It is none-
theless aggravating to find inconsistencies such as those in Smith's Handbook of Plant Names where he gives 'Linum' as 'LIE-num' yet 'liniflorus' as 'LIN-i-flor-us,' when they both derive from the same root. If someone can explain this to me, I'd appreciate it. We also have the rule as outlined by H.A. Kelly that stressed vowels followed by a single consonant are pronounced long, whereas those followed by two or more consonants are pronounced short.

Many names exist about which it is difficult to know for certain how they should be properly pronounced. Vowels in particular present problems. An English 'a' can be pronounced as in 'car,' or 'cat,' or 'call,' or 'care,' or 'came,' or 'capacity.' An 'e' can be pronounced as in 'pet,' 'penultimate,' 'personal,' or as the second 'e' in 'pendent.' Depend-
ing on how one pronounces the 'a' in the penultimate syllable, it could be either 'FY-lah-
go' or 'fy-LAY-go' and 'PLAN-tah-go' or 'plan-TAY-go,' and if you just see the name written down without knowing from what it is derived, you would have no idea whether the 'a' should be long or short or something in between. A European gardener would probably choose the former in many cases, while an American gardener would choose the latter, but both would be bound to follow the pronunciation of the root word. Each individual when confronted with a name that could be pronounced in two or more ways should make a choice as to which he prefers and then should try to develop a uniform style. Roland W. Brown in Composition of Scientific Words says "... the essential problem is not What is right or wrong? but What is the best, consistent usage." And the Jepson Manual says, "... practice what sounds good to your ear; conviction is important." I will probably continue to pronounce Achillea as 'ak-ILL-ee-ah' and not 'ak-ill-EE-ah as Stearn says, because it sounds better to me. As a final point, and again quoting from the Jepson Manual, "When someone presumes to correct your pronunciation, a knowing smile is an appropriate response."

There is to my knowledge no complete authoritative list of California plant names (or other plant names for that matter) that gives their pronunciation. Stearn's Dictionary of Plant Names (1972) provides syllabification and a table (included below) of the sounds of various vowels, dipthongs and consonants which require explanation in both the Roman reformed academic system reflecting the classical Latin pronunciation and the traditional English system in customary use today. However many names have come into use since this work was published. Robinson's New Pronouncing Dictionary of Plant Names gives both accenting and phonetics, but is dated 1939. Borror's dictionary came out first in 1960. Additional information is available in the other sources mentioned at the end. Most of these sources are also either British or so old that they do not include many of our current names, and so are of limited use.

The table I include here is excerpted from Stearn's Dictionary of Plants Names (with additions from Borror) and will provide information about the sound of various vowels, dipthongs and consonants that will help to illuminate how plant names should be pro-
nounced. The user of Stearn's work should be aware that as the Jepson Manual says, "As a classical scholar, he clearly prefers the system of 'restored' academic pronunciation." This may explain the occasional oddity in his pronunciations.

One final caveat (and it's a big one): The rules as expressed above by Stearn regarding accentuation and syllabification reflect I believe the more scholarly classical system and are in accord with most European botanists, thus they may not be completely applicable in the traditional English system. I have not been able to determine this to my satisfaction. The question of whether vowels are to be said long or short is another fly in the ointment ('vul-GARE-is' or 'vul-GAH-ris') because the above system depends on knowing if the penultimate syllable contains a long or short vowel sound. The Jepson Manual gives the following guidelines for the European pronunciation of letters which by and large corres-
ponds with the classical system:

People trained in or used to this system would therefore be inclined to say 'vul-GAH-ris,' whereas I prefer 'vul-GARE-is.'

Before I leave off this essay on pronunciation, I feel I must quote a lovely poem from Johnson and Smith's Plant Names Simplified about the genus Cyclamen, a Mediter-
ranean member of the Primulaceae, which goes like this:

And when struggling with what appear to be inconsistencies, it is best to recall the words of Ralph Waldo Emerson, "A foolish consistency is the hobgoblin of little minds," Oscar Wilde, "Consistency is the last refuge of the unimaginative," and finally, Aldous Huxley, "Consistency is contrary to nature, contrary to life. The only completely consistent people are the dead."

In the following list of accentuated generic names, an apostrophe succeeds each stressed syllable. First are the names that follow Stearn's guidelines, and then the few that don't seem to. In some cases, I have included a name in two places because contrary guidelines can produce differing pronunciations. I have also included some explanatory notes on pronunciation and accents which highlight the inconsistencies or contradictions apparent in the way certain names are spoken. This list represents as far as I know all the generic names currently in the flora of the Santa Monica Mountains. Eventually, I will create a list of all the generic names in California with phonetic spelling for each which will be linked from this page. The reader should be aware that I am not an authority, and for any particular phonetic pronunciation which I give, there could well be some arcane rule I am unaware of or some etymological reason why I am wrong.

A guide to the pronunciation of all the botanical names in my website may be found by clicking here.

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One-syllable names, pretty rare (actually the only ones that showed up in a quick run-through of the list of currently accepted Jepson names in California):

Lens
Phlox
Rhus

Two-syllable names, penultimate syllable either short or long, stress always on penultimate (1st) syllable:

A'cer ("A-ser" because 'c' before 'e' is pronounced as in "center")
Al'nus
As'ter
Ba'tis
Be'ta
Bi'dens
Bri'za
Bro'mus
Cac'tus
Ca'rex
Chlor'is
Cis'tus ("SIS-tus" because 'c' before 'i' is pronounced as in "center")
Cni'cus ("NY-kus or "NI-kus," because when a word begins with 'cn,' the C is silent)
Cor'nus
Cres'sa
Cro'ton
Cryp'sis
Dau'cus
Dra'ba
Em'ex
Garr'ya
Gau'ra
I'va
Jug'lans
Jun'cus
Lem'na
Li'num
Lo'tus
Ly'thrum
Mal'va
Mar'ah
Men'tha
Na'jas
Na'ma
Nu'phar
Phy'la
Pi'cris
Pi'sum
Pluche'a (Original French name pronounced PLOOSH, so "PLOOSH-a")
Po'a
Pru'nus
Quer'cus
Rham'nus
Ri'bes
Ro'sa
Ru'bus
Ru'mex
Ru'ta
Sa'lix
Scan'dix
Schi'nus (either "SHI-nus" or "SKI-nus" would seem to be correct)
Scir'pus
Se'dum
Si'da
Son'chus
Sor'ghum
Sta'chys
Sti'pa
Suae'da
Ty'pha
Vin'ca
Vi'tis
Yu'cca
Jaume'a ("ZHOME-a") (Pronunciation based on personal name derived from)

Three-syllable names, penultimate syllable short, stress usually on the antepenultimate syllable:
(*names that are exceptions to the general rule)

Al'lium
An'themis
Aph'anes
Ap'ium
Ar'abis
At'riplex
Bac'charis
Bas'sia
Ber'beris
Ber'ula
Bowl'sia
Bras'sica
Budd'leja
Ca'kile
Can'nabis
Clar'kia
Cir'sium ("SIR-see-um" because 'c' before 'i' is as in "center")
Cle'matis
Co'nium
Co'tula (This name along with Crassula, Mimulus and Populus puzzled me for a while until
  I realized that the final 'u' vowel was not really a long sound)
Cras'sula
Cy'nara (There is disagreement on this one with three sources going this way and two
  with "sy-NAR-a." "SY-nar-a" because 'c' before 'y' is as in "center." Applies to the
  following two entries)
Cy'nodon
Cy'perus
Dac'tylis
Ditax'is*
E'lymus
Frax'inus
Ga'lium
Gi'lia ("JEE-lee-ah" because 'g' before 'i' is soft and because 'i' in Italian is pronounced as
  'ee' and this name is derived from an Italian proper name)
Hoi'ta* (pronounced "ho-IT-ay" because it is a Native American name)
Ives'ia
Kick'xia
Ko'chia
La'mium
La'thyrus
Lay'ia
Li'lium
Lo'lium
Ly'cium
Ma'dia
Me'lica
Mi'mulus
My'rica
Ox'alis (Robinson, Bailey, Coombes and Johnson and Smith all accent it this way which is
  according to the rules, but I have heard it pronounced more often as Stearn gives it, "ox-
  AL-is" and I think I will continue to say it that way)
Pa'nicum
Pa'paver
Pas'palum
Pen'stemon
Phrag'mites ("FRAG-mi-tees")
Plan'tago (Could also be pronounced "plan-TAY-go")
Pla'tanus
Po'pulus
Ra'phanus
Ri'cinus ("ri-SI-nus" because 'c' before 'i' is as in "center")
Rom'neya (I think this sounds better than "rom-NEE-a" which is the way Stearn accents it
  and this is the way the proper name pronunciation is preserved)
Rup'pia
Sal'via
Si'lybum
Spar'tium
Sper'gula
Ta'marix
Tau'schia
Tor'ilis (This would be according to the rule, but I think "tor-I-lis" sounds better)
Tri'bulus
Tri'ticum
Ur'tica
Vi'cia ("vi-SEE-a" because 'c' before 'i' is as in "center")
Vul'pia
Xan'thium
Zos'tera

Three-syllable names, penultimate syllable long or two or more consonents dividing the last two vowels, stress on penultimate syllable:

Agros'tis
Ailan'thus
Alis'ma
Amor'pha
Arun'do
Ave'na
Azol'la
Brodiae'a
Capsel'la
Caulan'thus
Centran'thus ("sen-TRAN-thus" because 'c' before 'e' is pronounced as in "center")
Chaenac'tis
Cheilan'thes
Cleo'me
Cony'za
Cryptan'tha
Cupres'sus
Cuscu'ta
Datis'ca
Datu'ra
Dicen'tra ("di-SEN-tra" because 'c' before 'e' is pronounced as in "center")
Dichon'dra
Distich'lis
Ehrhar'ta
Eucryp'ta
Festu'ca
Fila'go
Githop'sis
Hibis'cus
Lactu'ca
Linan'thus
Lupi'nus
Malos'ma
Malvel'la
Micro'pus
Noli'na
Oenan'the ("ee-NAN-thuh")
Pellae'a (This is an odd one because it is clearly pronounced "pell-ee-ah" and the only
  question is where the accent falls. Stearn says Pella'ea which seems wrong. Robinson
  gives Pellea'a which would accord with the rules. I have always said "PELL-ee-ah" but
  that could well be wrong)
Phala'ris
Pholis'ma
Planta'go (Could also be pronounced "PLAN-ta-go")
Plectri'tis
Sile'ne
Sola'num
Romne'ya ("rom-NEE-a." This is the way the rule would dictate and this is the way Stearn
  accents it, but Robinson says "ROM-nee-a," and that's the way the pronunciation of the
  proper name would be preserved. I will probably continue to say it this latter way)
Rorip'pa
Ruper'tia (The accenting of this name is based on 'ti' within a word sounding like 'sh' in
  'nation' which would make the final two letters together into a single syllable that would   be separated from the penultimate syllable by two consonants, thus "ru-PUR-sha",   though it could also be pronounced "ru-PUR-tee-a" as a 4-syllable word)
Salso'la
Sambu'cus
Samo'lus (Stearn accents this name as "SA-mo-lus" as do Johnson and Smith, but that
  doesn't sound right to me and would seem to go against Stearn's own guidelines unless
  the 'o' is not pronounced as a long vowel, i.e. "SAM-uh-lus")
Sina'pis
Soli'va
Suae'da ("su-EE-da")
Thalic'trum
Triglo'chin
Verbas'cum
Verbe'na
Vio'la
Yabe'a ("ya-BEE-a")

Four-syllable names, penultimate syllable short, stress usually on the antepenultimate syllable:
(*names that are exceptions to the general rule)

Abro'nia
Achna'therum (This is one of those names that can be accented differently based on the
  pronunciation of the 'e,' either "ak-NA-theer-um" or "ak-na-THEER-um")
Acrop'tilon
Acour'tia
Ago'seris
Ambro'sia
Amman'nia
Amo'philla
Amsin'ckia
Apte'nia
Arge'mone
Aris'tida
Ascle'pias
Aspho'dolus (If it hadn't been for doing this research, I would probably have continued to
  say "as-pho-DEL-us" which sounds better to me, but numerous sources pronounce it   this way)
Asple'nium
Astra'galus
Athy'sanus
Bloome'ria
Boyki'nia
Brickel'lia
Calli'triche
Castille'ja*
Carda'mine
Corda'ria
Centau'rea ("sen-TOR-ee-ah" because 'c' before 'e' is as in "center." Also applies to next
  two entries)
Centau'rium
Cera'stium
Chloro'galum (This doesn't sound right to me but both Stearn and Bailey list it this way)
Chrysan'themum
Cicho'rium ("si-KOR-i-um" because 'c' before 'i' is pronounced as in "center")
Clayto'nia
Collin'sia
Convol'vulus
Cucur'bita
Delphi'nium
Diplota'xis*
Dryop'teris
Elat'ine
Ence'lia ("en-SEE-lee-ah" because 'c' before 'e' is pronounced as in "center"
Eri'geron
Ero'dium
Ery'simum
Eschschol'zia
Euphor'bia
Eutha'mia
Foeni'culum
Franke'nia
Gale'nia
Gastri'dium
Gera'nium
Gnapha'lium ("na-PHAY-li-um." I have not yet found a rule that accounts for this pro-
  nunciation, but it is like Cnicus or pneumonia)
Grinde'lia
Guille'nia
Hazar'dia
Hedyp'nois ("hed-IP-no-is")
Hele'nium
Hirschfel'dia
Horke'lia
Iso'meris
Juni'perus
Koeler'ia ("kee-LER-ia")
Lamarc'kia
Lastar'riaea
Lasthe'nia
Lepi'dium
Lessin'gia
Lewi'sia
Limo'nium
Lina'ria
Lobe'lia
Loeflin'gia ("lef-LIN-jee-a")
Loma'tium
Loni'cera ("lo-NIS-er-a" because 'c' before 'e' is pronounced as in "center")
Ludwi'gia
Maleph'ora
Marru'bium
Mentze'lia
Micro'seris
Mira'bilis
Nemo'phila
Opun'tia
Paeo'nia ("pee-OWN-ee-a")
Paraph'olis
Peri'tyle
Petu'nia
Phace'lia ("pha-SEE-lee-ah" because 'c' before 'e' is pronounced as in "center")
Pholi'stoma (I have no source that gives the accenting of this name. I have always said it
  this way, although if the final 'o' is pronounced as a long vowel, then the rule would make
  it "pho-lis-TOE-ma," but I've never heard it said that way)
Pilos'tyles
Pipe'ria
Pipta'therum (Could also be accented "pip-ta-THEER-um" depending on pronunciation
  of the 'e')
Platyste'mon*
Poly'gala (I've always said it "poly-GAL-a" but the rule and every source I have that list it
  are in agreement that it should be stressed this way)
Polyg'onum
Pteri'dium ("ter-ID-ee-um")
Ranun'culus
Sani'cula
Sarco'stemma (This is the way Stearn accents it, but the rule could be interpreted to allow
  either "sar-CO-stemma" or "sar-co-STEM-ma")
Sene'cio ("se-NEE-ci-o" because 'c' before 'i' is pronounced as in "center")
Seta'ria
Sherar'dia
Sisym'brium
Sporo'bolus
Stellar'ia
Tara'xacum
Thelyp'teris
Trichos'tema
Trifo'lium
Vero'nica
Wigan'dia
Woodwar'dia

Four-syllable names, penultimate syllable long or two or more consonents dividing the last two vowels, stress usually on penultimate syllable:

Achille'a (I have always thought this name should be accented "Ah-KI:LL-ee-ah" which in
  addition to sounding better to me retains the original pronunciation of the name Achilles
  on which it is based (and Coombes and Bailey do accent it that way), but Stearn,   Robinson, and Johnson and Smith all accent it this way)
Achnather'um (This is one of those names that can be accented differently based on the
  pronunciation of the 'e,' either "ak-NA-the-rum" or "ak-na-THEER-um")
Adian'tum
Allophyl'lum
Amaran'thus
Anagal'lis
Andropo'gon
Anemop'sis
Antirrhi'num
Apias'trum
Apocy'num ("a-po-SY-num" because 'c' before a 'y' is pronounced as in center)
Aspido'tis
Barbare'a (Robinson, Bailey, Taylor and Johnson and Smith all accent it this way which
  would be in accord with the rule, but Stearn goes against his own guidelines to accent it
  "bar-BARE-ee-ah," which is the way I've always thought it should be said)
Calochor'tus
Cameli'na
Carpobro'tus
Ceano'thus ("see-a-NO-thus" because 'c' before 'e' is pronounced as in "center." Applies   also to the following entry)
Cercocar'pus
Chamaesy'ce
Chamomil'la
Chorizan'the
Ciclosper'mum ("sik-lo-SPER-mum" because 'c' before 'i' is pronounced as in "center")
Cordylan'thus
Coreop'sis
Corono'pus
Dendrome'con (Could also be Dendro'mecon depending on the pronunciation of the final
  'e')
Dithyre'a (I have always said this name as "di-THEER-y-a," but that may be incorrect)
Emmenan'the
Epipac'tis
Equise'tum
Eragros'tis
Erechti'tes
Erias'trum
Eucalyp'tus
Galinso'ga
Glycyrrhi'za
Haplopap'pus
Helian'thus
Holodis'cus
Hypochaer'is (Could also be pronounced 'hy-POK-er-is" based on pronunciation of the   final syllable)
Ipomoe'a ("ip-oh-MEE-a")
Isoco'ma
Keckiel'la
Lagophyl'la
Lavate'ra* ("lav-a-TEER-a") (Perhaps this reflects a retention of the original proper   name)
Leptochlo'a
Lithophrag'ma
Malaco'thrix
Matthio'la
Meconel'la
Medica'go
Melilo'tus
Modio'la
Monardel'la
Mucrone'a
Nemacla'dus
Notho'laena ("no-tho-LEE-na")
Oenothe'ra ("ee-no-THEER-a")
Oroban'che ("oro-BAN-kee" if pronounced classically, otherwise ?)
Osmorhi'za
Parapho'lis (Could also be accented "pare-A-pho-lis" depending on pronunciation of the
  'o')
Pectocar'ya
Pennise'tum
Pentachae'ta ("pen-ta-KEE-ta")
Pentagram'ma
Phoraden'dron
Phyllospa'dix
Piptathe'rum (Could also be accented "pip-TA-ther-um" depending on pronunciation of
  the 'e')
Polycar'pon
Polypo'gon
Porophyl'lum
Portula'ca
Potentil'la
Psilocar'phus
Salpichro'a (Stearn oddly accents this as "sal-PI-crow-a")
Sanguisor'ba
Sature'ja (Robinson pronounces this as "sa-tur-EE-ya' but Stearn says that 'j' should be
  pronounced as in 'jam' except in the reformed academic system)
Saxifra'ga
Solida'go
Stenota'phrum
Stylocli'ne
Stylome'con
Tragopo'gon
Tropaeo'lum ("tro-pee-O-lum")
Uropap'pus
Verbesi'na (The '-ina' ending requires stress on the penultimate syllable)
Wolfiel'la
Zigade'nus

Five-syllable names, penultimate syllable short, stress usually on the antepenultimate syllable but sometimes on the penultimate syllable:

Arctosta'phylos
Adenosto'ma (Could be accented "a-den-o-STO-ma" if the final 'o' is pronounced as a
  long vowel)
Arena'ria
Artemi'sia
Calandri'nia
Calyptri'dium
Calyste'gia
Chamaeba'tia
Camisso'nia
Chenopo'dium
Cortade'ria
Descurai'nia
Digita'ria
Eleo'charis
Epilo'bium
Erio'gonum
Fritilla'ria
Helian'themum
Hemizo'nia
Hernie'ria
Heterome'les
Heterothe'ca
Hiera'cium ("hi-er-AK-ium"
Hydrocot'yle
Lepechi'nia ("lep-eh-CHIN-ee-ah" or "lep-eh-KIN-ee-ah")
Leptodac'tylon
Lobula'ria
Lycoper'sicon
Minuar'tia
Muhlenber'gia
Monolo'pia
Navarret'ia
Oligo'meris
Pickerin'gia
Polypo'dium
Probosci'dea ("pro-bos-KID-ee-ah")
Pteroste'gia
Rafines'quia
Salicor'nia
Scrophula'ria
Scutella'ria
Sisyrin'chium
Soleiro'lia ("sol-aye-ROL-ia")
Spergula'ria
Stebbinso'seris
Tetrady'mia
Tetrago'nia
Venega'sia

Five-syllable names, penultimate syllable long or two or more consonents dividing the final two vowels, stress on penultimate syllable:

Achyrachae'na ("a-ky-ra-KEE-na")
Adenophyl'lum
Adenosto'ma (Could be accented "a-den-O-sto-ma" if the final 'o' is not pronounced as a
  long vowel)
Agerati'na ('-ina' is a word ending that according to Borror requires stress on the
  penultimate syllable)
Amblyopap'pus
Ancistrocar'phus
Bothriochlo'a
Cardione'ma
Dichelostem'ma
Dimorphothe'ca
Echinochlo'a
Echinodo'ros
Eremocar'pus
Eriophyl'lum
Hesperocni'de ("hes-per-oke-NY-dee" or "hes-per-o-NY-dee")
Hesperoli'non
Lepidospar'tum
Malacotham'nus
Monanthochlo'e
Nicotia'na ("ni-ko-shi-AIN-ah")
Osteosper'mum
Pedicula'ris
Pityrogram'ma
Plagiobo'thrys
Potamoge'ton (If the 'e' is pronounced as a long vowel, then this would be accented as
  "po-tah-mo-JEE-ton," the 'g' being soft because it precedes an 'e', but if not it would be
  accented as po-tah-MO-jet-on")
Selaginel'la
Symphoricar'pos
Thysanocar'pus
Toxicoden'dron
Tropidocar'pum
Zannichel'lia

More-than-Five-syllable names, penultimate syllable short, stress on antepenultimate syllable:

Comarosta'phylis
Ericame'ria
Eriodic'tyon
Gutierre'zia
Heliotro'pium
Krascheninniko'via
Mesembryan'themum
Parieta'ria
Stephanome'ria
Umbellula'ria


-----------------------------------------------------------------------------------------
Names that don't follow the above general guidelines:

Three-syllable names, penultimate syllable long, stress not on penultimate syllable:

Car'duus
Dud'leya
Hor'deum
O'lea
Stan'leya

Four-syllable names, penultimate syllable long, stress not on penultimate syllable:

Sidal'cea ("si-DAL-see-ah")
Trite'leia (Johnson and Smith pronounce it "tri-tel-EE-ah" and Coombes pronounces it
  "tri-tel-AY-ah". 'Ei' should be pronounced as in height, which would make it tri-tel-aye-
  ah" which sounds awkward. I've always said it 'tri-TEL-ee-ah" and I think I'll just keep
  on with that)

Five-syllable names, penultimate syllable long, stress not on penultimate syllable:

Dodeca'theon

-----------------------------------------------------------------------------------------
Questions and inconsistencies

The word ending '-ina': Ageratina, Camelina, Nolina, and Verbesina all should sound the same but apparently they don't, so perhaps the pronunciation of the penultimate syllable derives from usage of different systems. I see no reason why they should not all have the same-sounding endings and I am going to strive to pronounce them consistently as "eena." Names beginning with 'Sc' are pronounced differently and I have no idea why. Examples are Scilla (SILL-a) and Scirpus (SKER-pus). Names beginning with 'Th' also are pronounced differently and I don't understand this either. Examples are Thymus (TY-mus) and Thysanotus (thy-san-OH-tus). Maybe it goes back to the way their word roots were pronounced in Greek. If anyone can explain this, please let me know.

Sources

Books and journal articles:
Bailey, L.H. How Plants Get Their Names (1933)
Bailey, L.H. Manual of Cultivated Plants (1949)
Borror, Donald J. Dictionary of Word Roots and Combining Forms (1960)
Brown, Ronald W. Composition of Scientific Words (1956)
Coombes, Allen Dictionary of Plant Names (1985)
Else, G.F. "The Pronunciation of Classical Names and Words in English" The Classical Journal (1967) 62:210-214
Gledhill, David The Names of Plants (1985)
Hickman, James (Ed.) The Jepson Manual: Higher Plants of California (1996)
Jaeger, Edmund C. A Source-book of Biological Names and Terms (1944)
Johnson, A.T and Smith, H.A. Plant Names Simplified (1931)
Kelly, J.A. "Pronouncing Latin Words in English" Classical World (1986) 80:1, 33-37.
Neal, Bill Gardener's Latin (1992)
Robinson, E.R. New Pronouncing Dictionary of Plant Names (1939)
Smith, A. W. A Gardener's Handbook of Plant Names, Their Meanings and Origins (1963)
Stearn, William T. Botanical Latin (4th Edition, 1992)
Stearn, Wiliam T. Stearn's Dictionary of Plant Names for Gardeners (1992)
Taylor, Norman Taylor's Encyclopedia of Gardening (1948)

Websites:
Fine Gardening's Guide to Pronouncing Botanical Latin
Pronunciation of Biological Latin
The Pronunciation of Classical Latin