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PPDC Botany lab

Evolution and systematics of Rhododendron occidentale

Dr. Fred Hrusa
published in: 2012, Madroño 59(3)

Morphological and isoenzyme variation among populations of western azalea, Rhododendron occidentale (Torr. & A. Gray) A. Gray, were examined. Three regional parapatric groups were revealed:
1) the northern California outer North Coast Ranges; 2) the northern California and southern Oregon Klamath Ranges; and 3) the central California Sierra Nevada and southern California Peninsular Ranges. A highly variable but generally intermediate fourth group is restricted to ultrabasic substrates
(serpentine) in the middle and inner North Coast Ranges of California. It is comprised of populations with recombined morphologies and alleles that were otherwise restricted to one or more of the three groups above. A revised intraspecific treatment is proposed, with the three regional groups above recognized as varieties. These are: Rhododendron occidentale (Torr. & A. Gray) A. Gray var. occidentale (outer North Coast Ranges), Rhododendron occidentale var. paludosum Jeps. (Klamath Ranges), and Rhododendron occidentale var. californicum (Torr. & A. Gray) Hrusa comb. et stat. nov. (Sierra Nevada and Peninsular Ranges).
Lectotypifications of Azalea occidentalis Torr. & A. Gray isotypes at PH and GH, Azalea californica Torr. & A. Gray in Durand, and Rhododendron sonomense Greene (NDG) are also provided.

Hrusa, G.F. 2012. Morphological and Isoenzyme Variation in Rhododendron occidentale (western azalea) (section Pentanthera; Ericaceae). Madroño 59(3): 128–142.

Systematics of Salsola (russianthistle, tumbleweed) in western North America

Dr. Fred Hrusa
published in: 2008, Madroño 55(2)

Since its introduction to North America in the mid 19th century, the invasive weed Salsola tragus sensu auct. (russianthistle), has become a widespread and troublesome plant pest. Early biocontrol attempts had achieved only partial success. Efforts to improve chances for success in renewed biocontrol research efforts revealed that there are actually two distinct, often sympatric, genetic entities which comprise what has been called Salsola tragus. Recent publications have referred to these as Salsola tragus and Salsola ‘type B’. Continued biocontrol efforts required the identification and characterization of ‘type B’. In the process, a third form was recognized and called ‘type C’. We examined Salsola tragus and Salsola ‘type B’ using morphometrics and ‘type C’ using DNA sequence data and morphometrics. Salsola tragus and ‘type B’ were morphologically distinct; ‘type C’ was morphologically intermediate between them and contained a combination of DNA sequence haplotypes mostly exclusive to S. tragus and exclusive to ‘type B’. We present a taxonomic and morphological characterization of Salsola tragus, Salsola ‘type B’ and ‘type C’ using discriminant analysis with DNA sequence genotypes as its taxonomic framework. We provide a pre-existing name, Salsola australis R. Br. for ‘type B’ and propose Salsola ryanii sp. nov. for ‘type C’. Morphological variation, habitats and dispersal behaviors among these Salsola taxa were examined in the herbarium and in the field. These are compared and discussed from both ecological and biocontrol viewpoints.

Hrusa, G.F., & J.F. Gaskin, 2008. The Salsola tragus Complex in California (Chenopodiaceae): Characterization and Status of Salsola australis and the Autochthonous Allopolyploid Salsola ryanii sp. nov.  Madroño 55(2): 113-131.

The Salvinia auriculata Aubl. complex (Salviniaceae)

Dr. Fred Hrusa
published in: 1999, California Plant Pest and Disease Report, 18(1-2)

The genus Salvinia comprises approximately 10 species of heterosporous floating aquatic ferns native mostly to South America and with vicariant species in Europe, Asia and N. Africa (Mabberley 1997). Some are grown as ornamentals and via anthropogenic introductions into wild areas have become naturalized on all continents except Antarctica.

All Salvinia species have a dense covering of water-repellent trichomes ("hairs") on the adaxial leaf surface. In the Salvinia auriculata complex, a group of three morphologically similar species native to subtropical and tropical South America (S. biloba Raddi, S. herzogii de la Sota, and S. auriculata Aubl. sensu stricto) plus one widely naturalized and aggressive sterile hybrid (S. molesta D.S. Mitch.), these trichomes are distinctive and serve to identify the group (Thomas & Mitchell, 1972). Fortunately, when completely developed, the characteristic trichome form is easily visible under only moderate magnification. These trichomes have a lantern-like structure, also sometimes referred to as “eggbeaters” from their similarity to a common kitchen appliance, and are formed from four slender connate “hairs” raised above the leaf surface with the upper half diverging then again converging and united at their distal tips. Such trichomes are often as much as three or four mm. tall and completely cover the lamina adaxial surface. However, if the individual plant is growing in shade or if the leaves are aged or deteriorated, these trichomes may develop into or be reduced to no more than small papillae-like structures less than 1 mm tall. On the same or different plants in the same colony, on somewhat younger leaves they may be developed into rather stout 2-4 branched structures more than a mm. in length, but not convergent at the distal tips. This is a common reaction in the S. auriculata complex to poor growing conditions, both in the wild and in cultivation. When moved into and maintained under more favorable growing conditions newly developed leaves on these same plants form the typical slender 4-branched trichomes described above.

Comparison of leaf venation patterns has also been proposed as a method of distinguishing these four species (Forno, 1983) but the discontinuities among the venation forms are not well-marked and determinations thus based are not highly robust. The most discontinuous feature among the species is chromosome number. However diploid numbers range from 2n = 36 to 2n = 63 (Schneller, 1981), and technical difficulties associated with obtaining reliable counts on numbers this high make routine identification using chromosome counts infeasible.

Leaf shapes within and among most species of Salvinia, and particularly the Salvinia auriculata complex, are highly variable and the leaf lamina may be as little as 1 cm. in width (or less) or up to 3-4 cm. wide (or more) and vary in shape from nearly ovate or orbicular with a ± cordate base and acute tip to obovate with a cuneate base and obtuse tip. Plants with leaves on the smaller end of the size range and with concurrently reduced trichomes, as described above, could be easily confused with the relatively non-invasive Salvinia minima Baker or possibly even Salvinia natans (L.) All. non Pursh depending upon the individual leaf observed. Thus leaf shape and size are of little help in distinguishing the species. Careful observation of young, vigorously growing leaves with fresh trichomes is necessary to separate the complex from these two relatively benign species.

The species of the complex are variably fertile; based on the proportion of filled microspores fertility varies from partial sterility in Salvinia herzogii (hexaploid with 2n = 63) to essentially complete sterility in Salvinia molesta (pentaploid with 2n = 45) (Schneller op. cit). Sporocarp clusters are produced only sporadically in cultivation and the clusters vary so much in form and arrangement that the different species often appear much like each other. The most aggressive and invasive species, Salvinia molesta, forms mostly empty microsporangia, the dominant presence of which may serve to separate this species. However, even in this species some microsporangia are filled, and although appearing fertile, these are the result of the same meiotic irregularities in the pentaploid that caused complete abortion of the remaining microspores, and effectively renders them inviable. Thus, based on the reproductive organs, Salvinia molesta is not readily separable from the other species even when these organs are produced.

Under favorable conditions Salvinia molesta has been reported capable of doubling its biomass in under two days (Cross, 1999), four days (Thomas & Mitchell 1972), or a week (Mitchell & Tur 1975) and can spread rapidly via fragmentation followed by wind, anthropogenic or other biotic dispersal mechanisms. In any case the consequences for slow moving warm-water rivers, artificial ponds, or even large lakes (Mitchell 1972) are dire. The plants can form solid mats up to a two feet or more thick, exclude surface-utilizing organisms, shade out other aquatic vegetation and dead or dying Salvinia plant material can raise the biological oxygen demand (BOD) to levels critical to the survival of all other aquatic organisms (Thomas & Room 1986).

Although among these species Salvinia molesta has the most notorious reputation as an invasive weed, all are considered potentially pestiferous, and because of the difficulty distinguishing among them using external morphological characteristics the entire complex is considered a threat to California wetlands.

At present, no plants referable to the Salvinia auriculata complex are known to be growing outside of cultivation anywhere California. A lack of perennating structures or dormant spores make Salvinia survival dependent upon the protection of the meristematic areas. The meristems are generally situated in surface plants within two cm. of the water surface (Whiteman and Room, op. cit.) and are thus protected to some extent according to the thermal mass of the habitat, that is, larger bodies of water will be more likely to protect the meristems during winter cold. Tests have shown the meristems are capable of surviving temperatures of –3°C (~27°F) but the plant does not survive where ice forms on the water surface. Surface air temperature of 40°C (104°F) caused dieback of the leaves, but regrowth rapidly occurred when temperatures declined (Whiteman and Room, op. cit.). The species is thus capable of surviving, growing or rapidly increasing in various regions of California depending on the minimum and to a lesser extent the maximum water surface temperature of a given site.


  1. Cross, Gene B. 1999. "Detection of Salvinia molesta in North Carolina" in Memorandum, Feb. 3, 1999, North Carolina Dept. of Agriculture and Consumer Services, Plant Industry Division.
  2. Forno, I.W., 1983. "Native Distribution of the Salvinia auriculata complex and Keys to Species Identification", Aquatic Botany 17: 71-83.
  3. Mabberley, D.J. 1997. "The Plant Book" ed. 2. Cambridge University Press, 858 pp.
  4. Mitchell, D.S. & N.M. Tur, 1975. "The rate of growth of Salvinia molesta (Salvinia auriculata auct. non Aubl.) in Laboratory & Natural Conditions." Journal of Applied Ecology 2: 213-225.
  5. Mitchell, D.S., 1972. "The Kariba weed: Salvinia molesta" Brit. Fern Gaz. 10 (5): 251-252.
  6. Schneller, J.J., 1981. “Cytotaxonomic Investigations of Salvinia herzogii de la Sota” Aquatic Botany 9: 279-283.
  7. Thomas, P.A. & D.S. Mitchell, 1972. "Ecology of water weeds in the neotropics" in Contributions to the International Hydrological Decade, Unesco, Paris. 13-21.
  8. Thomas, P.A. & P.M. Room, 1986. "Taxonomy and Control of Salvinia molesta." Nature 320: 581-584.
  9. Whiteman, J.B. & P.M. Room, 1991. "Temperatures lethal to Salvinia molesta Mitchell". Aquatic Botany 40: 27-35.

Limnobium spongia L. sensu lato (Hydrocharitaceae)

Dr. Fred Hrusa
published in: 1999, California Plant Pest and Disease Report, 18(1-2)

The Limnobium spongia L. group (spongeplants, frogbit) (Hydrocharitaceae) are monocotyledonous floating or terrestrial aquatic plants composed of two monotypic genera, Hydromystria (or sometimes Trianea) and Limnobium; two species in Limnobium; or one species with two subspecies in Limnobium, depending on the generic or species concepts and taste of the particular taxonomist (Cook & Urmi-Konig 1983). For this discussion we will consider Limnobium to consist of two species, Limnobium spongia L. sensu stricto and L. laevigatum (Humb. & Bonpl. ex Willd.) Heine. Limnobium spongia is native to the southeastern United States where it is found along the eastern seaboard as far north as Delaware. Along the gulf coast it extends west to eastern Texas and inland up the Mississippi to extreme southern Illinois. Limnobium laevigatum is found from Central Mexico and Cuba south to Argentina, and has been introduced into Puerto Rico. It ranges widely in elevation, occurring from sea level where it is most common, to 2000 meters in central Mexico and to 2800 meters in Colombia (Cook & Urmi-Konig op. cit.).

In habit these taxa are similar to the floating aquatic species Hydrocharis morsus-ranae L. (also often called frogbit). These two genera can be difficult to distinguish when in vegetative condition and this difficulty probably has been responsible for both the sometimes shared common name and the labeling for sale of Limnobium as Hydrocharis morsus-ranae. Careful attention to a few easily seen differences can distinguish H. morsus-ranae from L. spongia or L. laevigatum. Flowers of both Hydrocharis and Limnobium are unisexual and the plants monoecious with staminate and pistillate flowers formed on the same or different rosettes. Staminate flowers of both genera have sepals and petals and the pistillate flowers may or may not have petals. In Hydrocharis the petals are showy and generally at least 1.5 times the length of the sepals while Limnobium petals are generally no longer than the sepals and not showy. In Limnobium the leaf form changes according to growth stage from floating and horizontal with a slightly cordate base and petiole shorter than to a little longer than the leaf lamina, to vertical with a tapered base and long petiole often 5 times or more the length of the lamina while Hydrocharis leaves remain always of the floating type. When in this condition Limnobium can be distinguished from Hydrocharis morsus-ranae by its distinct aerenchymatous pad on the leaf undersurface which serves as a float. When aerial leaves are formed the petiole becomes aerenchymatous and provides the necessary buoyancy. Roots of Limnobium are dimorphic (branched) and while those of Hydrocharis are monomorphic (unbranched).

Limnobium spongia and L. laevigatum are a bit more difficult to distinguish. The most reliable characteristic is stamen number. Generally there are six in L. laevigatum and 9-12 in L. spongia. In the absence of flowering material leaf tip shape is useful; the leaf tip is more or less acute in L. spongia but decidedly rounded in L. laevigatum. Otherwise these two species are quite similar appearing and both have the same aggressive habit when introduced into areas without natural controls.

Limnobium are aquatic perennial herbs which grow in dense floating mats or rooted in mud on wetland edges. The flowers are held above water and pollination is probably via wind currents. The seeds are shed above water, but germinate submerged and the seedlings float to the surface where they grow rapidly (Cook & Urmi-Konig op. cit.). Although occurring up to 2800 meters in elevation, Limnobium laevigatum is marginally hardy in California, however it survived temperatures to approximately –4°C (25°F) in the San Francisco Bay Area in the 1998-1999 winter. Limnobium spongia is hardy to temperatures well below those known in low elevation California. Both reproduce rapidly by both seed and stolons, quickly filling newly colonized sites with both clones and new individuals, and both are often considered pestiferous even in their native ranges. Limnobium spongia sensu stricto has become established in several states west of its native range, although at present only Limnobium laevigatum has been found introduced into the wild in California, likely because it is the only one currently sold as an ornamental. Individual seeds are covered with small spinules and the seeds when shed are contained in a gelatinous mass; both forms readily attach to watercraft and if they should become established in navigable waterways are likely to spread rapidly and widely.


  1. Cook, Christopher D.K. & Katharina Urmi-Konig, 1983. "A Revision of the Genus Limnobium including Hydromystria (Hydrocharitaceae). Aquatic Botany 17: 1-27.

The North Coast Biodiversity Arena in Central California: A New Scenario for Research and Teaching Processes of Evolution

G.L. Stebbins & G.F. Hrusa
published in: Madrono 42 (2); 269-294, 1995

ABSTRACT - The factors responsible for variation in evolutionary rate and pattern among related plant taxa remain in large part obscure. Neutral or selectionist theories alone cannot account for divergent rates of morphological and molecular evolution among related lineages. Interactions among population histories, inherent genetic capacities and environment influence divergence rate. The development of a concise modern evolutionary synthesis, incorporating molecular data and transcending the neo-Darwinian synthesis, is complicated by interaction of these highly complex variables. Control or limitation of these variables is necessary if further evolutionary research is to result in a new synthesis. To this end a biogeographic region is delimited within north-central coastal California and termed the 'North Coast Biodiversity Arena' (NCBDA). The NCBDA has a relatively well understood geological and environmental history. It is a region of marked microclimatic and edaphic discontinuities, but not one of extreme environments. In general, the climate displays a gradient of decreasing precipitation west to east with simultaneously decreasing equability. It is restricted by definition to elevations below 500 meters. Without high mountain ranges within its boundaries killing frost is essentially unknown. Its topography is varied and in some areas rain shadows have strong local effects. The NCBDA supports a highly diverse flora including both habitat specialists and broadly adapted species. The diversity of major vegetation types, floristic associations, edaphics, and geologies within the NCBDA is introduced. Evolutionary problems critical to the development of a modern evolutionary synthesis and whose solutions may be accessible within the NCBDA are described.

Catalogue of non-native vascular plants occurring spontaneously in California beyond those addressed in the Jepson Manual – Part I

Fred Hrusa, Barbar Ertter, Andrew Sanders, Gordon Leppig & Ellen Dean
published in: Madrono 49(2); 61-98. 2002

ABSTRACT - A catalogue of 315 non-native vascular plant taxa documented as occurring spontaneously in California beyond those addressed in The Jepson Manual: Higher Plants of California is presented. The catalogue was compiled from new collections by the authors and others, previously existing herbarium specimens, formal publications, other printed reports, and direct communications with field botanists. Only reports backed by herbarium vouchers are accepted as adequately documented. Of the 315 species, 58 are fully or sparingly naturalized in relatively undisturbed wildland habitats, 53 are naturalized in disturbed areas, 34 are tenuously established or locally persisting, 13 are non-escaped weeds of greenhouse or similarly cultivated environments, 43 are presumed to be non-persisting casuals (waifs), for 110 there is no current information or observations available, and 4 have likely been extirpated. In addition, 13 reported taxa are here specifically excluded as based on erroneous information. Taxa highlighted as already being fully naturalized or potential pests are Amaranthus rudis, Brassica fruticulosa, Boehmeria cylindrica, Calystegia silvatica subsp. disjuncta, Cabomba caroliniana, Cotoneaster lacteus, Crataegus monogyna, Dittrichia graveolens, Fumaria capreolata, Geranium purpureum, Geranium rotundifolium, Hedera canariensis, Limnobium laevigatum, Maytenus boaria, Pyracantha crenatoserrata, Salvinia molesta, Trifolium tomentosum, and Verbascum olympicum.