The Most Common Non-Native Plants of Southern California

As of 27 August 2005, there are 90 trails and 47 floras (lists for short in the following) in my Master List. As part of giving feedback to Cal-IPC for their invasive weed list, I sorted the Master List to find the most common non-native taxa.

I selected the species in two ways:

The final list contains 67 taxa, which are clearly widespread in Southern California. The specific lists on which they occur can be found by using the search feature here. In addition, most of the trail guides retrieved from such a search contain an estimate of the abundance of each species on that trail.

The primary reason that more taxa were selected using the number of floras is that floras always cover a much larger area than that of a single trail list. For example, the flora of the San Gabriel Mountains includes all the taxa found only at the lowest elevations, whereas none of the plant trail guides at high elevation contain taxa found only at the lowest elevations. This is a large effect, since the highest elevations are nearly weed-free (see, for example, the first plot here. Hence the selection criterion of half the maximum number found reaches deeper into the counts from the floras than into counts from the plant trail guides.

The relationship between the counts from the floras and the counts from the plant trail guides is shown in the next plot:

There is a very high correlation between the counts at high values, although there is significant scatter due primarily to the plant trail guides located at high elevation with few non-natives. Hence the most common non-native taxa selected using just the floras will be very similar to the most common non-native taxa selected using just the trail lists.

In contrast, uncommon non-native species are picked up much better in the floras, as would be expected due to the larger area covered by the floras.

For the curious:

Of course, widespread does not mean they are also abundant (see the discussion of the many ways to define the "most common taxa"). However, nearly all the species on this list are in fact also abundant taxa where they occur, as might be expected of widespread non-native weeds.

A few caveats on the table below:

The numbers presented in the table below are:

#allnumber of all trails and floras in the Master List that contain this taxon (137 maximum)
#trailsnumber of our trails that contain this taxon (90 maximum)
#florasnumber of all floras in the Master List that contain this taxon (47 maximum)

The Most Common Non-Native Plants of Southern California

The table is sorted by declining number of #all.

#all#trails#florasLatin NameCommon Name
965541Erodium cicutariumredstem filaree
905139Bromus diandrusripgut brome
895237Bromus madritensis ssp. rubensred brome
885137Hirschfeldia incanashortpod mustard
814536Centaurea melitensistocalote
773938Avena barbataslender wild oats
754233Lactuca serriolaprickly lettuce
754134Sonchus oleraceussow thistle
754035Medicago polymorphaCalifornia burclover
714526Filago gallicanarrowleaf filago
703634Hordeum murinumfoxtail barley
703238Marrubium vulgarehorehound
694326Vulpia myurosrattail fescue
694029Bromus hordeaceussoft chess
693534Hypochaeris glabrasmooth cat's ear
693336Silene gallicawindmill pink
673235Anagallis arvensisscarlet pimpernel
602634Avena fatuawild oats
602535Salsola tragusRussian thistle
592435Melilotus indicussourclover
583028Cynodon dactylonBermuda grass
582533Stellaria mediacommon chickweed
561937Nicotiana glaucatree tobacco
552332Rumex crispuscurly dock
543024Sonchus asper ssp. asperprickly sow thistle
542133Lamarckia aureagoldentop
522032Erodium moschatumwhite-stemmed filaree
521933Malva parvifloracheeseweed
502228Polypogon monspeliensisrabbits-foot grass
501832Polygonum arenastrumknotweed
491831Senecio vulgariscommon groundsel
482226Schismus barbatusMediterranean schismus
471928Cerastium glomeratummouse-ear chickweed
471928Piptatherum miliaceumsmilo grass
462026Sisymbrium officinalehedge mustard
461927Plantago lanceolataEnglish plantain
451728Cotula australisAustralian brass-buttons
441925Chamomilla suaveolenspineapple weed
441133Raphanus sativuspurple wild radish
431528Ricinus communiscastor bean
431231Sisymbrium orientaleOriental mustard
422121Carduus pycnocephalusItalian thistle
421626Conyza bonariensislittle horseweed
421131Chenopodium albumwhite goosefoot
412120Bromus tectorumdowny brome
411724Cirsium vulgarebull thistle
401327Foeniculum vulgarefennel
401129Erodium botryslong-beaked filaree
391227Amaranthus albustumble pigweed
361620Gnaphalium luteo-albumcommon cudweed
351124Poa annuaannual blue grass
35926Silybum marianummilk thistle
35926Capsella bursa-pastorisshepherd's purse
341123Pennisetum setaceumfountain grass
33726Plantago majorcommon plantain
321022Atriplex semibaccatacreeping Australian saltbush
31922Sisymbrium irioLondon rocket
31823Melilotus albuswhite sweetclover
31823Arundo donaxgiant reed
301020Euphorbia pepluspetty spurge
30822Spartium junceumSpanish broom
30525Chenopodium ambrosioidesMexican tea
29623Lobularia maritimasweet alyssum
28523Brassica nigrablack mustard
28523Conium maculatumpoison hemlock
27324Chenopodium muralenettle-leaved goosefoot
24222Brassica rapafield mustard
23320Apium graveolenscelery

The complete list of all non-native species present in my Master List in either a plant trail guide or a digitized flora is also available, but keep in mind that this list simply counts the number of lists in which they occur. The abundance of these species may be quite different from that count.

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Copyright © 2005 by Tom Chester
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Last update: 28 August 2005