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:

• First, I sorted the Master List by the number of trails in which each taxon appears. The maximum number was 55. I then selected all species that appeared on at least half this number of trails. It turned out that there was a break in the number of occurrences between an observed number of 26 and an observed number of 30, so this was a quite clean breakpoint. There were 17 taxa which were selected in this step. All of these taxa appeared in at least 24 floras, so these were frequently-found taxa in all lists.

• Second, I sorted the Master List by the number of floras in which each taxon appears. The maximum number was 41. I then selected all species that appeared in at least 20 floras, which was roughly half of the maximum number of 41. I selected the number of 20 in part to make sure I listed some species that were on the Cal-IPC list. This added another 50 taxa to my selection.

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:

• The taxon present in 15 floras but not found in any plant trail guide is barnyard grass, Echinochloa crus-galli. Interestingly, I just discovered a population of this species in one of the trail guides recently, but the Master List has not yet been updated to include it.

• The taxon present in 13 plant trail guides but only 3 floras is cut-leaved geranium, Geranium dissectum. Its overpopulation in the plant trail guides is due entirely to the high number of trails covered at the Santa Rosa Plateau Ecological Reserve, where 13 of the 16 trails there contain this species.

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:

• Desert trails and floras are vastly undersampled in the Master List, and hence non-native plants in the desert are vastly undercounted relative to their true abundance in Southern California. Only 4 of the 90 trails and 1 of the 47 floras, all from the Anza-Borrego Desert, are included in the Master List presently. Hence this list should properly be considered primarily a list of non-natives observed in the Southern California section of the CA-FP.

  For example, Asian mustard, Brassica tournefortii, is a very widespread, very abundant species in the DSon, but doesn't make this list due to the undersampling of the DSon in my Master List. This species is found on all 4 of those trails in Anza-Borrego, and even in four floras in addition to the Anza-Borrego Flora.

• The number for the Avena species from the plant trail lists is under-reported, since A. barbata and A. fatua can only be reliably distinguished for a short period. Hence if a trail wasn't covered during that period, the trail guide will list only A. species, and not entered in my Master List.

• Species that have several common subtaxa in Southern California will be underreported, since many floras and some trail guides list only the species, and some floras list only the genus name. I have not attempted to combine the subspecies or varieties in general, but I have for the following two species, which are very common species:

  • Vulpia myuros is much more widespread than would be indicated if one just considered the varieties alone. Doing that would list only V. myuros var. hirsuta, found in 18 trails and 20 floras.

  • Hordeum murinum, which wouldn't even appear on the list below if one treated the subspecies separately.

  If anyone at Cal-IPC wishes me to get the accurate total count for any other such species with subtaxa, I can easily do that manually.

The numbers presented in the table below are:

Title	Meaning
#all	number of all trails and floras in the Master List that contain this taxon (137 maximum)
#trails	number of our trails that contain this taxon (90 maximum)
#floras	number 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	#floras	Latin Name	Common Name
96	55	41	Erodium cicutarium	redstem filaree
90	51	39	Bromus diandrus	ripgut brome
89	52	37	Bromus madritensis ssp. rubens	red brome
88	51	37	Hirschfeldia incana	shortpod mustard
81	45	36	Centaurea melitensis	tocalote
77	39	38	Avena barbata	slender wild oats
75	42	33	Lactuca serriola	prickly lettuce
75	41	34	Sonchus oleraceus	sow thistle
75	40	35	Medicago polymorpha	California burclover
71	45	26	Filago gallica	narrowleaf filago
70	36	34	Hordeum murinum	foxtail barley
70	32	38	Marrubium vulgare	horehound
69	43	26	Vulpia myuros	rattail fescue
69	40	29	Bromus hordeaceus	soft chess
69	35	34	Hypochaeris glabra	smooth cat's ear
69	33	36	Silene gallica	windmill pink
67	32	35	Anagallis arvensis	scarlet pimpernel
60	26	34	Avena fatua	wild oats
60	25	35	Salsola tragus	Russian thistle
59	24	35	Melilotus indicus	sourclover
58	30	28	Cynodon dactylon	Bermuda grass
58	25	33	Stellaria media	common chickweed
56	19	37	Nicotiana glauca	tree tobacco
55	23	32	Rumex crispus	curly dock
54	30	24	Sonchus asper ssp. asper	prickly sow thistle
54	21	33	Lamarckia aurea	goldentop
52	20	32	Erodium moschatum	white-stemmed filaree
52	19	33	Malva parviflora	cheeseweed
50	22	28	Polypogon monspeliensis	rabbits-foot grass
50	18	32	Polygonum arenastrum	knotweed
49	18	31	Senecio vulgaris	common groundsel
48	22	26	Schismus barbatus	Mediterranean schismus
47	19	28	Cerastium glomeratum	mouse-ear chickweed
47	19	28	Piptatherum miliaceum	smilo grass
46	20	26	Sisymbrium officinale	hedge mustard
46	19	27	Plantago lanceolata	English plantain
45	17	28	Cotula australis	Australian brass-buttons
44	19	25	Chamomilla suaveolens	pineapple weed
44	11	33	Raphanus sativus	purple wild radish
43	15	28	Ricinus communis	castor bean
43	12	31	Sisymbrium orientale	Oriental mustard
42	21	21	Carduus pycnocephalus	Italian thistle
42	16	26	Conyza bonariensis	little horseweed
42	11	31	Chenopodium album	white goosefoot
41	21	20	Bromus tectorum	downy brome
41	17	24	Cirsium vulgare	bull thistle
40	13	27	Foeniculum vulgare	fennel
40	11	29	Erodium botrys	long-beaked filaree
39	12	27	Amaranthus albus	tumble pigweed
36	16	20	Gnaphalium luteo-album	common cudweed
35	11	24	Poa annua	annual blue grass
35	9	26	Silybum marianum	milk thistle
35	9	26	Capsella bursa-pastoris	shepherd's purse
34	11	23	Pennisetum setaceum	fountain grass
33	7	26	Plantago major	common plantain
32	10	22	Atriplex semibaccata	creeping Australian saltbush
31	9	22	Sisymbrium irio	London rocket
31	8	23	Melilotus albus	white sweetclover
31	8	23	Arundo donax	giant reed
30	10	20	Euphorbia peplus	petty spurge
30	8	22	Spartium junceum	Spanish broom
30	5	25	Chenopodium ambrosioides	Mexican tea
29	6	23	Lobularia maritima	sweet alyssum
28	5	23	Brassica nigra	black mustard
28	5	23	Conium maculatum	poison hemlock
27	3	24	Chenopodium murale	nettle-leaved goosefoot
24	2	22	Brassica rapa	field mustard
23	3	20	Apium graveolens	celery

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.

Go to:

• How Common Are The Plants Of Southern California?
• Native and Introduced Plants of Southern California