Southern California's Invasive Plants

Quercus agrifolia.  This specimen may be several hundred years old:

Here's a link to the Los Angeles and San Gabriel River Watershed's invasive plant guide (pdf).  This is a good list to look through because it has decent photos of the plants, which are useful for identification.  Additionally, many of the entries also explain why the plant is considered a baddie, and specify the regions where it is a problem.  It's enlightening to see what is troubling our local area, even more interesting to see how and why.  I already knew wind and birds will spread the seed of invasive plants, but I didn't realize storm water run off does as well. 

I wrongly assumed exotics merely crowd out or steal the water from native plants.  There is more to it than that.  I did not realize some exotics hybridize with native plants, destroying the native gene pool (London Plane Tree vs. our native Planus racemosa).   You'd eventually think of that that with exotics, but how about from another native?  According to the list, a Prunus native to Catalina Island, Prunus ilicifolia ssp lyonii, is considered a baddie because it's overtaking or hybridizing with the California mainland's native Prunus ilicifolia ssp. ilicifolia.  An invasive native.  Who would have thought?


Some plants considered invasive do little or only modest damage, while others cause serious problems.  What exactly is the damage they do?   Crowding out native plants (Vinca minor) is the first thing that comes to mind, but clogging storm drains and other waterways (Washingtonia robusta, Arundo donax), causing flooding and erosion, is also a major issue.

I see I've been growing a couple of plants that I need to rethink:  Limonium perezii, so handy for purple color in dry spots, is a problem along the coast, and that handy winter-annual, the garden nasturtium, Tropaeolum majus, "will directly compete with native plants and dominate the landscape".  Oopsie!


It should be pointed out that some of these plants are far less of a problem in the solidly suburbanized areas of Southern California, where they are less likely to escape and spread to surviving wild areas that still support native plants.   Our garden happens to be close to an area that still holds native vegetation, so I am one of those gardeners who needs to be considerate.  

Oh dear, time to go pull some plants.


So look through the list.  It's worthwhile even if you only learn to identify all the weedy trees in your neighborhood, or if you shun an invader or two at the local home improvement store.  And if you decide to pull out all your Washingtonia robustas, which not only crowd out native species and clog storm drains, but is also a serious fire hazard, an annoying re-seeder and and an ugly weedy piece of cr-p, you may just possibly, though a series of fortunate events, save an Oak.


  1. Fabulous photos. Such majesty. Also, thanks for the reminder about the complexity surrounding the invasive plant issues. It reminds me of the way that we learn and re-learn to be sensitive to ethnic groups, and others.

    First we recognize the problem, then we start to refine the problem, the information hits the mainstream, people become aware, and become more sensitive, then we pass through a stage of feeling a little tired at having to maintain all the new info, then we assimilate it and go on with our lives and forget that there was a time that we were so insensitive.

  2. I'm beginning to realize I need to remove about 60% of what's growing on our property. What many of us don't realize as we try to landscape difficult sites is that those plants that grow and flourish for us - and fill us with pride in our fledging gardening abilities - are oftentimes the very worst plants for the environment.


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