(Editor's Note: This article was originally published on February 28 2011. Your comments are welcome, but please be aware that authors of previously published articles may not be able to respond to your questions.)
Hundreds of years ago, Spain claimed the land that is now the state of California. They did virtually nothing with it until the mid 1700s, when they felt that the northern edge of their empire was being threated by the Russians, who had a colony in what is now Sonoma County. The Spanish plan was to turn the Natives into good Spanish citizens by making them learn the "civilized" arts of farming, ranching, pottery-making, brick-making, weaving, and so on, and converting them to the Catholic faith. In 1769, the Spanish started their conquest. Along with the Old World methods came Old World seeds, some introduced on purpose and some by accident. The human conquest happened quickly and usually with little resistance. Apparently the plant conquest did too. Seeds of non-native weeds have been found in adobe bricks at Mission San Juan Bautista, which was started in 1803 and completed in 1812. Even places where agriculture was never performed have not remained untouched. Until fairly recently (and it still may be occurring in some places), after a wildfire, it was the policy to sow seeds of non-native grasses over the burned areas to provide a fast-growing cover to prevent erosion. People were ignorant of the fact that a vast seed bank of native plants was in the soil, just waiting for the next fire to allow them to germinate.
Wild oats (Santa Monica Mountains National Recreation Area)
It would be naively romantic to think that California of 1768 was some kind of primeval, untouched, virgin land. People have been living here for tens of thousands of years. Pre-columbian California probably had the highest population density of any area north of Mexico. These people did not farm, but some used fire for land management. When fire favored food plants, fires would be set at intervals more often than would occur naturally. This probably resulted in some areas being grassland or open forest, rather than some form of scrub. Although the plant species found in 1768 were all native, the species distribution was likely not identical to what it would be without humans.The non-native "weeds" have flourished in California for several reasons. First of all, California has a similar climate to the weeds' Eurasian homeland, but without the pests and diseases of their homeland. Also, most native California plants do not stand up well to Eurasian settlement style disturbances, that is, plowing, intensive grazing, road-building, and city-building. Farming and livestock raising have been going on in the Mediterranean and Middle East for 5,000 years or more. It is only natural that weeds introduced in that area would be resistant to or compatible with those practices. Any native Eurasian plant that was not compatible would have gone extinct or been relegated to the wilderness. The non-native plants flourish in disturbed places like farm fields, road cuts, construction sites, and to an extent can handle very heavy grazing.
Hardinggrass (Santa Monica Mountains National Recreation Area)
Also, the abundant non-native plants are better competitors. Many of the native plants are perennials, which spend most of the first few years of their life growing roots and not an abundance of vegetation and they do not produce seed in their first few years. The invasive non-native grassland plants are annuals. They can produce a vast amount of vegetation and they produce seed in one growing season. Thus they can quickly shade out and out-reproduce native plants. There are California native annual grasses, but most are now rare and some are endangered species.
Mustard, wild oats, and milk thistle have all turned gray by late fall (Upper Las Virgenes Open Space Preserve)
- Wild oats (Avena fatua)
- Ripgut brome (Bromus diandrus)
- Hardinggrass (Phalaris aquatica)
- Black mustard (Brassica nigra)
- Yellow star thistle (Centaurea solstitialis)
- Milk thistle (Silybum marianum)
- Redstem filaree (Erodium cicutarium)
- Fennel (Foeniculum vulgare)
New grass and dead, last season's mustard (Santa Monica Mountains National Recreation Area)
Since the non-native plants are highly compatible with farming, livestock raising, frequent burning, and other disturbances, it might seem that if those pressures are removed, that perhaps the native plants would come back. However, I have not seen that to be the case. Places that now are parkland and have not been grazed or otherwise disturbed for 20 to 40 years are still chock-full of invasives. With a lot of work, like timely weed pulling and strategic mowing, the thistles can be eradicated, but the grasses and mustard seem to be here to stay. They seem to have the biological edge and it would seem that barring some herculean effort, we are stuck with them.
Perhaps flowery fields like this were more common before the 19th century (Gorman, CA)
It is sad to think the grasslands are potentially forever changed, and probably not for the better aesthetically, either. However, let's learn from this when it comes to places that are not beyond hope or rescue. Find out what is aggressively invasive in your area and do not plant those species. Check your state's agriculture department web site and any native plant or nature web sites relevant to your area. Just because you see something for sale, it does not mean that it is well-behaved. The nursery lobby can be powerful and prevent plants from being prohibited. Even if you promise to yourself that you will not let the plant spread, there are things beyond one's control. Seeds can be spread by birds, rodents, ants, wind, and floods. If you move or after you pass on, who will look after that plant? Please plant responsibly. We don't want the wildlands of the 21st century to be a mystery to future generations.