Most people think they know all they need to know about weeds. They come up in the middle of the flowerbeds, through cracks in the sidewalk, and all over the lawn. They get stuck in your socks and in your dogâ€™s fur and, occasionally, even under your skin. Weeds are just one of lifeâ€™s little annoyances, but certainly not something to get all worked up about. The weeds in your garden may actually fall into the â€œdonâ€™t sweat the small stuffâ€? category.
The problem is, the weeds havenâ€™t stayed in your garden. Even your plants havenâ€™t stayed in your garden, but instead, have become â€œplants out of placeâ€?â€”weeds. The same story could be told of your parentâ€™s garden, and their parents before them, and their parents before them. With each generation, more and more plants have found their way into the wild, and all too many of them have thrived in the absence of their native competitors, predators, and diseases. Weeds that once occupied a small corner of the landscape have since spread exponentially and now carpet millions of acres. They have become one of the most serious and pressing environmental challenges of our times.
Some may object to the idea of calling a cultivated plant a â€œweed,â€? but in fact many of the worst invaders were first introduced as garden ornamentals. Pampas grass, arundo, tamarisk, cape ivy, French broom, iceplant, hemlock, and periwinkle certainly fall into this category. Others were introduced for agricultural or medicinal uses, such as fennel, Himalayan blackberry, milk thistle, and castor bean. Still others happened here by chance. Star thistle, for example, was most likely introduced to California during the Gold Rush through alfalfa contaminated with the seed. Star thistle now infests more than 12 million acres in the state and continues to spread. The number of infested acres increased from 10,000 to 60,000 between 1985 and 2002 in our county alone.
You may be wondering why some introduced plants have escaped cultivation and become â€œweeds,â€? whereas others have not. It appears that some plants have characteristics that make them unusually adaptive or â€œinvasive.â€? These include traits such as exceptionally long seed viability, prolific seed production, minimal germination requirements, and the ability to recover quickly from natural disturbances. For example, Italian thistle is a common invader of oak woodlands and rangelands in our county. A single plant can produce 20,000 seeds in one season, and the seeds can remain dormant in the soil up to 10 years. Similarly, a large star thistle can produce up to 75,000 seeds, and they can still sprout two to five years later. Cape Ivy is another aggressive weed that has overtaken many of the streams along our coast. It reproduces by seed and by sprouting roots from stem nodules. Studies have shown that a single node will successfully root 95 percent of the time, and that a node can still root after being dried in the sun for a full 10 weeks!
As you might suspect, the characteristics that have helped these plants survive in the wild are the same ones that make them such a menace. Their adaptive strengths combined with the absence of their natural enemies give them an enormous competitive advantage over the native plant communities. Once they move into an area, they will gradually crowd out native plants and eventually overrun them. They also crowd out all of the animals that have come to depend on those plants as well. A field full of star thistle isnâ€™t particularly tasty for livestock or wildlife, nor is it much fun to walk through. Other invasives, such as hemlock, are highly toxic to animals, causing sickness or death if eaten. Still other invasives, such as tamarisk, rob birds and small mammals of the nesting sites to which they are accustomed. Over time, such disruptions result in a tremendous loss of biodiversity and ultimately even extinction for some plants and animals. The spread of invasive species is second only to direct habitat loss as a leading cause of extinctions.
Itâ€™s impossible to put a price tag on the loss of a species, and yet this cost is probably the greatest of all. We can put a price tag on the cost to agriculture in this country, and it is staggering. Damage and control measures cost tens of billions of dollars each year. Invasive weeds are responsible for billions of dollars in crop losses annually and soak up billions more in herbicides and other control measures. Many of these dollars come directly from you and me in the form of taxes, and others come indirectly in the form of higher prices.
So what can you do about this? A lot! Each of us can make an enormous difference in solving this problem. First, educate yourself on which plants are invasive in our county. There is a lot of information available on the Internet, as well as through your local Resource Conservation District, Cooperative Extension, and County Department of Agriculture. Next, make sure that you donâ€™t make the problem worse by planting any invasives in your yard. Get rid of any that you have. Then look beyond your cultivated garden into the forgotten corners of your yard and more naturalized areas. Make sure that things like star thistle, mustard, or fennel arenâ€™t lurking there, and yank them if they are. If you go for a hike, keep your eye out for invasives. Thereâ€™s not much you can do if you run across a huge infestation of them. But if you happen to run into just a few, you can prevent them from getting established by pulling them out. Or, if you are feeling really virtuous, you can join a group of â€œweed warriorsâ€? and spend your weekends pulling weeds. You would be surprised at how engaging and satisfying this endeavor can actually be. It can take you to the most scenic of places with the best of company and leave you with a very real sense that you have lent a healing hand and helped to nurture this wounded earth back to health. âˆ†
Holly Sletteland is a conservation program manager for the Upper Salinas-Las Tablas Resource Conservation District and a â€œweed warriorâ€? in her spare time.