Walt Whitman once said that a weed is a plant whose useful purposes have not yet been discovered. My dad, on the other hand, always said that anything growing in the yard that isn’t grass is a weed.
Botany professors say that weeds are plants, native or non-native, whose populations readily grow in severely disturbed sites. For botanists the key phrase is “severely disturbed sites”: i. e. lawns, landslides, gaps in a forest from a fallen tree, roadsides and abandoned ranchland. These areas are typically open and flooded with light. Many weeds are intolerant of shade and therefore cannot colonize undisturbed sites. (Muir Woods for example does not have many exotic plants on the forest floor.) Because disturbed sites are new, they are often free of competitors. Countless weed sends drift in on the wind and settle. They readily germinate, grow fast, and produce thousands of seeds.
I define a weed as not only a plant that thrives in disturbed sites but also as an alien that invades, outcompetes and drives out the native flora. Most of the weeds in North America are Eurasian in origin; they accompanied the European invasion of this continent. These plants have already spent several thousand years adapting to the severe changes that agrarian societies made in the European environment. In the New World the settlers cleared the primeval forests and plowed the native grasslands. The European weeds were pre-adapted to these “severely disturbed” conditions and took a firm hold. They have been expanding their range ever since.
California has a Mediterranean climate: hot, dry summers and mild, wet winters. Many of our cultivated, ornamental and weedy plants come from regions of the world that have a similar climate – Australia, South Africa and, of course, the Mediterranean. Legions of these plants now dominate our landscape. Indeed many Californians cannot imagine their state without the golden hue of annual Eurasian grasses or the immense stands of blue gum eucalyptus trees.
Undeniably some of our more prolific weeds – multicolored radishes, brilliant yellow mustards, radiant ox-eyed daisies and blushing sheep sorrel – add a certain beauty to the fields. At the edge of the forest the tall stately spikes of foxglove seem to stand quietly on guard. But some of this loveliness comes at the expense of native California plants. The California Native Plant Society monitors the encroachment of weeds in our state. Many of our unusual and rare plants are losing ground to the aggressive invaders. Seven of the worst offenders are gorse, broom, pampas grass, eucalyptus, European beach grass (dune grass), artichoke thistle and tamarisk. Most of us consider a plant that was found in California before the Europeans came is a native. I am often asked how long does a plant have to be here until it is considered native. My answer “Never!”
To read a great book on successful weedy species (white Europeans among others), try Guns, Germs and Steel by Jared Diamond.