Santa Clara Valley Chapter

Weeds

Weed Removal at Edgewood Preserve: See work parties listed in the current issue of The Blazing Star or call Ken Himes, 415 591-8560.

Articles from The Blazing Star:

• Weeds & Invasive Exotics
• Total Taxa & Introduced Plants
• Biocontrol of Yellow Starthistle
• Do You Sell Broom?

• CNPS. State Office. Invasive Exotics.
• University of California. Hastings Natural History Reservation. Landowner's Guide to Native Grass Enhancement and Restoration, Coastal California.
• Introduced Tidal Marsh Plants in the San Francisco Estuary from the San Francisco Bay Area EcoAtlas.

Oh you who plow and reap and sow,
guard well your acres from this foe
nor vigil cease, nor labor spare
lest weeds become harsh tyrants there.

Ecologically, a weed might be defined as a pioneer species, a colonizer of open habitats. It is a plant that takes advantage of disruptions of natural plant communities. To the extent disruptions are caused by human activity, weeds are like camp followers. Like the cockroach, the rat, and the starling, they are uninvited guests in a niche carved by humankind.

Many plants developed their weediness as cultivated plants. Under selective pressure by farmers and horticulturalists they became fast-growing and highly productive. These attributes allowed them to invade areas where they were unwanted, thus taking the short step from cultivar to weed. Many other weeds, while never intentionally cultivated themselves, evolved as persistent associates of cultivated plants. Seeds of these are harvested and dispersed along with those of the cultivar. Some weeds are derived through hybridization producing a broad environmental tolerance; the common dandelion (Taraxacum officinale) is a notable example. The cosmopolitan characteristic of many weeds is a tribute both to the ubiquity of man's modification of the environment and his efficacy as an agent of dispersal.

While some natives display weedy characteristics or otherwise act as pioneer species in disturbed areas, e.g., Baccharis pilularis, most of our weeds come from outside of North America, and were introduced either intentionally or by accident. Intentional introductions include horticultural and food plants. Among the most serious weeds are extremely invasive horticultural introductions that replace native species on a large scale. These weeds are known as Invasive exotics, and include species of Cytisus and Genistra (brooms), Eucalyptus, and Cortaderia (pampas grass). Another widespread weed that is the bane of local grasslands is starthistle (Centaurea solstitialis).

Weeds may be handsome, or tasty (Urtica dioica, Rumex crispus), but invasive exotics are threats to our native flora.

Adapted from Ronald Taylor (1991) Northwest Weeds, Mountain Press