January 14, 2017
As University of California professor and agricultural economist Doug Larson says, “A weed is a plant that has mastered every survival skill except learning how to grow in rows.”
To gardeners, a weed is a plant that is in the wrong place. Most weeds are introduced accidentally, brought in by people, animals, equipment, packing materials or as a contaminant in soil. Others, introduced intentionally, enter through the nursery trade as ornamental landscape species. Weeds take all growth forms including herbaceous or woody plants, climbing vines, aquatic or parasitic plants, and can have annual, biennial or perennial life cycles.
More formally, weeds are native and non-native plants that impact crop production in home fruit and vegetable gardens or commercial settings that can cause health problems in livestock, pets and humans, or are aesthetically unpleasing in turf and urban landscapes. Weeds serve as hosts for insect pests and pathogens, provide cover for rodents and are allergens to many people. Invasive weeds are generally non-natives that infest natural ecosystems, including wildlands, rangelands and pastures.
No matter what definition is used, weeds are plants whose undesirable characteristics outweigh their beneficial qualities. Weeds compete for water, light, soil nutrients and space. Weeds persist because they can disperse, establish and spread without human assistance. Weeds out-compete their neighboring plants because they produce an abundance of seeds that can lie dormant in the soil yet remain viable for decades. Weeds spread rapidly and have the propensity to occupy sites with poor soil, especially areas undisturbed by human activity.
On the positive side, weeds are important for soil stabilization; as a source of habitat and feed for wildlife and bees; and ultimately, organic matter when they decompose.
Common weeds that plague our gardens in Marin are annuals like bluegrass, crabgrass, mallows, purslane and spotted surge, and perennials like Scotch, French and Spanish broom, pampas grass, Ehrharta erecta, English ivy, cotoneaster, vinca, bermudagrass, bindweed, iceplant, dandelion, nutsedge and oxalis among others. To view photos of common weeds and help identify yours go to UC’s integrated pest management website: ipm.ucanr.edu/PMG/weeds_intro.html.
Every square inch of your garden soil contains weed seeds, but only those in the top inch or two get enough light to germinate. Whenever you dig or cultivate the soil, hidden weed seeds move to the surface.
Manage weeds by preventing known invasive plants from entering your garden and by removing those already present. Prevention generally focuses on weed suppression through garden design, habitat modification or horticultural controls. Control measures include crowding plants to choke out emerging weeds by shading the soil, limiting access to water or by adding mulch to deprive weeds of sunlight. Sheet mulching is used to smother perennial weeds and interrupt the annual weed seed cycle. It is done by layering and wetting newspaper about two sheets thick on a weedy surface, and by adding a 2- to 4-inch layer of mulch on top of the newspaper; be sure to keep mulch at least 6 to 12 inches away from the base of trees and shrubs.
A more painstaking method is hand pulling or hoeing offending weeds when the soil is moist. Lop off flower heads of larger weeds to prevent seed dispersal.
Turning under weeds, especially before they flower, provides organic matter when laid on top of the soil to dry out. Grasses that spread by rhizomes or stolons present problems if not completely dried up. It is worth noting that composting may not destroy weed seeds if the pile doesn’t reach temperatures approaching 140 degrees.
As a last resort, herbicides may be used in and around the home garden, but special care must be used to insure it is applied safely. Herbicides should always be used according to label instructions and only for crops listed on the label. The wrong herbicide can destroy a garden’s productivity for years.