Published in the Contra Costa Times on December 6, 2003

Q. I have a green carpet of weeds that have come up after the rains. What is the best way to control them?

A. The plants that one gardener calls weeds may be called wildflowers by another. Weeds are often described as such because they are competitive and persistent, producing abundant numbers of seeds and spreading voraciously. Weeds compete for sunlight, water and soil nutrients, affecting the growth of desired plants. Weeds are often hosts for certain insects pests and diseases, and they are allergens for many people. Winter weeds are plants that germinate in late summer or fall, growing during the winter months and dieing in the late spring or early summer.

Winter annual weeds can be controlled with a variety of options. Your first step needs to be weed identification, which can be a bit difficult when the weeds are immature and small. The majority of the weed identification guides show photos of mature plants, however seedling photos are available in the ‘Growers Weed Identification Handbook’ a UC DANR publication # 4030. This book can be seen in the Master Gardener Research Library from 9-12 am., M-F.

As your weeds have germinated, your first step needs to be removal. This can be done physically or with chemicals. In home gardens, physical control is most often the most appropriate and least damaging method of controlling the weeds. Hoeing, cultivating and hand-pulling work well as long as weeds are removed below the soil surface. A circular hoe is effective pulled just beneath the surface of the soil. The cut seedlings can be left on the soil surface to compost. This method is non-polluting, does not require special safety equipment or potentially dangerous chemicals. After the weeds are killed, a mulch should be applied. Mulches are layers of opaque material, organic or inorganic, applied over the soil surface, preventing light from hitting any remaining dormant seeds. Many weeds species have seeds that are light specific, meaning that they must be exposed to sunlight to germinate. Mulches also prevent weed seedlings from reaching the sunlight and photosynthesizing. The coarser the mulch material the more thickly it needs to be applied to be effective, 3-6 inches for shredded plant material such as chipped trees. Mulches should be kept several inches away from the crowns of trees and shrubs as they reduce soil temperatures and may increase the chance of disease problems.

The pesticides that kill plants are called herbicides, Until the 1960’s, these chemicals were rarely used in home gardens. If you choose to use herbicides, be certain that you follow all label instructions exactly, using the proper equipment at the right time. Master Gardeners often diagnose herbicide damage on ornamentals that appears on spring growth following winter herbicide applications. Before spraying you need to identify exactly the plants you are trying to control; know whether is it an annual, biennial or perennial; know the type of soil; the duration of the chemical; and the kind of equipment needed. Hoeing and mulching is often a faster and easier alternative.

A list of common garden weeds in California and recommended controls is available from the Master Gardener office.