Here in the Central Valley, our brief winter has ended and soil temperatures have already risen to 55 degrees and above. Warm enough to plant transplants of cool-season vegetables and spring-blooming annuals and just warm enough for weed seeds to germinate. If you didn’t get around to applying a pre-emergent herbicide last month to prevent our springtime weed crop of poa annua or annual bluegrass, crabgrass and the highly-invasive yellow-flowered oxalis from sprouting, you’ll need to take steps now to control weeds before they become full-sized and set seed for another weed crop next year.
In years past, many gardeners depended on herbicides to control for weeds, but since trace amounts of herbicides including glyphosate, the active ingredient in Roundup herbicide and other brands, have been discovered in drinking water supplies, home gardeners need to try other methods of weed control.
There are two non-chemical means of weed control. The first is to keep your wiggle hoe handy and cut off weed heads when they are tiny, before the weed heads can set seed. The second and most efficient, less labor-intensive method is to prevent weeds from ever germinating.
Weed seeds can lie dormant on or just under the soil surface for several years. And weeds are prolific seed producers. It takes sunlight to germinate weed seeds and the best way to reduce the weed population in your garden is to prevent sunlight from touching the seeds. A 3- to 4-inch layer of wood bark, compost or humus mulch must be replaced or topped to maintain that thick layer every few months but they are really effective. Less effective mulches include cedar shaving mulches, which tend to mat together and become waterproof so that no water reaches the soil, cocoa hull mulches, which are toxic to dogs, and lightweight mulches such as mushroom and redwood mulches, which are quickly blown or raked away.
There are many new types of mulches or mulch-like products being developed that also prevent sunlight from reaching weed seeds. All are permeable, allowing air and water to pass through. Like wood chip and compost mulches, they also raise soil temperatures and speed up ripening or flowering times and reduce evaporation of soil water.
Engineered plastic films with microscopic holes are permeable but completely block sunlight. The plastic film supposedly (I haven’t tested any) will not degrade for several years, making it less work to maintain than wood chip or compost mulches.
Biodegradable paper mulch blankets will decompose within a season and can be tilled into the soil. There are mulches made of organic cellulose fibers that also decompose naturally. The Burpee seed catalog advertises a cellulose mulch that contains a 3-3-3 time-release fertilizer. Landscape fabrics that do not degrade naturally can be cut to fit awkward spots in the garden. Plain black garbage bags will work in a pinch as a weed control covering. Just remember to poke a lot of tiny holes in the plastic to allow water and air to pass through.
As a time and labor-saving bonus, you can lay down mulches on top of small weeds and the mulch will smother them.
Elinor Teague: firstname.lastname@example.org