How to control weeds without using chemicals

Pat D. writes: Help! Weeds are overtaking my garden. How do I control them without chemicals?

Answer: Weeds become aggressive garden competitors if you don’t control them early in their life cycle. Spring is a great time to control weeds without chemicals, and the more you know about the life cycle of your weeds, the more successful you’ll be at managing them. The main thing to know is whether your weeds are annuals, biennials or perennials.

Annual weeds complete their life cycles within a year and produce many seeds that can survive in the soil for years. Winter annuals are maturing in our gardens right now. Summer annuals start their life cycle in spring and produce seeds in the summer and fall. The best way to control annual weeds is to prevent seed production.

Annual weeds you’re probably seeing in your yard right now include bittercress (Cardamine spp.), common groundsel (Senecio vulgaris), petty spurge (Euphorbia peplus), chickweed (Stellaria media) and scarlet pimpernel (Lysimachia arvensis). Bittercress has a pretty white flower that quickly turns into a seed pod. When that pod dries out, it pops open and flings seeds through the garden. Control these annual weeds by pulling or hoeing them before they flower and go to seed.

Biennial and perennial weeds live for more than one year. In the winter, they may look dead above ground, but under the ground, their root systems are still alive. To control them, you need to dig up their entire root system, whether they have tap roots, rhizomes or masses of little bulbs called bulblets.

Mallows (Malva spp.) can be annuals or biennials. Geranium (Geranium spp.), filaree (Erodium spp.), prickly lettuce (Lactuca serriola) and dandelion (Taraxicum officinale) are perennial weeds. The best time to dig up all of these is as soon as you see them, in winter or spring, before the taproots have time to develop and the ground has dried out.

Bermuda buttercup (Oxalis pes-caprae) and Italian arum (Arum italicum) are especially invasive perennial weeds. Buttercups have bulblets and arum has thick underground stems called tubers. You can remove these by hand or dig them out. You many not find them all at first, but repeated digging over time will reduce their numbers.

Annual and perennial grasses grow vigorously in the spring. Pull them out now, when the plants are young, before they develop flowers and while the soil is moist.

Here are some actions you can take to control weeds now:

• Remove weeds when the plants are small and the soil is moist.

• Throw weeds in the trash or green bin, not in your home compost pile. Weeds cannot survive the high temperatures that commercial composting operations use to decompose organic materials.

• Keep your garden gloves and tools clean to avoid spreading weeds around the garden. Weed seeds stick to gloves and tools. And always use aged compost and “clean” soil that is free of weed seeds and diseased material.

• Add organic mulches to suppress weed growth. Apply 1-3 inches over the soil to block sunlight. To further inhibit weed growth, try sheet mulch. First, lay down overlapping sheets of cardboard, then cover them with a mulch layer. In fire-prone areas, use nonflammable mulches such as aged compost, and do not apply them within five feet of buildings.

• For severe infestations, try soil solarization, a low-cost, highly effective way to control weeds. Place clear plastic sheeting over the infested area and leave it in place for four to six weeks during the warm summer months.

You may not succeed in controlling all of your weeds in a single year, but with persistence you’ll see progress over time.

For further information on soil solarization, identification and photos of weeds and the control of specific weeds, see these links:

Soil Solarization for Gardens: bit.ly/3r9ROP1

Weed Photo Gallery: bit.ly/3ctnIjE

Weed Control: bit.ly/3tbPvvv

Ann K. asks: Do local nurseries and big box stores still sell invasive plants such as Scotch broom, pampas grass and ivy that overrun native species? If so, is there something we gardeners can do about it?

Answer: While many retail nurseries and garden centers no longer sell invasive plants, others do. The nonprofit organization PlantRight partners with California’s nursery industry to promote the sale of noninvasive alternatives. PlantRight has successfully worked with the industry to remove 15 species of invasive plants from inventory, including Scotch broom and pampas grass. English and Algerian ivy are on the PlantRight watchlist. For information on how you can help, visit: plantright.org.

Contributor to this week’s column was Janet Bair.

The UC Master Gardener Program of Sonoma County (sonomamg.ucanr.edu) provides environmentally sustainable, science-based horticultural information to Sonoma County home gardeners. Send your gardening questions to scmgpd@gmail.com. The Master Gardeners will answer in the newspaper only questions selected for this column. Other questions may be directed to their Information Desk at 707-565-2608 or mgsonoma@ucanr.edu