Fall Weed Strategies

Indoors, it’s time to get ready for daylight savings and Halloween candy. Outdoors, the changes come more slowly. Each day the sun is a little bit lower, the night is a few moments longer, and the plants in our Mediterranean, summer-dry climate, breathe a sigh of relief.

San Francisco already has recorded more than an inch of early rain, and the newly moist soil, the clear sparkling days, and the release from fog and wind mark a new beginning. Our long growing season lasts from now (with a brief pause for cold weather around the winter solstice) until late May or early June when the soils dry out for next summer’s drought.

With the beginning of a new season, gardeners also change gears. Fall clean-up accelerates, as more plants finish their blooming cycles and can be cut down in preparation for a new season. Plants like Shasta daisies, which we have been selectively deadheading to prolong the blooming season, can now be cut all the way back to their basal leaves. The tall blooming stems, while they still may have a bud or two on them, will never look good again until next year.

As we move through the garden like the Grim Reaper, it’s also a good time to selectively remove those plants which didn’t do well enough to earn their spot in the garden. Some plants may be chronically troubled by insects or diseases, some may never have adapted to the spot they occupy, some may just be the wrong color or size. By removing the stragglers, the shape of the garden becomes clearer. Sometimes the newly opened space is welcome, allowing surrounding plants to expand freely. Sometimes the new space calls for a good soil amendment and a new planting. When choosing new plants, consider repeating a species that has done well in your garden and delighted you. Gardens often look more abundant and natural when successful elements are repeated.

Out with the old, in with the new - filling in gaps in the border.

Out with the old, in with the new - filling in gaps in the border.

The new growing season also means an abundance of seedlings in the garden soil. In well-tended old gardens, many or most of these seedlings will be desirable plants, cool season annuals emerging from last year’s seeds. After a few more good rains our gardens will be full of seedlings from forget-me-nots, California poppies, red lychnis, borage, Cineraria, sweet alyssum, and honeywort. All these plants return reliably year-after-year from seeds.

Just as reliable are the weedy seeds. Dandelions, dock, mustard, cheese weed, petty spurge and annual grass weeds, including the dreaded Erharta grass, will also be popping up everywhere. Good gardening calls for an organized strategy to combat the new weed season.

Weeds galore.

Weeds galore.

The first step in regaining control of your garden from weedy invaders is a positive identification of the weed that’s driving you crazy. Once you know the identity of the naughty little plant, you can do a little research and find out what its reproductive strategies are. Different weeds respond to different control methods, and the smothering mulch that eradicates one weed, might make the next weed multiply and spread everywhere. Many of our most common garden weeds can be found in Pam Peirce’s Golden Gate Gardening, which includes an encyclopedia of weeds along with pictures and eradication strategies. Once you’ve identified your weed, it’s time for action.

One weed in particular haunts my Halloween dreams. Oxalis pes-caprae is perhaps the most common weed in San Francisco’s gardens. It has three-leaf clover type leaves and a yellow five petalled flower in late winter. Because the plants disappear completely during the dry months of summer, it can almost be forgotten as it sleeps underground in the form of tiny brown bulbs. But with the first rains it emerges, ready to devour your garden and mine.

Ideally, this weed is removed by digging out the bulblets. Heavily infested soil can even be screened to remove the bulbs. Both these approaches are labor intensive and difficult to rely on because the chances of missing some of the tiny bulbs are so great. The first year I worked at Garden for the Environment, we tried to dig the Oxalis, and the Oxalis was the clear winner that year. The digging was time-consuming and we simply couldn’t move fast enough to get ahead of the Oxalis. The next year we tried another approach.

Each Oxalis plant has the same anatomy. From a tiny brown bulb a white stem emerges underground. As this stem reaches the light, it forms a single node from which leaves emerge, followed by a flowering stalk. Instead of trying to dig deep enough to reach the bulbs, we just pulled the top off each Oxalis plant, being sure to get the node from which the leaves emerge. Then the little bulb has to start all over producing another white stem from deep underground. If we are thorough about removing the tops (which we can do much faster that digging the bulbs) we will eventually exhaust the bulbs and they will die. This has actually been working, and the last two years have seen us the clear winner in the Oxalis battle.

Hilary pulling up  Oxalis .

Hilary pulling up Oxalis.

But winning a battle and winning a war are two different things. We have to keep at it each year, because as the days shorten and the Jack-o-Lanterns emerge, so do the first little green Oxalis leaves, and they are just waiting for an opening. If we ignore them, they will take their evil revenge on us, and cover our garden beds in clover-leaved triumph.

Next month, From the Border will give some tips on controlling the dreaded Ehrharta grass, also rightfully known as “panic grass.”