The rains are welcome but the bumper crop of spring weeds isn’t. In our neighborhood, thick clumps of clover-leafed oxalis (yellow wood sorrel) are already 2 feet high with a proliferation of yellow flowers bobbing their pretty heads in the spring breezes. We’ve got crabgrass and annual blue grass filling in all the bare spots in lawns as well as drought-tolerant landscapes – and a lot of work ahead of us.
Spraying herbicides on weeds used to be the first method of control, but concerns about possible carcinogenic properties and trace amounts of herbicide runoff in our drinking water supplies have forced us to reconsider using them on a regular basis to suppress weeds.
It’s clearly too late to apply pre-emergents to prevent spring weeds from germinating. Weed seeds have survived five years of severe drought, lying dormant on the soil surface until sufficient rainfall this season allowed them to germinate – all at once. The weeds are reproducing seed as fast as they can. Our control methods should begin by trying to prevent seed heads from forming.
Oxalis is one of the hardest weeds to get under control. Those pretty yellow flowers will soon develop seed pods that can spurt seeds for several feet whenever they are touched. Oxalis’ thin fibrous roots form mats that break apart when pulled and then regrow. To reduce next year’s crop of oxalis, try to mow or cut down down oxalis plants and the flowers before they set seed pods. Use a spading fork to loosen the soil to make it easier to pull out most of the root systems. Covering and smothering the remaining oxalis with several layers of biodegradable, water-permeable black and white newspaper or a sheet of cardboard and a 3- to 4-inch layer of mulch, humus or compost will prevent sunlight from reaching the plants or any viable seeds. This method won’t kill pre-existing weeds completely, but will slow or prevent seed germination. Pull out any oxalis plants that resprout when they and their root systems are tiny.
Set mower blades as low as possible to cut off seed heads that form on low-growing poa annua or annual blue grass before the seeds are mature. Crabgrass seed heads form later in the summer or early fall, but crabgrass also spreads by stems that root from their nodes. Use a weed extraction tool or a screwdriver to dig out crabgrass tufts and the root system. Lay down and maintain a 3- to 4-inch layer of mulch or compost over the bare spots to block seed scatter onto bare soil later in the summer and fall.
Weed cloth barriers will also prevent seed germination. The cloths should be perforated to allow water to pass through but completely block light. Weeds can thrive in just a few inches of dirt. Keep the surface of the weed cloths covered with mulch, not dirt.
Weed seeds sprout easily in cultural conditions that deter competing plants’ or grasses’ growth. Correcting any irrigation problems, improving soil texture and drainage and leveling low and high spots in the garden will reduce weed populations. But, as you can see in your own neighborhood, heavily mulched areas have far fewer weeds.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).