Weeds can seem hard to control because they have such a vigorous reproductive system.
Many weeds can grow from root or other weed fragments. Other weeds that propagate primarily by seed can produce literally thousands of seeds per plant. For example, a single purslane plant (Portulaca oleracea) can produce 52,300 seeds. Weed seeds remain viable in the soil for years. Worst cases include seeds like those of chickweed (Stellaria media), which are viable in the soil for 600 years, and lambsquarters (Chenopodium album), which are viable for 1,700 years. So it’s important that you never let the weeds on your property go to seed.
To control your weeds, you first need to get to know them.
Types of weeds
Annuals grow, produce seeds and die out in a single year. So the only new weeds that grow are those that sprouted from the seeds produced or transported into the area. Examples are spotted spurge (Euphorbia maculata) and mallow (Malva parviflora).
Biennials have a two-year life span, growing in the first year, then flowering, producing seeds and dying out in the second year. An example is bristly oxtongue (Picris echioides).
Perennials possess a life cycle beyond two years and are the most difficult to get rid of because of their extensive root system, like bindweed (Convolvulus arvensis).
Weeds are generally also classified as either grasses or broadleafs (weeds with wide, thin, flattened leaves).
To help you identify the weeds in your yard, visit the Weed Gallery on the University of California, Davis IPM website www.ipm.ucdavis.edu/PMG/weeds_intro.html to view photos of broadleafs and grass weeds.
There’s also Weed ID Tool at http://weedid.wisc.edu/ca/weedid.php to help you narrow down the list of weeds for you to view in your identification process.
Mechanical: Hoeing, pulling, cultivating and tilling are easy ways to get rid of annual and biennial weeds when they are still small. Perennial weeds are more difficult to control. For simple perennial weeds you need to dig up the roots or cut the weed roots below the soil surface every two to three weeks. Creeping perennials like Bermudagrass have extensive root systems that are difficult to kill by mechanical methods.
Mulches: They work by blocking out the sunlight needed for weeds to grow. Several types of mulches can be used, such as wood chips, bark chips, gravel, pebbles, cardboard and landscape fabric. Make sure your layer of mulch is thick enough to prevent any sunlight from reaching the soil.
Solarization: Beneficial in controlling fungi, bacteria and nematodes in the soil, solarization can also control many annual weeds. Solarization involves placing clear plastic over the affected area for four to eight weeks during the summer to heat the soil to a high enough temperature to kill the weed seeds in the soil. You can find instructions on solarizing your soil at www.ipm.ucdavis.edu/PDF/PESTNOTES/pnsoilsolarization.pdf.
Dense plantings: This landscaping option helps minimize weeds in a given space. If you utilize all your available space for desired plantings, they can provide competition for the resource weeds also need. The weeds will have a hard time taking over.
Herbicides: If you do end up using a herbicide, there’s a lot you need to know about selecting a herbicide since they are all not the same. For example, pre-emergent herbicides need to be applied before new weeds emerge, whereas post-emergent pesticides are applied after the weeds emerge. Contact herbicides injure only the segments of the weeds in which they come into contact, whereas systemic herbicides move throughout the entire weed to kill them. Non-selective herbicides are toxic to all plants they come into contact with. Selective herbicides are toxic only to specific weeds. For more information on herbicides for weed control, see the document on the topic on the UC Davis IPM website at www.ipm.ucdavis.edu/QT/weedcontrolcard.html. Another document providing additional information on controlling weeds in the landscape is accessible at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7441.html.
Controlling weeds takes persistent vigilance, proper knowledge and action on your part. But the investment of your time and energy will be worth it. A garden is so much enjoyable without weeds. And remember, never let weeds go to seed.
Kathy Low is a Master Gardener with the University of California Cooperative Extension office in Fairfield. If you have gardening questions, call the Master Gardener’s office at 784-1322.