Air and soil temperatures are dropping signaling that fall is finally here! Now is the time to beware of winter weeds. Buried in the soil are winter weed seeds lying in wait for rainfall to trigger them to germinate and grow.

Weeds occur in all lawns but seldom become problems in ones that are taken care of properly.  A thick lawn usually crowds out weeds, since weed seeds need light to germinate. Poor maintenance practices, such as too much or too little fertilizer, watering, and mowing, can weaken and thin turfgrass stands and predispose the lawn to weed invasion.

Identifying weeds and knowing their life cycles are essential to management. The life cycle of weeds may be annual, biennial, or perennial.

  • Annual weeds are most commonly identified as either winter/cool-season or summer/warm-season and survive only one season. If not controlled before they flower, they can produce seed that will sprout the following year.
  • Biennial weeds survive for two growing seasons, reproducing vegetatively or by seed; however, seed is not produced until the second year. In general, there are only a few biennial weeds in landscapes and most can be managed as if they were annuals.
  • Perennial weeds survive many years, and though some may produce seed, many primarily reproduce vegetatively by creeping stems (stolons and rhizomes), tubers, or fleshy roots. Perennial weeds are the hardest to control once established.

The most troublesome winter annual weeds are annual bluegrass and Italian ryegrass and broadleaf weeds: annual clovers, chickweeds, filarees, groundsel, prickly lettuce, sowthistle, and swinecress. Perennial broadleaf weeds include dandelions, white clover, English daisy, and the always troublesome creeping woodsorrel (commonly called oxalis). See pictures of weeds in the UC Pest Note called "Weed Management in Lawns" at:  http://ipm.ucanr.edu/PMG/weeds_intro.html

When weeds invade:

  1. Identify the weed species especially to learn if the weed is a grass or a broadleaf and an annual or a perennial plant.
  2. Determine if there is an underlying lawn care problem, such as improper irrigation or mowing or inadequate fertilizer, and correct it. Renovate or replace weak areas of lawn with vigorous new turf.
  3. Remove weeds by hand as you see them, when they are young and before they flower, set seed, form vegetative parts, and spread into patches.
  4. Use herbicides as a last resort, and combine with proper cultural control.
  5. If used, choose an herbicide labeled for the species of weed you are trying to control and one that is safe for use on your type of lawn.  Fertilizer products containing herbicides must be carefully chosen and applied at the right time of year or you could end up fertilizing the weeds.

If you do have a weedy lawn you may wish to start knocking down the weed seeds in the soil by using a preemergent herbicide.  Several products are available that control many of the winter annuals and perennials.  To be most effective these products should be applied in the early fall before weeds emerge from the soil; they kill weed seedlings as they germinate and try to emerge. These products will not kill existing plants, but if used in the fall, be aware that you cannot reseed your lawn until the spring.  

Postemergent herbicides are applied after weeds have emerged from the soil; they control actively growing weeds and they work best on small weeds. There are several products and combinations formulated for lawns that kill broadleaf weeds (both annuals and perennials).  The majority of these products have systemic activity.  When using herbicides always follow directions, especially with respect to how much to apply and using them around tree roots or ornamental plants.

An integrated weed management program can reduce most weed populations to tolerable levels and prevent large, unsightly weed patches. Total eradication of weeds is not realistic or necessary for most lawns, and with good management practices a lawn can be practically weed-free without the extensive use of chemicals.

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