Master Gardener: Race against weed seeds is on – Steamboat Pilot and Today


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Gwen Swenson-HaleFor Steamboat Pilot & Today

Knotweed preparing to seed. (Courtesy photo)

While weeding is a task that ideally is done continually — and early in the growing season — summer is also a critical time to tackle weeds before they go to seed. Effective weed control requires a three-pronged approach: prevention and cultural and chemical control.PreventionPrevent weeds by encouraging the rapid establishment of desirable plants and keep large areas of your yard free of weeds through the use of three inches of mulch. The other major way to prevent a worsening weed situation is by getting rid of the plant before it drops its seeds — which, in the case of a pigweed, is about 117,400 seeds — that can live in the soil for 20 to 40 years.The first step is to identify the weed and determine if it is annual, biennial or perennial. This is important because control methods are different.Cultural controlCultural weed control methods include: mulching, mowing and hand-weeding. Mulching suppresses annual weeds by limiting light required for weed establishment. Mowing helps limit seed production but must be done when flower heads are produced around April for winter annuals and in summer for summer annuals. Frequent hand removal of annual weeds is time consuming but effective if done before flowering. Hand removal of perennials won’t get rid of the weed unless you can pull up all of the root, some of which grow as long as two feet, so it is usually very difficult.Chemical controlBefore using any chemical product, carefully read the product label. Pre-emergent herbicides are used primarily to control grassy weeds like crabgrass before they germinate. Apply two to four weeks before the plant emerges. Post-emergent herbicides kill weeds present at the time of application. Weeds must be actively growing when the herbicides are applied. Selective, systemic, post-emergent herbicides such as 2, 4-D and dicamba offer broadleaf weed control without harming lawngrass. Some, like glyphosate, are nonselective and systemic, killing any plant contacted. Others, like diquat, are nonsystemic or contact herbicides, killing only plant parts contacted by the herbicide. Contact herbicides are effective against annual weeds but only “burn off” the tops of perennial weeds. Many organic herbicides are contact herbicides, and, therefore, frequent reapplication through the season may be necessary.August triageIn general, it’s best to use chemical and cultural controls when the weeds are young, but by August, if nothing else, pull or cut off the flower/seed heads. If the plant is a perennial, it will return next year, but you will have prevented it from spreading its thousands of seeds to your landscape.Gwen Swenson-Hale is a CSU master gardener apprentice from the Class of 2021 and is regretting that she didn’t work harder in the spring to control weeds.
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