By Michael Roll, Colorado State University Cooperative Extension Agent, Horticulture
A common philosophical definition of a weed is "A plant whose virtues have not yet been discovered."
For the home Gardener, another definition might be more appropriate, "A plant out of place."
Whichever definition you prefer, A.J. Koski, Cooperative Extension turfgrass specialist at Colorado State University, suggests starting with a relaxed perspective about weed control. "A totally weed-free lawn is rarely attainable, even with herbicides," Koski says. "It is better to maintain a healthy lawn and tolerate a few weeds rather than to make many applications of herbicides in an attempt to eliminate all weeds. Indiscriminate use of herbicides can cause problems for trees and other landscape plants. Herbicides are expensive in time and money, and actually may reduce the vigor of the lawn," he adds.
A weed management program should be based on the type of turf and ornamentals as well as specific weeds in your yard. Control will be achieved by a three-pronged program: prevention (by far the most important), non-chemical (cultural control) and chemical control.
Weeds are opportunists. They flourish in areas with little or no vegetation, or in spots where vegetation is highly stressed. It is their adaptability to difficult cultural conditions that makes them a weed. If weeds don't get started in the first place, you won't need to worry about controlling them. And, you will win 90 percent of the pre-control battle by quickly establishing ornamental plants or a dense, healthy stand of grass. You also can use weed barriers fabrics in areas that aren't planted.
But if you have weeds already, what do you do? The method of control depends on the plant's biology, so it's critical to correctly identify the weed and understand its life processes.
Weeds are annual, biennial or simple perennial. Summer annuals, such as Crabgrass or Puncturevine, germinate in the spring, develop and produce seed during the summer and die with a killing frost in the fall.
Winter annuals, such as Shepherd's Purse or Blue mustard, germinate in late summer or fall, live over winter as small tufts or rosettes, resume growth in spring and mature seed early in the summer.
Biennials require two seasons to complete their growth cycle. During the first season, they store food and their foliage is limited to rosettes. During the following season, these plants draw heavily upon their stored food and grow vigorously. They produce mature seeds in summer and fall before a killing frost does them in. Examples of biennials are common Burdock, Houndstongue and Musk Thistle.
Simple perennials, such as dandelions, possess large tap roots that store food in winter and a root crown that produces new shoots every year. Some perennials, such as Canada Thistle and Field Bindweed, spread by creeping horizontal roots that reproduce by seeds and roots.
Non-chemical or cultural control methods include mulching, mowing, hand-weeding and cultivation.
Mulching suppresses annual weeds by limiting light that weeds need to become established. Mowing also can limit seed production. If the weed is a winter annual, it will produce mature seed in early summer. This means if you begin mowing flower heads off in April, the problem should be history by June. Annuals that produce seed all summer must be mowed from mid-May through August to control seed production.
It's time-consuming to remove annual weeds by hand, but -- if done before flowering -- this is an effective means of weed control. Hand removal of perennials, however, seldom is effective. Often a large part of the root remains in the soil, and it will quickly regenerate a top.
While cultivation can kill some weeds, creeping perennials often are the exception. They can be spread by cultivation because their horizontal roots get chopped up and then sprout as new plants.
Before using any chemical, identify the weed, then read the label on the package. If the weed is not listed on the label, don't use it because no one herbicide will control all weed species. You may need to use a combination of two or more herbicides to control specific weeds.
Pre-emergent herbicides are recommended primarily to control grassy weeds, such as crabgrass or foxtail, before they germinate. Crabgrass is a summer annual that germinates from late March to early April in southern and western Colorado and mid-April to mid-May along the Front Range. Apply a pre-emergent herbicide two to four weeks before these dates. Post-emergent herbicides kill weeds present at the time of application. Weeds must be actively growing when the herbicides are applied. Selective post-emergent herbicides, such as 2,4-D, 2,4-DP, MCPP, MCPA and dicamba, offer broadleaf weed control. If the post-emergent herbicide is systemic, the most effective time to apply it is late summer or early fall. That's when plants are moving photosynthesis materials rapidly to the roots, effectively carrying the herbicide to them. Non-systemic or contact herbicides, such as crabgrass killers, provide a quick-kill but will not move into the roots. They are effective only on sprouting annual weeds. Mature perennials will re-grow after contact herbicides are applied. Because weeds are opportunists, you will need to quickly fill in their space soon after treatment. New weeds will quickly re-infest areas left open from recent weed kills. So, spread new grass seed, plant ornamentals or lay down weed barrier fabric and mulch. The use of these best management practices will help you grow a healthy landscape thereby avoiding future weed invasions.
Some weeds have been accorded legal status and are defined by the law as 'noxious.' These plants include leafy spurge, Canada and musk thistle, diffuse and spotted knapweed, field bindweed, hoary cress, yellow and Dalmatian toadflax, to name a few. For a complete list go to the Colorado Department of Agriculture web site www.ag.state.co.us/DPI/home.html. Most weeds found in the urban landscape are not noxious weeds as defined by law, but are no less noxious to the homeowner.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010