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Preventing Weeds in Established Turf Using Best Management Practices

 

When preexisting weeds are not properly managed, establishing turf becomes problematic.

The goal of proper lawn care is to assist the grass in outcompeting weeds using management techniques that favor turf. Cultural practices that include appropriate mowing height, efficient irrigation scheduling, proper fertilization practices, and aeration help create an environment in which turf can almost always outperform weeds. For specific details on lawn management access the following fact sheet entitled Basic Turfgrass Care. A brief summary of proper turf management practices is provided below.
 

Consistent Observation:

Weeds in turf are objectionable because they disrupt the uniformity of the lawn and may add color (yellow dandelion to green lawn) or different looking leaves (i.e., quackgrass, tall fescue, or orchardgrass in Kentucky bluegrass). Being observant during mowing, irrigation, and fertilization can permit a property owner to see a small infestation of an invading weed, allowing the weed to be controlled before it takes over large areas.

 

Herbicide Options:

Tall fescue and other perennial weedy grasses are often confused with crabgrass. Positive identification a weed to be controlled is often critical to success.

 Herbicides applied to turf can be classified as either pre-emergence or post-emergence types. Some herbicides are selective (kill broadleaves but leave grasses or kill grasses and leave broadleaves) and some are non-selective (kill all plants that they contact). Pre-emergence herbicides are preventative and suppress or control germinating seeds. They are not selective for just weed seeds, however, but will also kill or suppress desirable seeds, including newly seeded turfgrass. 

 

Pre-Emergence Herbicides :

 Pre-emergence herbicides are an excellent weed control option when their application is timed correctly. They are often sold in granular form and may be bundled with fertilizers. Certain pre-emergence herbicides are better suited to control newly germinated annual grasses and some broadleaf weeds in established turf. Common residential herbicide formulations provide between 30 and 120 days of control. Some organic herbicidal products also exist. See Table 1 for summaries of specific products. Locally, it is common to apply a pre-emergence herbicide to lawns in the spring to control warmseason annual grasses like crabgrass. Springtime soil temperatures of 50-55°F indicate that crabgrass will begin to germinate within a few weeks. Beware that these specific weedy grasses emerge from seed in the spring and die at or near the first hard frost in the fall. Existing weedy grasses such as tall fescue are not controlled by pre-emergence herbicides. Often times a second application of pre-emergence herbicides is needed to control summer and autumn germinating weeds if they are excessively problematic. 

 

Post-Emergence Herbicides for Broadleaf Weeds:

Many post-emergence products (Weed-BGone type products) selective for broadleaf plants can be applied to turf to control weeds such as dandelions. As long as they are used according to label, they do not harm turf. A limitation of selective broadleaf weed killers is that they must be applied under low wind conditions and at temperatures between 55-85° F to be effective and avoid damage to desirable plants. Labels also provide cautions when using them in the drip line of desirable broadleaf plants or simply state to not use them in these areas at all. Depending on the product, some of these herbicides may remain soil active for several weeks, meaning they can be absorbed through the soil by roots.

 

Additionally, weeds are best controlled when they are young, actively growing and before they flower. This means that applications of post-emergent products are best made in mid to late spring or early fall depending on the types of weeds to be controlled. See Table 2 for summaries of specific products. 

Further, these weed killers should not be applied when temperatures exceed 85 degrees F. They will volatilize, drift in the wind and will sometimes drift up to a few miles. Results from both types of exposure include distorted growth in branches and leaves that may last for a few years, depending on the severity of exposure.

Some commercial grade, soil persistent, broadleaf herbicides are used to control weeds outside the scope of homeowner weed control. Most are not labeled to be applied in residential situations. In general, use by homeowners should be avoided. Post-emergence herbicides are available in both granular and liquid forms. Granular forms are commonly called “weed-and-feed” and are easy to apply, but usually less effective than liquids due to low percentage of active ingredient. To be effective “weed-and-feed” type products should only be used when: 

 

Weed infestations are light:

When weeds are at an easy-to-kill stage (before the blossoming stage) 

Mainly treating annuals and few to no perennial species

These products should also be applied to slightly wet turf, after sprinklers have run or after a rain storm, so that the granules stick to the weeds instead of dropping off.

 

Post-Emergency Herbicides for Grassy Weeds:

There are few post-emergent, herbicides that can selectively remove even some weedy grasses from desirable turfgrass. These include the few products labeled for Bermuda grass control and other products labeled to control crabgrass and other warm season annual grasses. Otherwise, once a weedy grass spreads, a nonselective herbicide such as glyphosate is usually the only herbicide options to control the infestation. But, remember that such herbicides will also kill the desirable turfgrass.

 

Timing Herbicide Applications:

Use preventive herbicide practices in the spring to control annual weed seeds as they germinate. In the fall, when annual weeds have already gone to seed, spraying dead/dying plants has little effect on weed populations for next year. However, applications of herbicides to perennial weeds in the fall may prove very successful due to the fact that the chemical is translocated down to the roots during this time, providing a more efficient kill. Also keep in mind that turf is not adapted to grow under certain conditions, such as in extreme shade. Unfortunately, there are weeds that tolerate lower light levels than turf. In these situations, weeds can still be controlled but consider some other landscape options such as shade-tolerant groundcovers, mulch, or landscape rock. After all is said and done, if the weed problem is still as bad as it was or worse, know when to admit defeat and start over with new turf, following the establishment steps above. Note that in most cases, though, poor quality turf can be reinvigorated and, over a period of time and with some effort, can come back into full functionality and beauty.