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Battling weeds

May 5, 2011
By Stephanie Bethke-DeJaeghere , Marshall Independent

Usually the middle to end of May is the prime time for putting down preemergence weed killer for crabgrass. In general, early in the month is appropriate for the southern one-third of Minnesota while later in May is fine for middle to northern sections of the state. But what if I miss the prime window of application, how do I know if it's too late to apply the product? That's a good question. For all practical purposes, once the crabgrass seedlings have emerged from the ground it is too late for a preemergence product to effectively be put down. There is one notable exception and that is the preemergence weed killer known as dithiopyr. It is known in the trade by its product name Dimension. It is a common ingredient in many homeowner formulations.

This product does provide some control of seedling crabgrass plants up until the 2 or 3 true leaf stage. That is still a pretty small plant. It is important to distinguish between tillers and leaves. Tillers are secondary shoots that also arise from the crown of the plant. Applying dithiopyr at the two to three tiller stage is useless. It must applied prior to or at the two to three leaf stage to have any control effect.

For creeping Charlie, second to fall treatment, spring time, at or during full bloom, is a very good time to apply postemergence herbicides to control this common lawn weed.

However, rather than just simply reaching for an herbicide to kill the plant, stop and consider for a moment as to why this weed seems to be getting worse or expanding in your lawn.

Creeping Charlie does best in a moist, partly shaded to fully shaded environment. As shade increases it becomes less and less favorable for sustaining a turfgrass cover and more and more favorable for weeds like creeping Charlie to encroach and take over.

It might be that doing some pruning or other practices to get more light to the soil surface will improve growing conditions such that turfgrass can survive and thrive there. Then, if creeping Charlie is controlled with an herbicide you can come back and overseed with a shady lawn mix and expect it to be more competitive and vigorous thus helping to keep creeping Charlie from reestablishing. If the area is simply too shady to grow turfgrass, why fight it, consider other ground covers or a shade garden of perennial flowers. They are not necessarily less work than a lawn, but can provide a very attractive area in the landscape. If you are considering the use of an herbicide for controlling creeping Charlie, select one that contains the active ingredient triclopyr.

Research has shown that the addition of this product to a broadleaf herbicide is more effective than those not containing it. You can even purchase products where triclopyr is the only ingredient.

They are usually sold as a weed control products specifically for creeping Charlie, white clover, oxalis and other difficult weeds

Since spring is a very active period of grass growth, there is greater demand for nutrients, particularly nitrogen (N), to sustain that growth. However, it is equally important to not be over zealous with the application of lawn fertilizers, especially N, in the spring. Remember that most of the growth emphasis of mature grass shoots (which will be most of the lawn) is to produce a flowering stem and then die.

Most of us see little of the actual grass flowers because we are always mowing them off. Nonetheless, a light application of nitrogen fertilizer can be beneficial. In most cases this means about three-quarters to 1.0 pound of actual N per application. If possible, about half the fertilizer should contain 'slow release' nitrogen along with some that is readily available. Once water is applied, the nutrients dissolve and move into the soil where they can be taken up by the grass roots.

The slow release fraction provides an additional supply of N over a longer period of time. Very high application rates of nitrogen fertilizers can cause too much top growth at the expense of root growth. Weakened and stressed root systems are less able to supply shoots with the necessary nutrients and water creating even more stress in the grass plant. This also sets up the plant to be even more vulnerable to injury from mid-summer heat and drought stresses likely to come.

Higher mowing heights in combination with good turfgrass density can sufficiently shade the soil surface such that weed seeds won't receive the necessary sunlight to begin germination.

Even though other factors such as moisture and nutrients may be favorable for weeds to grow, the reduced sunlight prevents those seeds from germinating and beginning to grow. Mowing should follow the "1/3" rule which states that no more than one-third of the turfgrass height be removed at any one mowing.

Since growing back that one-third will take more time at a higher height of cut, mowing frequency can also be reduced. In other words, if the desired mowing height is two inches, it will take longer for a grass plant to grow from 2 inches to 3 inches, than it will from a mowing height of 1.0 inch to 1.5 inches; both of which are increases of one-third in grass plant height.

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