Bob Mugaas, University of Minnesota Extension Educator
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 1/3 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. See accompanying picture for a comparison of a two to three tiller stage compared to a two to three leaf stage. 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. It should also be noted that dithiopyr will continue to have its preemergence effect once it’s applied as well as very early postemergence effect.
Photo 1: The crabgrass plant on the left side of the picture is the 2-3 true leaf stage while the one on the right is at the 2-3 tiller stage. Bob Mugaas
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. For a listing of available home lawn and landscape weed control products, see the chart included in the update article in the May 15, 2008 Yard and Garden Newsletter.
Photo 2: Typical growth habit and foliage of creeping Charlie (Glechoma hederacea). Bob Mugaas
Hot, dry periods during the latter half of May are not uncommon. Since spring is a very active period of growth for grass plants plus we are often putting down fertilizers and weed killers, it’s important to keep grass out of drought stress. Broadleaf weed killers such as those used for dandelions can injure or even kill lawn grasses that are under drought stress. A general rule of thumb for lawn watering is about 1 inch of water per week which includes what we receive through rainfall. Remember, it may be necessary to apply a couple of ½ inch applications on compacted soils, heavier clay soils and sandy soils rather than a single 1 inch application. In the compacted and heavier clay soils, too much water per application may cause ponding or water saturated conditions on the soil surface due to very slow water infiltration rates. This can damage or even cause dieback of roots in a matter of hours that are growing in the shallower regions of the soil. Sandy soils usually allow for rapid water infiltration and drainage; so much so that a significant amount of water can be lost down through the soil and unavailable to grass roots. Hence, for lawns growing on sandier soils a couple of ½ inch applications of water during the week may be more beneficial to the grass plants and less wasteful of the water applied. If you’re uncertain about how much water your sprinkler or sprinkler system is putting out, place a few flat bottom cans (tuna fish cans work well) in the water distribution area of the sprinkler. Leave the sprinkler on for about an hour or one cycle of an automated system then go measure the amount of water in the bottom of the can. That will give you the rate at which water is being applied either through a manual or automated system.
Photo 3: Spring flowering stems of creeping Charlie. Bob Mugaas
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 ¾ 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.
If you have been mowing your lawn a little shorter during the early spring period, now is the time to increase mowing heights prior to the onset of summer stresses. This helps encourage a more robust and deeper root system allowing the plant to access a greater volume of soil and therefore larger soil water and nutrient reserves. 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 1/3 of the turfgrass height be removed at any one mowing. Since growing back that 1/3 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 1/3 in grass plant height.
While there are many other competing interests in the yard and garden during this time of year, regular attention to basic lawn care should still allow ample time for the flower and vegetable garden activities that many of us also enjoy.