Bob Mugaas, UMN Extension Educator
Photo 1: Newly emerging crabgrass seedlings. Bob Mugaas.
1. In many areas of the state, crabgrass, along with other warm season annual grasses are or will soon be germinating in lawns. See Picture 1. Once crabgrass has emerged from the ground, control strategies need to be changed. Herbicides directed at controlling crabgrass above ground are known as postemergence products. Be sure to select those products that are specific to visible, actively growing crabgrass plants only. Those products labeled as grass killers are usually designed to kill all kinds of grasses, including lawn grasses, not just crabgrass. Products sold as broadleaf weed and grass killers may or may not be safe for use on lawns. Be sure to read the product label carefully as to whether or not it is a product that is safe for use on lawn grasses. When treating crabgrass after it has emerged from the ground, be sure that the desirable lawn grasses are not under any kind of heat and/or drought stress as you can sometimes cause temporary yellowing. Under severe stress, permanent injury can also occur to our lawn grasses. As with most weed control options, treating the plants while they are small and tender is generally more effective than trying to control larger, more mature plants.
Photo 2: An easier to control or remove young lambsquarter weed
2. By early to mid-June, many of our annual broadleaf lawn weeds will also be starting to germinate. One of the best times for controlling these weeds is in this early growth stage before they have started to mature and initiate flowering. Examples of our common annual broadleaf weeds include prostrate or spotted spurge, erect and prostrate knotweed, lambsquarter, redroot pigweed and purslane. See Picture 2. The clover like plant, black medic as well as yellow woodsorrel are usually considered annuals although some plants may overwinter and behave like a biennial or short lived perennial. Taking a few minutes to walk your lawn in order to determine when some of these weeds may be germinating allows one to easily remove them by hand and/or treat them with an herbicide. Remember, removing and/or treating plants with an herbicide is much more efficient when they are small, even at the lower application rates stated on the product label. For more weed identification help and possible control options for lawn weeds see our 'Is this Plant a Weed?" section on our Extension website: http://www.extension.umn.edu/gardeninfo/weedid/index.html
Photo 3: Raise mowing heights to encourage larger, deeper grass root systems.
3. Since this is the time of year when many of our weed seeds are germinating, including those of crabgrass, it is generally best to avoid lawn care operations such as dethatching or core aerating. These operations disrupt the surface of the soil and can bring additional weed seeds into a more favorable environment for germination and growth, hence creating additional competition for our lawn grasses. For that reason, it is best to wait until late summer or early fall to resume these operations. However, be sure to leave at least four to six weeks of good growing conditions for the grass to fully recover from this injury before consistently cold weather arrives.
4. Along with our cooler than normal temperatures for much of the spring, many areas of the state have also received adequate to abundant amounts of rainfall. Those cooler temperatures and ample moisture supplies have encouraged vigorous, lush growth of our lawn grasses. Vigorous growth and rainfall will have likely depleted at least some of the available nitrogen in the soil making less available for plant growth. Therefore, most lawns will benefit from an additional ½ pound of nitrogen per 1000 square feet applied in the early to middle part of June. This will help replenish the supply of N used by the plant or lost due to leaching or returning to the atmosphere in a gaseous form through a process known as volatilization. This is about ½ the normal amount of nitrogen applied per application. On the other hand, too much nitrogen encourages more rapid and excessive growth that can compromise the overall health of the grass plant going into the summer months.
5. Lawn grasses will usually have better stress tolerance when they are mowed higher from the middle of spring through early fall. See Picture 3. Higher heights of cut usually mean at or above 2.5 inches for most lawn grasses. This helps encourage deeper, more robust root systems capable of extracting water and nutrients from a greater soil volume. Access to more soil moisture and nutrients increases the plant's capacity to tolerate and survive the warmer, drier conditions often experienced late spring through the summer months.
Photo 4: Shorter than expected mowing height due to softer ground conditions. See text for more explanation. Bob Mugaas.
6. While most walk-behind rotary mowers adjust mowing heights by resetting the four wheels to the desired height, it is occasionally a good idea to see how close that setting really is to the actual height of cut. The easiest way to do this is to simply take a ruler and gently push it through the turfgrass canopy until it rests firmly on the lawn/ground surface. Then look across the grass plants just in front of the ruler and see what the height is. For example, if the ground is firm then mower wheels will ride higher and consequently the height setting will more closely approximate the actual cutting height. However, where the ground is soft or there is a significant thatch layer present, the wheels will sink more deeply into the lawn and hence the mowing height is actually less than the wheel settings would indicate. In fact, where there is significant thatch present mower wheels can ride so much lower that the lawn surface between the wheels is actually scalped. See Picture 4. Remember to take the time to adjust your mower correctly, periodically verify that the mower height settings are actually providing the desired height of cut, and always mow with a sharp blade.
7. With so many yard and garden chores needing to be completed during a typical Minnesota May and June, it is very easy to overlook the water needs of our lawn grasses. However, it's important to remember that May and June are very active growth months for our lawn grasses. While much of the growth is directed at producing flower stems, our bluegrasses and perennial ryegrasses will grow much better with ample supplies of water during this period. An 'ample supply' usually means that the lawn receives about ¾ to 1.0 inch every seven to ten days including rainfall. See the May 15, 2011 Yard and Garden News blog for more information on Lawn watering practices that encourage healthy lawns and help protect water resources.