This week Iowa State University Extension garden experts have answers to questions about caring for the home lawn. Extension horticulturists address questions about pre-emergent herbicide use, mowing recommendations and control of violets. Gardeners with additional questions can contact the experts by emailing or calling the ISU Extension horticulture hotline at firstname.lastname@example.org or 515-294-3108.
The key to the successful control of crabgrass in lawns is the correct timing of the pre-emergent herbicide application. Crabgrass seeds begin to germinate when soil temperatures reach 55 to 60 F and continue to germinate over several weeks from spring into summer. Pre-emergent herbicides normally should be applied in early to mid-April in southern Iowa, mid-April to May 1 in central Iowa and late April to early May in the northern portion of the state. Spring weather often varies considerably from year to year in Iowa. Accordingly, gardeners should make minor adjustments in the timing of the pre-emergent herbicide application. If the weather in March and April is consistently warmer than normal, apply the pre-emergent herbicide early in the normal period. Apply the herbicide late in the recommended period if the state is experiencing a cool early spring. If you're still uncertain when to apply a pre-emergent herbicide, Mother Nature provides some helpful (colorful) clues. Pre-emergent herbicides should be applied when the forsythia blossoms start dropping or when redbud trees begin to bloom. Crabgrass seed germination typically begins after these events.
Kentucky bluegrass lawns should be mowed at a height of 2 ½ to 3 inches in the spring and fall months. Mow bluegrass lawns at a height of 3 to 3 ½ inches in June, July and August. A higher mowing height in summer helps to cool the crowns of the turfgrass plants, encourages deeper rooting and provides more leaf area for photosynthesis during the stressful summer months.
Mowing below the recommended range may scalp the turf and cause the turfgrass to deteriorate. Extremely low mowing heights decrease the total leaf area, carbohydrate reserves and root growth, creating a situation where the turfgrass plants are unable to produce enough food to meet their needs. This makes the plants more susceptible to drought, high temperature and wear injury. In addition, the bare areas created by a decrease in turfgrass density increase the chances of weed problems.
Mowing too high also can create problems. Mowing above the recommended range reduces tillering and causes matting of the grass. Reduced tillering results in fewer and coarser plants, while matted grass creates a micro-environment that encourages disease development.
Mowing frequency is based on the growth rate of the turfgrass and mowing height. As a general rule, never remove more than one-third of the total leaf surface at any one mowing. Shorter mowing heights require more frequent mowing. A lawn maintained at a 2-inch height should be mowed when the grass reaches a height of 3 inches, while a lawn maintained at a 3-inch height should be cut when it reaches a height of 4 ½ inches. One inch of growth is removed when the lawn is maintained at a 2-inch height, one and one-half inches of growth is removed when the grass is maintained at a 3-inch height. During favorable growing conditions it may take grass five to six days to grow one inch, eight or nine days to grow one and one-half inches. Irrigation and fertilization practices, weather conditions and other factors determine the growth rate of the turfgrass.
When the lawn is mowed properly, grass clippings do not need to be removed or bagged. Small clippings filter down into the turf and quickly decompose, returning essential plant nutrients to the soil. Lawn clippings do not significantly contribute to thatch development.
Grass clippings may need to be bagged or raked and removed when mowing extremely tall grass. You also may want to bag the grass clippings and use them as mulch in vegetable and flower gardens.
Violets are difficult to control in turfgrass areas. Digging up the plants is an option for home gardeners with a small infestation of violets. Broadleaf herbicides are the most practical solution when dealing with large numbers of violets. Broadleaf herbicides containing triclopyr usually provide good control of violets. Applications can be made in spring (when the violets are blooming) or fall. Two applications, two to three weeks apart, are usually necessary to achieve good control.