Mild growing conditions in the spring and early summer produced beautiful lawns across Iowa, but excessive heat and rainfall have produced some weary and weedy lawns that will need assistance to recover this fall. Statewide precipitation rates well above normal provided ample water for lawn growth, but while your sprinklers may have been growing cobwebs, lawns in Iowa were being set up for decline from diseases, weeds, insects, and summer stress.
In June there was a rather unusual outbreak of Ascochyta caused by, of all things, dry conditions and heat. (Information and Photos in HHPN, June 23, 2010) Lawns damaged by Ascochyta recovered in varying degrees but it is important to remember that summer stress accumulates and lawns are generally weakest by late August. Dollar spot and red thread were active through June, but the more deadly brown patch and Pythium have reared their ugly heads in late July and August to finish off some of the weaker lawns.
This was a terrible year for crabgrass and yellow nutsedge. Pre-emergence herbicides generally give 85 to 100% control of crabgrass, but this year's excessive rain and high temperature reduced efficacy of crabgrass control products. High moisture and high temperature are two factors that increase the activity of soil microorganisms that ultimately ingest the herbicide and render it inactive for season long weed control.
Lawns inundated with crabgrass by August 2010 will benefit from pre-emergence crabgrass control in spring 2011 to reduce the infestation of crabgrass that is eminent; seed from this year's heavy infestation will germinate next summer and the cycle of crabgrass will continue.
Should you try to kill the heavy infestation of crabgrass now? When crabgrass covers less than 25% of the turf area, do nothing. Crabgrass will die after the first frost and the Kentucky bluegrass will usually fill in the areas through the dead crabgrass. However, if the Kentucky bluegrass is being smothered beneath a layer of crabgrass that covers 50 to 100% of the visible lawn surface additional action is needed. The thick, uncontrolled mat of crabgrass will dominate the turf until the first killing frost that usually occurs in October; then it will be too late to establish Kentucky bluegrass from seed. Contact your lawn care company for assistance to suppress or kill the existing crabgrass to aid re-establishment. Power rake and reseed in early September.
White grubs and bluegrass billbugs are our two major lawn insects. There were some bluegrass billbugs this year but damage was very limited compared to past years with drier conditions. Annual white grubs of the masked chafer and Japanese beetle are showing up in ample supply and right on schedule for mid-August. If grubs are actively feeding in August and early September then curative treatments with fast acting insecticides such as trichlorfon are required (thoroughly watered in for effective control). Grub damage may be concealed by ample rainfall in late summer, only to appear during a dry spell. Curative insecticides are only effective between now and early October.
High temperature and excessive moisture are a deadly combination that cause stress for cool-season grasses grown in Iowa lawns (Kentucky bluegrass, tall fescue, perennial ryegrass, fine fescue). High temperatures favor warm-season grasses such as crabgrass while cool-season grasses suffer. This partially explains why crabgrass can overtake Kentucky bluegrass as the summer progresses.
Excessive moisture also contributes to lawn decline in the summer. Roots need air to survive. Oxygen is displaced in soggy or flooded soils and the anaerobic conditions cause the roots to not function properly. Imagine lying down in the sun of your front yard in the hottest part of the day on a sunny, soggy and humid bed of grass. You wouldn't last but a few minutes. Grass plants in the sun can't get up and move to the shade. They're stuck, and when the evaporative cooling system begins to shut down the grass plant, thatch, and soil surface quickly heat up to the existing air temperature or higher. Plants can be literally cooked to death by direct heat injury as plant tissue temperatures rise above 95 degrees F. Temperatures this year were sufficient to cause rapid injury directly from high temperatures and indirectly from prolonged periods of high temperatures that eventually depleted stored carbohydrates. Weakened plants with slow growth were often overcome by brown patch and Pythium diseases that flourished when night time temperatures were greater than 72 degrees F.
What to do
The bad news is that several lawns have succumbed to the various woes of summer described above. The good news is that now is the best time to rejuvenate damaged lawns. This may be a good time to kill the existing mess and start over with improved grass varieties suitable for your lawn. Consult your local professional lawn care company to develop a plan to recover your lawn through aerification, slicing, seeding, and fertilizing. Also see "Late Summer Lawn Care" in the August 11, 2010 issue of Horticulture and Home Pest News.