Summer Swelter Stresses Lawns
Inside our company, we’ve been discussing mother nature’s effect on our lawns. It’s no surprise that it’s been a strange season for weather. Honestly, every season seems strange. This one started out with over 400 funnel clouds early in the season, followed by plenty of rainfall, and an extended period of hot, humid weather. The item that’s caught our attention is the bumper crop of weeds. We expect a lot of weeds during hot, dry weather, but not in a season with plentiful rainfall.
So, why the bumper crop of weeds? It shouldn’t be a surprise when you hear that on August 30th, it was announced that Minnesota farmers are expecting all time record crops this year. It’s obvious that growing conditions are excellent.
Crabgrass is a particularly tough problem. Crabgrass crowds out healthy turf and a single plant can leave behind an ugly purple skeleton along with 3000 seeds to germinate in your lawn for the next few years, so control is critical. Most companies apply crabgrass control once in the spring. We’ve taken the proactive stance of applying it twice as a pre-emergent and have spot treated during the summer as a third treatment.
Let’s recap the year:
Mild growing conditions in the spring and early summer produced beautiful lawns, 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 Minnesota were being set up for decline from diseases, weeds, insects, and summer stress.
Disease
Dollar spot and red thread were active through June, but the more deadly brown patch and Pythium have reared their ugly head in late July and August to finish off some of the weaker lawns.
Weeds
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.
Insects
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. 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.
Summer Stress
High temperature and excessive moisture are a deadly combination that cause stress for cool-season grasses grown in Minnesota 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 lawn care professional company to develop a plan to recover your lawn through aerification, slicing, seeding, and fertilizing.
By Dave Minner, Department of Horticulture & Steve Hoogenakker, Concierge Landscape Environments
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