Bob Mugaas, UMN Extension Educator
With the recent rains and storms across the state, most lawns have had sufficient moisture to remain actively growing and green through the month of June. In fact, in some instances there has been too much rain causing lawns to remain in excessively wet conditions for several days or more at a time.
Photo 1: Temporarily ponded water from excessive rainfall.Bob Mugaas.
Under moderate temperatures and partly sunny or cloudy conditions, water that temporarily (a day or two) remains at or near the lawn surface is usually not a problem. (See Picture 1). Once the water drains away and soil oxygen levels rise such that normal root functions can continue, the grass will resume normal activity and growth with little to no evidence of having been temporarily submerged. However, under sunny conditions and high temperatures, lawn areas remaining in saturated soil conditions or submerged for even a few hours can suffer serious damage and even death. Once the moisture does recede, grass plants will appear dark brown to black indicating they are no longer alive. These areas often have a foul smell associated with the dead and dying plant tissue. Following those conditions and once the area has dried out, it will be necessary to reseed or resod the area due to the grass that has been killed.
Moist conditions in the lawn have also given rise to the random appearance of many different kinds of mushrooms. These are the result of fungi feeding on the dead and decaying organic matter in the soil and thatch layers of the lawn. As these fungi continue to grow and carry out their decomposer role in the soil and thatch, they will periodically, especially under moist conditions, send up fruiting structures that we know and see as mushrooms. See Pictures 2 & 3.
Photo 2: Lawn mushrooms emerging during moist conditions.Bob Mugaas.
Photo 3: Another species of mushroom growing in the lawn. Bob Mugaas.
These are not indicative of problems in the lawn or of an impending lawn disease outbreak. Most of these fungi carry out the beneficial process of decomposing soil organic matter which ultimately helps recycle nutrients back to the grass plants for growth and development. There are fungi that cause the appearance of darker green circles or arcs in the lawn. This pattern or symptom is commonly known as fairy ring. See Picture 4. If these are what you are seeing, you can check out turfgrass disease section of Extension's Gardening Information page: http://www.extension.umn.edu/gardeninfo/diagnostics/turf/circular.html for more information. One should not eat mushrooms appearing in the lawn! Where there is concern about children or pets possibly consuming them, simply break them off with a rake, pick them up and dispose of them in a manner that keeps them completely out of the reach of children or pets.
Photo 4:"Fairy ring" in lawn. Bob Mugaas.
With the excessive amount of rainfall and the continued vigorous growth of our grasses all during June, it is very possible that our lawns will benefit from a light application of nitrogen fertilizer. Nitrogen can be lost in a number of ways from the lawn including leaching (i.e., carried with water draining through the soil), gaseous loss back to the atmosphere and taken up and used by grass plants for growth. Hence, under the moist conditions and rapid grass growth that we have been experiencing lately, many of our lawns will benefit from ¼ to ½ pound of actual nitrogen, per thousand square feet. It's important to not fertilize excessively going into and during the potentially hot summer months of July and August. That can unnecessarily stress the grass plants and perhaps result in injury during those hot, dry summer periods we expect to encounter in Minnesota during July and August.
Photo 5: Excessive clippings from waiting too long to mow. Clippings should be removed from lawn surface before causing damage to underlying grass. Bob Mugaas.
While it's tempting to mow the grass much shorter once it has gotten too long, that will be extremely stressful for the grass plant and actually results in a slowing or even stoppage of growth and recovery, and perhaps even the death of some plants. That slowing or stopping of growth may seem like just the result you are looking for, but, it's really not. Our lawn grasses are much healthier, competitive, and stress tolerant when they grow at relatively uniform rates. Under those conditions, the amount of food produced by the leaves is sufficient to meet the plants growth needs as well as create some additional stored reserve. When large amounts of leaf and stem tissue are removed at a single mowing, shoot and root growth slows or even stops. That additional stress can open the door to certain disease and insect problems as well as increased potential for weed invasion while the grasses are recovering. If the excessive clippings are left on the lawn, there can be enough sunlight reduction to the plants underneath the clippings that they can be injured or even killed causing a thinning out of the lawn. (See picture 5.)
The loss of large amounts of leaf tissue all at one time forces the use of stored plant reserves just to survive. As a general rule, when our grass growth gets a little too far ahead of us, initially mow as high as your mower will safely allow. Then, begin lowering the height of cut by mowing more frequently and gradually reduce the mowing height back to the desired level. This is much healthier and less stressful for our lawn grasses.