Home & Garden: When to water the lawn
Free Press Gardening Columnist
You have most likely heard plant watering at night increases the potential for disease. At the same time you have heard watering during the day results in an excess loss of water due to evaporation. You also have been told the magnification of the sun’s rays burns the grass. So what should you believe?
During the late evening and early morning hours, our lawns become moist due to accumulation of moisture through condensation. As the air cools during these hours, the amount of moisture in the air becomes too great for the air to sustain (especially as it touches the cooler grass blades). This results is dew forming on the grass. During these late evening hours is also the time when guttation occurs.
Leaves have glands that excrete sugars, starches and amino acids onto the leaf surface. This guttation fluid is an ideal source of energy for fungus and bacteria that are on the leaf. When mixed with the dew from condensation, this creates a perfect environment for the fungus and bacteria that causes plant disease. When we water during the late evening and early morning hours, we not only dilute this guttation fluid but also wash many of the fungal spores and bacterial cells off the leaf surface. When these organisms mix with the thatch or soil, beneficial organisms have the opportunity to attack and destroy these disease-causing pathogens. In other words, watering between 10 p.m. and 6 a.m. reduces the chance of the turf becoming diseased.
We know that many of these disease organisms require a certain length of wet leaf surface in order for it to invade and colonize the plant tissue. For some disease organisms that may be nine hours of wet leaf surface; for many 12-14 hours of wet tissue is necessary for the pathogen to invade the plant.
When we water in early evening, i.e. between 4-10 p.m., we increase the length of time the leaf tissue is normally wet thus increasing the chance of the turf being wet long enough for a pathogen to cause a disease. Watering in early evening also stimulates the guttation process, thus increasing the energy supply available to disease-causing pathogens. Watering in early evening therefore is not recommended.
Watering in the morning before the grass dries off also increases in the length of time the grass blades are wet resulting in an increased chance of disease. By allowing the grass to dry off for several hours before watering again, you break the life cycle of the pathogen preventing it from becoming a problem.
Now let’s look at the recommendation of watering between 10 a.m. and 3 p.m. Obviously watering at this time will result in greater losses of water due to evaporation. However, if you missed your night watering or discover the turf is severely stressed, it is better to turn on the irrigation system for a few minutes during the heat of the day than delay watering until night.
When your lawn starts to turn bluish-gray due to inadequate water, it also starts to lose roots. When this occurs, waiting to water until later that night can result in severe root loss as well as attack by diseases such as Ascochyta leaf blight and dollar spot. Giving the lawn a three to five minute syringe during hot days, especially when the grass is showing symptoms of stress, will cool the grass, replenish water in the upper layer of soil, and reduce root death. If you delay watering when the turf is stressed, you might need to water even more to help the grass recover from root death.
If you do decide to water in the heat of the day, always turn the water off soon enough in the afternoon to allow the grass to dry out before it again becomes wet due to condensation and guttation fluid.
Oh, by the way, a droplet of water on the grass blade can not burn the grass by magnifying the rays of the sun.
GJ Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.
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