These July weeks can be the hottest of the year. Tree and plant roots are not always able to take up enough water to compensate for the moisture lost by transpiration through their leaves. Low humidity, long days, shorter hot nights and high temperatures can cause wilting, leaf scorch and plant death even when the soil is kept evenly moist.
Wilting is the first sign of heat stress in plants. Cool-season lawn grasses develop a bluish hue and foot imprints remain visible for several minutes in lawns that are stressed by high temperatures. Branch tip dieback and early leaf fall are symptoms of heat stress in mature landscape trees, but those symptoms may not be visible for several weeks after severe heat stress.
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All automatic irrigation timers should be reset in July. Bermuda and other warm-season lawns will require 85 minutes of irrigation per week on average; fescue and other cool-season lawns need approximately 113 minutes.
Watering times for lawns and planting beds depend on soil type and irrigation methods. Clay soils hold water longer than sandy soils and well-amended soils hold water better than non-amended soils.
Planting beds that are covered by a three- to four-inch layer of mulch will lose 70 percent less water from evaporation during hot spells. Plants watered by a single drip emitter or overhead sprinklers will need more irrigation time than those watered by multiple emitters, soaker hoses, micro sprinklers or bubblers. Plants with similar irrigation requirements should be grouped together in the same irrigation zones to ensure that every plant receives the optimum amount of water.
Citrus, fruit and nut trees will need deep irrigation every three to four days during the hottest spells-when the top 3 to 4 inches of soil has dried; mature landscape trees should be deep irrigated at least monthly during the hot summer months. Deep slow irrigation over a period of several hours will soak the trees’ root system which usually lie about a foot deep right underneath the tree canopy.
Container plants, especially those in smaller pots or barrels, might need daily hand watering, even twice a day when temperatures rise above 100 degrees. The soil inside containers quickly dries out during hot weather and the containers themselves conduct heat to plants’ roots, especially when placed in full sun on hard, hot surfaces. Placing your container plants on a raised platform or on saucers with wheels allows for a cooling air flow under the containers. Consider moving container plants into full shade under trees, eaves or patio arbors for the next few weeks. Take care of container plants that require full sun – the long summer days of 12 to 14 hours is too much even for these plants. They may not grow well or produce flowers in the shade, but they will have a better chance of survival.
Master Gardeners – The lawn watering times provided above come from the Fresno County Master Gardeners’ “A Gardeners’ Companion for the San Joaquin Valley.” The guide was compiled and written by our local Master Gardeners and gives detailed gardening information specific to our hot, dry climate. The third edition is now is available; call the Master Gardener hotline at 559-241-7534 or check the MG website, ucanr.edu/sites/mgfresno, for more information.
Send Elinor Teague plant questions at firstname.lastname@example.org.