Our extended cool, wet spring can easily deceive us into thinking that our garden soils are holding enough water to sustain our plants. In our coastal area, it may be gray and drippy for days, but the rainfall totals are usually not significant enough to replenish soil moisture. Even though our heavy clay soils may feel damp, they still may not have enough water to supply plant needs.
Hot, dry weather affects plants differently. Some plants wilt. Wilt results when water loss from the leaves is greater than uptake by the roots. It’s easy to diagnose wilting in flowers and many vegetables. Their leaves collapse, droop and don’t appear turgid or stiff. Blossoms fade and die soon after opening. Tomatoes and corn exhibit symptoms seldom seen in other vegetables. Tomato leaves roll under, while corn leaves roll up. Many garden crops abort their flowers if the weather is too hot and dry. Tomatoes, peppers, squashes, eggplants, melons and pumpkins are notorious for this condition.
Established trees and shrubs are less likely to show wilt symptoms. Instead, trees and shrubs exhibit characteristics called scorch. Scorch shows up as browning of leaf margins and the tissue between the veins. Leaves feel crisp or leathery. The contrast between the normal green color and brown of scorch is obvious.
One of the best methods for telling if plants have enough water is to use the “screw driver test”. Simply press the screwdriver into the soil. It will easily penetrate moist soil, but stops when it its hard and dry. Since the top layer of soil will dry out faster than the rest of the soil profile, make sure you sample soil at a depth of between 4-8 inches, or below this depth, if you are concerned with deep moisture. With so much variation among water systems and the amount of water distributed by sprinklers and soaker hoses, it’s impossible to specify exactly how long a sprinkler should be left on in order to replenish soil moisture to a particular depth. This is best determined by leaving the sprinkler on for a specified period of time and then measuring the depth of soil moisture.
Trees and shrubs should be watered to a depth of 18-20 inches. The amount of water to apply in any situation will depend on the soil type. Sandy soils absorb water faster (about 2 inches per hour), followed by loam soils (3/4 inch per hour). Clay soils have the slowest absorption rate (1/2 inch per hour). By allowing water to penetrate deeper into the soil profile you are encouraging deeper rooting and a more drought tolerant plant. Frequent, light irrigations will lead to plants that have a shallow root system and are more prone to water stress. The amount of water required will vary with the number and size of plants, as well as the soil type. It takes about ¾ inch of water on the surface to penetrate to a depth of one foot in dry sand. It takes 1 and ½ inches of water on the surface to penetrate a one foot depth in loamy soil, and 2 ½ inches to penetrate to a one foot depth in clay soil.
The best procedure is to water only when needed, as the soil becomes dry. Daily sprinkling is not the method of solving a plant’s moisture needs. The best method is to run the sprinkler for a number of intervals in one 24-hour day. Soak the area deeply, and then do not re-water until necessary. Intervals may be only two days in hot weather if the soil is sandy. A heavier clay soil on the other hand, should easily carry four or five days between waterings.
Water running off the surface is not an indication that the soil is soaked. One possibility is that the type of sprinkler you are using is putting out water too fast. An oscillating arm sprinkler is one of the best to apply water slowly. Dedicated gardeners use soaker hoses to do deep watering. For prized plants, placing the hose on the root mass and letting the water trickle for several hours will do a good job of soaking the root zone.
Fortunately, lawns are equipped for hot, dry conditions. Instead of wilting and dying like many plants, they simply go dormant. Like a hibernating bear, a dormant lawn does not need food or water. When conditions are more ideal, usually in September, the grass will awaken and turn green. A newly established lawn may need to be watered throughout the summer to reduce drought stress and to prevent the plants from actually dying. Lawns with a thick thatch layer are also more prone to dying and thus may need watering.
Don’t water on hot, windy afternoons. You’ll lose more than half of your water to evaporation. Early morning watering lets the water soak into the soil. It also allows the plant foliage to dry off fast once the sun rises which helps prevent plant diseases. Remember that anything planted this year and any trees or shrubs planted over the last two years need extra water. Even plants listed as drought tolerant will need extra water if newly planted. Vegetables and fruit plantings need water, especially during fruiting. In addition, plants in containers will need to be watered more often, perhaps daily, depending on temperature and wind.
One of the best investments home gardeners can make, to help retain overall soil moisture, is through the liberal use of mulches. Mulching materials placed over the soil reduce evaporation from the soil surface, may reduce some water run-off, allow better water penetration into the root mass, and limit weed growth. Mulches may be organic (shredded leaves, bark, sawdust) or inorganic. Plastic mulches are especially effective in limiting evaporation from the soil surface, but also limit water absorption.
This column is printed as a service of the WSU Extension Office at Elma. Donald D. Tapio is the regional gardening and farming specialist for Grays Harbor and Pacific counties. He can be reached at (360) 482-2934 or firstname.lastname@example.org.