Trees are the most valuable of all landscape plants and are often the most neglected. Properly placed trees provide shade, reducing the need for air conditioning in the summer, while buffering the cold winds in the winter. Besides saving you money on energy costs, they provide oxygen and clean the air. Correctly watering trees during the summer is one of the most important things a homeowner can do to keep them healthy and less attractive to pests.
Many people plant trees in their lawns and assume when the sprinklers go on that their trees are being adequately watered. Wrong. Another problem occurs with trees that have been planted outside of the lawn. Often these had only a few emitters installed near their trunks at planting time. After a few years, water isn't getting to their feeder roots. These spread with growth and are located near the edge — or drip line — of the canopy and beyond. Or, the number of emitters hasn't been increased to accommodate the root area after a few years of growth. If your trees are in any of these situations, they are not receiving adequate water to sustain good health.
Symptoms of drought-stressed trees can be wilted leaves, browning and curling of leaf edges or browning between their veins, smaller than normal leaves or leaves that turn brown but remain on the tree. Older leaves can also turn yellow, red or brown and prematurely drop off. Conifer needles can turn yellow, red or purple or have brown tips.
Over-watering should be suspected if the youngest leaves become light green or yellow, young shoots are wilted, leaves are a dull green and brittle, the soil surface never dries out or you can see moss, mushrooms, or algae growing where it's damp.
Water trees slowly so that the water goes down into the soil instead of running off. Soaker hoses wrapped in a spiral beginning several inches away from a mature tree's trunk and extending several inches beyond the canopy drip line is an ideal way to deliver water evenly where it can be used with the greatest efficiency. Avoid watering the tree's trunk and leaves with sprinklers.
Roots grow where the water source is. On average, 50 percent of tree roots are in the top 6 inches of soil and 95 percent are in the top 18 inches. Trees that aren't watered deeply enough, such as lawn trees, may develop surface roots that can be damaged by lawnmowers. When you shallowly water your lawn, tree roots will compete with the grass for every drop they can get, which usually isn't enough during the hot summer months.
So how do you get it right? Amount and frequency of watering is determined by the type of soil you have, the evaporation rate due to air temperatures and wind, and your tree species and its maturity, says Rob Griffith of Southern Oregon Nursery in Medford. Granite soils drain and dry quickly and trees growing in it need to be watered more often, but for less time than trees in clay soils require. One inch of water can penetrate 12 inches of sandy soil, 7 inches of loam soil, and about 4 inches of heavy clay soil. The less penetration you have, the longer you need to water to get the moisture deep enough. Use tuna cans placed around the drip area of your trees to measure how much water you are actually applying to the area.
A good rule of thumb is to water mature, non-native trees that are between 18 inches to 3 feet deep every 10 to 14 days in granite soils and every 17 days to 4 weeks in clay soils during the summer heat. Soils that hold a lot of moisture don't need to be watered as often. A soil probe or long, shanked screwdriver that won't penetrate more than 4 inches of soil gives a good indication that water is needed.
Know the trees on your property and find out what special watering needs they have. Native trees such as black and white oak and madrone don't need summer watering or they will develop root rot, warns Daryle Hawkins, certified arborist and owner of Hawkins Tree Service in Medford. But if winter and spring are dry, water the trees in April and May to make up the difference.
Some of our favorite ornamentals have even more complicated needs. Japanese maples don't like to get dried out at all, but must have excellent drainage since they are also prone to root rot. It pays to find a specific plant's watering needs from your nursery, or a source like Sunset's Western Garden Book. With wise watering, your valuable trees can grow healthy and you'll enjoy them for a lifetime.