Drought stress… we all feel it: dirty cars, fallow fields, hurried rain showers, brown lawns and now our landscape trees are suffering too. Without trees, our ecosystem changes, creating a hotter valley with less oxygen, wildlife, privacy and beauty. Because trees are also a valuable investment of money and time, trees should be preferred when water supplies are limited.
“Old landscape trees are worth saving! Recognizing early signs of drought stress is important because irreversible damage can occur that no irrigation can correct,” says Janet Hartin, horticultural consultant at UC Cooperative Extension. Most residential landscape trees are surrounded by lawns. When local residents eliminate or reduce water for lawns, trees do not adapt easily because they have never developed extensive, deep roots due to the constant shallow watering.
Check trees regularly for common symptoms of drought, including:
• wilting or drooping leaves that do not return to normal by evening,
• curled or yellow leaves that may fold or fall off,
• Foliage that turns greyish and loses its green luster or is already brown,
• new leaves that are smaller or stem sections that are closer together than normal.
Trees stressed by drought are more susceptible to damage from diseases and insects, especially during long years of drought like the one we are experiencing.
1. PREVENT SOIL COMPACTION around trees. The compaction restricts water movement and reduces the oxygen demand for tree growth.
2. REMOVE COMPETITION from other plants by removing turf and weeds under trees. Plants compete for water, nutrients, light and space.
3. USE ORGANIC MULCH in a circle around the tree, 2 to 6 inches thick (depending on material) and at least 3 feet apart. Spread mulch like a donut, not like a volcano, leaving an 8-inch circle around the truck to prevent rot. Avoid using rocks as mulch as they absorb heat and stress the roots.
4. AVOID pruning, fertilizing, and any other maintenance that promotes tree growth, resulting in increased water requirements. Prune only to remove dead and diseased wood, dangerous branches, and shoots from the base of the tree.
5. Water trees slowly and deeply to a depth of 2 to 3 feet. To check how deep the water goes, stick a metal rod, thick wooden dowel, ground probe, or straight coat hanger into the soil a day or two AFTER watering. It moves easily through moist soil but becomes more difficult to push as the soil gets drier and deeper. Young trees need to be watered more often than old trees. “You need to be aware of the situation and stay in tune with your trees,” recommends arborist Roger Poulson.
The tree roots of a mature tree are in the top 3 feet of the ground and extend OUTSIDE of the tree canopy. This outer area of the tree roots is the most active water intake area. Therefore, trees stressed by drought should be watered slowly, deeply and widely. Surrounding a tree with drip tubing is an inexpensive option. Space each circular ring 1 foot apart as you wrap the rings around the tree, starting about 2 feet from the trunk.
Many tree species are showing signs of stress, particularly trees not native to our valley. Coast redwoods resemble “a fish out of water” in the San Joaquin Valley. Our Tulare/Kings Master Gardener Ornamental Tree Guide lists them as “Problem Trees: Proceed with Caution or Don’t Plant” because planting in the valley is risky. Coast redwoods prefer acidic soil, cool temperatures, and high humidity (the OPPOSITE of valley climates) in summer. Extreme salt sensitivity causes brown leaves. Trees usually perform well for 8-10 years; they require large amounts of watering in summer; and they almost always show summer stress.
New studies from UC Berkeley Forest Experts show that even sequoias in their native region (a narrow strip of California coast from parts of Monterey County to the southwestern Oregon border) are showing signs of distress due to the extended drought. Several coastal towns are already using recycled water for landscape irrigation, but it’s usually higher in salinity than well water, so they don’t use it for the redwoods. Local redwoods can die off during the summer heat but often recover in winter. However, our winter temperatures have also increased, so the trees are not fully recovering. Winter and spring rains, which are slightly acidic, not only provide distressed redwoods with moisture, but also help leach harmful salts from the root zone.
Apply the above strategic tips to your sequoias. Also, let the branches grow down to the ground to shade the roots. Sequoias do not have a taproot and have a very wide network of fibrous surface wet roots.
Even some of our prized oak trees are showing the effects of the drought. William Tietje, natural resources specialist at UCANR, suggests these three options: deep water, mulching, or doing nothing.
“If the soil under your oak tree is 12 to 18 inches deep dry and crumbly, then the oak is out of water. Deep watering will revitalize the drought-stressed tree,” says Tietje. Low flow water seeping into the ground. Do this once or twice in the summer.
The best mulch is the natural leaf litter of the oak tree, but other plant-based mulches can also be used. Cover the area under the tree canopy to at least the drip line and keep mulch away from the trunk.
Doing nothing may be best if your tree appears healthy, with dense and green leaves throughout most of the canopy. Blue oak and valley oak are deciduous (shed their leaves in winter). Both can respond to drought by browning and dropping as early as July. This is a natural water protection response. The tree is probably not dead and should be fine.