Watering trees during the drought
Watering cutbacks forced by drought conditions have a lot of people looking downward, focused on yellowing lawns. It’s much more important to look upward, examining trees to see if they’re suffering from the lack of water.
After all, a lawn can be replaced with drought-tolerant plants, decorative rocks, bark or mulch – or, when the drought ends, new grass, if that’s important for children and pets.
But trees we simply can’t live without. And as drought conditions and high summer temperatures make them more susceptible to pests and disease, they require care to stay healthy.
Sycamore and oak are among the many urban species that provide excellent homes for birds, bees, squirrels and other wildlife, according to the group Tree People.
Regarding the tens of thousands of mature trees in Modesto, “People need to understand that these are resources, and if not protected, they’ll never see them again in their lifetime,” said Ray Wilson, certified arborist and operations crew leader with the Community Forestry Division of the Modesto Parks, Recreation and Neighborhoods Department.
Similarly, certified arborist Joseph Anderson, owner and operator of Modesto tree and landscape company California Roots, said trees that grow in nature are one thing “but we planted all these and have a responsibility to them.”
The Modesto Bee spoke with Wilson and Anderson – both members of the International Society of Arboriculture – and Modesto Solid Waste Manager Jocelyn Reed, who oversees forestry, about indicators of tree health, how to keep trees healthy and why they’re worth protecting.
The most obvious signs a tree is thirsty are leaves that are wilting or have brown edges. A larger sign of poor health is crown dieback – dead branch tips in the upper portion and outer edges of the tree’s crown.
Reed also noted that this summer’s heat has brought a “huge spike in sudden limb drop” – trees cracking and dropping large branches.
Sudden limb drop happens in very hot weather, when a tree is trying to cool itself, she said. But in trees damaged by drought, the inside of the limbs can be dead and dry. “The leaves on a tree are like the radiator of a car,” Reed said. “They carry the water up and push it out to the leaves. But if it collects in leaves faster than it can evaporate, you end up with tons of water weight above you.” If the tree is weakened, limbs can break under that weight.
Wilson said that when examining trees to see if a limb is at risk, “we look for a nice, even canopy.” If a limb is clearly separated from the rest – enough, say, to see sky – that’s a bad sign, he said. Prune perhaps 20 percent of that limb and it could correct itself, Wilson said. He likened it to holding a brick at arm’s length from your body. It grows heavy after a bit, but is much easier to hold close to the body with arm bent.
But pruning is an art form, so simply cutting away canopy to take weight off isn’t the answer. Those canopy leaves are essential to photosynthesis, for one. “You’re taking away its nutrition, its carbohydrates,” Anderson said. And removing canopy can expose bark to sun damage, Reed said.
Trees properly placed around buildings can reduce air-conditioning needs by 30 percent and can save 20 percent to 50 percent in energy used for heating, according to the U.S. Forest Service.
Also, drought is not an island entire of itself. The stress put on trees – especially old ones – by drought and heat can weaken their immune systems, making them more vulnerable to insect infestation, disease and parasites such as mistletoe. “That one thing, lack of water, can lead to numerous problems,” Anderson said
A tricky thing about drought symptoms, he said, is that by the time they present themselves, it could be too late. He noticed crown dieback in a small tree on his own side yard, in an “out of sight, out of mind” spot. But by the time he began tending to it, the damage was done, it’s not recovering and he expects to have to remove it.
Anderson and Reed said drought stress symptoms can be hidden for up to a couple of years in evergreens such as the redwoods that are grown here but really shouldn’t be. “A redwood isn’t drought-tolerant,” Anderson said. “They’re surviving in our climate by our watering.”
How much to water, and how
A general rule of thumb, Anderson and Wilson agreed, is to apply 10 gallons of water for every inch of trunk diameter.
“Let’s say you have a tree 4 inches in diameter,” Anderson said. “With a medium flow of water, maybe you get 10 gallons of water in five minutes. You would multiply that by four, so water 20 minutes.”
That said, other factors – type of soil, and whether a tree is surrounded by lawn, landscaping rocks or paved concrete – play into how the water is applied. Anderson, who said he almost always waters by hand, has a large Modesto ash near his sidewalk, growing with shrubs in a bed of river rock. “I’m only able to water a minute or two before I get runoff,” he said, so he’ll let it soak in as he waters elsewhere in the yard, returning to the ash periodically.
“And depending on your soil, if it’s real sandy, you’re going to water less more often,” he said, “and if it’s clay, you’re going to do longer, and less often.”
Tree roots go where the water is, so those planted in lawns tend to develop a system close to the surface. If your tree has become dependent on years of competing with grass for sprinkler water and then you stop watering the lawn because of drought, Wilson said, you’ll likely need to drip irrigate.
A young tree that isn’t babied from the start with surface watering will send tap roots down to find water, Reed said, but an older, established tree can’t change its ways.
“These mature trees are just living at this point, not growing vigorously,” Wilson said.
You can roughly gauge the width of a tree’s root system by its drip line. To irrigate, “buy an inexpensive soaker hose and lay it around the edge of the tree’s canopy,” Reed said.
She also suggested that homeowners can inexpensively build a deep water jet to nourish roots. Instructions on building and using one can be found at https://santabarbaraarborist.wordpress.com.
Anderson is a proponent of putting in a deep-watering pipe when planting a tree. It’s simply a 2- to 3-foot length of PVC pipe, 4 inches in diameter, with holes drilled along its length and sunk into the ground about 6 to 10 inches from the base of the new tree. Fill it with rock and cap it with a screen to keep debris out. “Deep soaking will give you a better tree in the long run,” he said.
To make the most of watering, use compost, Reed said. A layer 2 inches deep – and not laid right up to the trunk – will help the soil retain moisture. It also promotes earthworms, “which act like miners, creating holes in soil that allow water to percolate and help oxygen flow, too,” she said.
The city of Modesto has an online “Water-Wise Gardening Guide” that offers tips on watering habits and plants, shrubs and trees that grow well in the Central Valley. Among the suggested trees are holly oak, silver dollar eucalyptus, crape myrtle and Chinese pistache.
Oaks in general do well, Reed said, while not faring as well in the drought are maples, elms and some magnolias.
“Lots of trees here struggle when it gets above 100,” Anderson said. Though the city plants Modesto ash, he said, they are “prone to disease, and once a factor like prolonged lack of water is added, it snowballs.”
Wilson said people find fault with the Modesto ash, “but a lot of the trees they’re complaining about are 70 years old, in their decline.”
He said that when choosing a tree to plant, avoid high-maintenance varieties such as mulberries or willows that require pruning three or four times a year. And, he said, everyone needs to learn the lesson that “redwoods don’t do well in the Valley. Mature ones need 500 to 800 gallons of water a day.”
Benefits of trees
One obvious benefit of large trees is that when grown in the right spots, they can shade homes from the sun, helping cut the need for air conditioning. They also help cool the air through evapotranspiration. Reed said there’s a measurable temperature difference between areas of the city that are mostly asphalt and concrete pavement vs. those with a big tree population.
Trees also clean the air. They absorb carbon dioxide and potentially harmful gases, such as sulfur dioxide and carbon monoxide, and release oxygen.
A strong, continuous canopy of trees along a neighborhood street even prolongs the life of the asphalt, Wilson said.
They also are Modesto’s biggest ally in the mandate of Assembly Bill 32, the California Global Warming Solutions Act of 2006, to reduce greenhouse gas emissions, Reed said.
That’s why the city is aggressively pursuing its tree planting program even during drought, she said.
“We can’t afford to stop,” Reed said, noting that Modesto still is playing catch-up for many years of forestry being short of money and manpower, during which more than 20,000 city trees were lost.
“These trees are working for the city, for the residents,” she said.
Deke Farrow: 209-578-2327
Find out more
- City of Modesto forestry division: www.modestogov.com/trees
- City’s water conservation page: www.modestogov.com/water/conservation
- City’s “Water-Wise Gardening Guide”: www.modestogov.com/water/conservation/docs/water-book.pdf