Tree Watering Basics: Keeping Trees Healthy After a Drought

Headshot of Nan StermanIt’s nice to get a breather from extreme drought but we live in a water poor region and our gardens – along with our lifestyles – should reflect that daily. We’ve come a long way changing gardens to be lush and green, using plants suited to the region’s water reality.

The focus has been on removing lawns and replacing thirsty plants but in our haste, we’ve not done well by trees. Some people removed trees based on their misperception that trees waste water. Others simply stopped watering trees and expect them to do fine. Neither is the case.

The value of trees
Trees are extremely important in wildlands, rural environments, and urban areas. They provide many “services” that few of us recognize. For example, the carbon dioxide we exhale is the gas trees take in during photosynthesis. And as they photosynthesize, they recharge the air with the oxygen we need to breathe.

Tree roots capture, slow, and clean runoff. Their roots hold water that would otherwise flood our communities.

Tree leaves shade and cool our houses. They also filter the air and capture particulates we would otherwise breathe in. Trees serve as habitat for birds, good bugs, and other important critters.

Trees are one of the most powerful tools in the fight against global warming
Cities and suburbs are hot places thanks to paving, buildings, cars, machinery, air conditioning, reflective surfaces, and so on. But trees and other plants release moisture into the air; a sort of evaporative cooling that makes a huge difference. I recently measured the temperatures of different surfaces in downtown San Diego. When the air was in the upper 60s, the concrete sidewalk was in the 80s, the asphalt street in the 90s, the rubber playground surface was in the 120s and artificial turf was more than 130 degrees (really).

Yet the soil under a shade tree was in the 50s!

Trees serve other important roles too. There’s so much to be said for trees – it’s time we stopped ignoring them, and started caring for them.

Tree watering basicstree
Trees, like all plants, take up water through their roots. Equally important, trees need to grow deep, drought resistant roots.

Trees don’t belong in lawns. That’s fine in climates where there’s plentiful rainfall and soil stays moist deep into the earth. Here, trees planted in lawn develop shallow roots that take up surface water but not the important, deep, drought-tolerant roots. To encourage those roots, trees need deep, infrequent irrigation.

Newly planted trees
In their first two or three years in the ground, trees should be watered about once a week, with 15 to 20 gallons each time (captured shower water or rainwater are fine). Before you water, stick a finger into the soil, about 4 inches deep. If you feel moisture, it’s not yet time to water. If it is dry, water.

At planting, build a 3 or 4-foot-wide basin around the tree’s trunk. If you have the patience, water from a bucket, adding it to the basin slowly. Alternatively install concentric rings of in-line drip irrigation around the trunk. Set the first ring 8 to 10 inches out from the trunk, add rings every 10 to 12 inches to just past the drip line (that’s the outer edge of the canopy). As the tree matures, the trunk will thicken. Over time, remove the inner irrigation rings and add more to the outer edges, keeping their spacing at 12 inches.

Established trees
After three years most trees are established, which means they’ve developed a pretty deep and extensive root system. Irrigate these trees less often — only once or twice a month — but with more water each time. The goal is to wet the roots 1 or 2 feet deep. This is when in-line drip irrigation is most important. Set rings of in-line drip irrigation a foot or 18 inches out from the trunk, and extend to just past the drip line. For a short term fix, use concentric rings of soaker hose. After a few years, their pores will clog. Recycle the old ones and replace them.

How often to water
You’ll need to experiment to figure out how long to run the irrigation. With clay soil, water moves into the soil slowly and stays a long time. With sandy soils, water moves through quickly and dissipates. Use a soil probe to determine out how deep water penetrates as you irrigate. Use the probe after you irrigate too, to see how long water stays in the soil. When the area around those deep roots to start to dry. That’s when you irrigate again.

Different kinds of trees need more or less water, too. Deciduous fruit trees like peaches and apples need water when they have leaves, flowers, and fruits on them. In winter when it rains, they may not need any irrigation at all. Citrus on the other hand, need irrigation year round. Don’t make the mistake of putting citrus and avocado trees on the same irrigation zone as peaches, plums, figs, or pomegranate. You’ll end up watering those less thirsty trees more than they need just to supply the citrus and avocado.

University of California recently posted directions for assembling your own “Tree Ring Irrigation Contraption” using standard in-line drip irrigation, along with a calculator for figuring out how long to run the water. Find it here tinyurl.com/hlwzeq3.

Mulch
Young or old, fruiting or not, all trees should be mulched. The best mulch is woody mulch applied at least 3 inches thick, starting 6 or 12 inches out from the trunk. Mulch insulates the soil to hold water in and keeps soil cool.

To learn more about the value of trees and caring for trees in urban environments watch A Growing Passion’s recent episode on Urban Forestry, tinyurl.com/ycnhyu48.

Nan Sterman is a garden expert, author, designer, and educator. Nan leads international garden tours and hosts the show A Growing Passion on KPBS TV. Learn more about Nan at plantsoup.com.