Give That Tree a Drink

Headshot of Nan StermanThe weather is finally (and thankfully) cooling as San Diegans await the El Nino rains.

After four long years of drought, even when El Nino arrives, it won’t bring us back to normal. So, we need to pay attention to trees. We’ve been so focused on killing and replacing lawns that we’ve overlooked the fact that trees are suffering.

Beyond beautifying our gardens, trees generate the oxygen we breathe. Trees remove CO2, one of the greenhouse gases, from the air (that’s the process of carbon sequestration). As trees respire, the moisture they release into the atmosphere humidifies it and cools it. Trees hold water in soils and “feed” the microscopic underground flora and fauna that keep soils healthy. Above ground, trees create habitat for wildlife. They shade and cool our homes. Some trees even do us the great favor of making delicious fruits.

It’s hard to see the effects of drought on established trees, according to Robin Rivet, La Mesa resident, certified arborist, and urban forester. A tree can suffer for four or five years before it shows visible symptoms like leaf drop, droopy leaves, or browning leaves.

Rivet says that while we can’t see it, all the landscape trees across San Diego are in crisis and desperate for water, especially those once surrounded by lawns. The most important thing, now, is to water all your garden trees. Yes, water them.

Does that sound like a conflict with watering restrictions? It isn’t. Otay Water District restricts watering times for old-fashioned overhead sprays, which is the worst way to water trees anyway.

Soaker hoses, drip systems, and highly efficient spray heads are not subject to the same runtime limitations, but are limited to running twice a week, between 6 pm and 10 am. But that’s a great schedule for watering a tree. Rivet recommends using drip irrigation, soaker hoses or seeping hoses placed slightly inside the tree’s drip line (the drip line is the outer edge of the tree’s canopy). Turn the water on in early evening and off in early morning. The goal is to slowly saturate the soil under the tree two or three feet deep. Saturating the soil, irrigates the roots.

How often to water depends on your soil and the tree’s age. Water penetrates through and drains away quickly from sandy soils, so you might deep soak established trees once or twice a month, newly planted trees more often. Water penetrates clay soils slowly and stays a long time, so deep soak only once every two months or less often for established trees in clay soils, more often for newly planted ones.

Rivet suggests using a straightened out wire clothes hanger to determine how deep the water has penetrated. Poke it into the soil as deep as you can, then pull it up and feel the hanger to see how much of it is damp. That tells you how deep the soil is moist. Wait till the hanger is dry most of the way down before watering established trees again, dry only several inches down before watering newly planted trees again.

Once the rains start and hopefully continue, hold back on watering. But if this winter ends up as dry as last winter, the winter before, the winter before that, and the one before that, you’ll be watering all through the season.

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.