In these hot days of summer, we take pleasure in watering vegetable gardens and fruit trees but we don’t often think about watering pines, liquidambar, magnolia, floss silk, and all the other trees that grow so well in our climate. Once tree roots are deep in the earth, we figure they’ll be fine on their own, especially when we have to be careful about using water.
As you can see in our episode about urban forests, trees are critical components of healthy communities. Likewise, the health of urban and suburban trees depends on us, in part on our irrigating them, especially after so many years of too little rain.
Tree watering basics
Trees, like all plants, take up water through their roots. One of our goals as good tree stewards is to encourage trees to grow the deep, drought resistant roots that come from deep, periodic irrigation (and to keep trees separate from lawns).
Newly planted trees
When you plant a new tree, build a three or four foot wide basin around the trunk. Install concentric rings of in-line drip irrigation around the trunk.
Set the first ring eight to ten inches out from the trunk, add another ring every 10 to 12 inches out from the first ring and continue so the last ring is just past the drip line (that’s the outer edge of the canopy). As the tree matures and the trunk thickens, remove the inner irrigation rings and add more outer rings, all spaced about 12” apart.
In the first two or three years, water trees once a week, using 15 to 20 gallons each time. Before you water, stick your finger about four inches deep into the soil. If you can feel moisture in the soil, it’s not yet time to water. If it is dry, water.
After three years, most trees are “established,” which means they’ve developed deep, extensive root systems so they need water less often, but more water each time.
The goal now is to wet the roots one or two feet deep. You can always add drip irrigation to an already established tree. Like with the young trees, create concentric rings of drip line, starting a foot or 18 inches out from the trunk and extending to just past the drip line.
If you can’t install in-line drip, use concentric rings of soaker hose. After a few years, soaker hose pores clog with debris and minerals from the solids in our water. When that happens, recycle the old lines and replace them with new soaker hose or with in-line drip.
How often and how long to water
Water when soil around the roots is dry. Established trees need irrigation less often than young trees — only once or twice a month.
Once or twice? Soils are not all the same. Sandy soils are made of big, irregular pieces with lots of space in between. Water moves through sandy soils quickly, then dissipates so trees in sandy soils need water more often. Clay soils are made of small particles that pack closely together. So, water moves through clay soil slowly and stays for a long time. Trees in clay soils don’t need irrigation as often.
Either way, a soil probe can help you monitor soil moisture deep in the ground. When you start out, test soil moisture every few days to get a sense of how long it takes for the deeper soil to dry out. When it’s dry, it’s time to water.
How long to run your irrigation system depends on whether it is clay (run in cycles, each slow and short) or sand (run fast and long) and the rate of the drip irrigation (drip line is available in different drip rates).
Not all trees are alike
Different kinds of trees need more or less water, too. Deciduous fruit trees like peaches and apples need water in the months when branches are covered with leaves, flowers, and fruits. When branches are bare, they may not need any irrigation at all. Citrus and avocado on the other hand, needs irrigation year round, except after a good rain.
Avoid the mistake of putting citrus and avocado trees on the same irrigation zone as peaches, plums, figs, or pomegranate. You’ll end up overwatering these less thirsty trees just to supply the citrus and avocado.
University of California recently posted directions for assembling your own “Tree Ring Irrigation Contraption” using in-line drip irrigation. That page also has a downloadable “calculator” you can use to determine how long to run the irrigation.
And always mulch. More on that in the future.