This series provides answers to selected questions recently submitted to the UC Master Gardeners of Yolo County Help Desk.
Question: How much water should I give my deciduous landscape trees?
Answer: The answer will vary by species and age of the tree, but there are guidelines that apply to most deciduous landscape trees. Trees establish root systems that rely on regular, but infrequent, moisture. Shallow watering can encourage roots to stay near the surface, leaving the tree vulnerable to drought and heat. The vast majority of landscape trees do best with deep, infrequent watering once established.
Here are some general recommendations. Young trees (first three to five years) need more consistent watering, usually once a week during the dry season especially during the hottest weather. Each watering should soak the soil deeply (twelve to eighteen inches). Established trees (older than five years) typically need deep watering about once every two to four weeks during hot, dry months. In very hot spells, every ten to fourteen days may be appropriate. A good rule of thumb is to apply ten to fifteen gallons per inch of trunk diameter measured at about 4 feet up the trunk per watering. For example, a six-inch trunk would need sixty to ninety gallons spread slowly over the root zone.
The best practices are to give the tree a deep, slow soak. Use drip irrigation, a soaker hose or a hose trickling for several hours that you move to cover the entire root zone. This prevents runoff in our clay loam soil and ensures water penetrates deeply. Most absorbing roots are one to three feet below ground and, for most trees, extend one-and-a-half times the distance from the trunk to the dripline (the edge of the canopy). There is a concentration of roots at the dripline so focusing at least half of the water there is a safe bet. A spiral of soaker or drip hose starting a few feet from the trunk and extending out is a good approach. Be sure to avoid wetting the trunk to prevent crown rot.
Check soil moisture after watering to be sure water is penetrating deeply. This can be done with a water meter that has a long probe or a metal rod, which should be able to be pushed relatively easily into wet soil. It is important that the soil dry significantly between waterings. Ideally, moisture should penetrate to a depth of two to three feet after a watering session. By the time of your next watering session, the soil should be fairly dry again.
A three-to-four-inch layer of organic mulch around the root zone helps conserve soil moisture and reduce heat stress. Keep mulch six inches away from the trunk to prevent crown rot. In winter, rainfall often provides enough moisture, so supplemental watering is usually unnecessary unless there is a prolonged dry spell. But in a winter drought, even established trees will need irrigation.
Pay attention to the warning signs of under- or over-watering. When trees are underwatered, leaves wilt, curl, or brown at the edges, the canopy thins, and premature leaf drop occurs. Overwatering is accompanied by yellowing leaves, dieback at branch tips, or soggy soil around roots. “Overwatering” is more often caused by too frequent watering rather than too large a volume of water at any one time.
In summary, set up a routine of deep watering about every 2–3 weeks through the summer, adjusting based on heat and soil moisture. Aim for slow, thorough soakings that reach at least two feet deep, supported with mulch to keep roots cool and moist. This steady schedule should keep your trees well hydrated and healthy.
For additional information, see UC Master Gardeners of Alameda County's article Watering Landscape Trees and Shrubs: The Good and the Bad.
This article previously appeared in the Winters Express and Davis Enterprise.
If you have a gardening question, contact the UC Master Gardener Help Desk at 530-666-8737. Or send an email, with information regarding watering, sun exposure, details about your problem, and photos, when possible, to mgyolo@ucdavis.edu.