The amount of water walnut trees need changes with age and season. Mature trees require 1 to 1.5 inches (2.5 to 3.8 cm) of water per week during the growing season from spring through early fall. Young trees in their first two years need even more attention with deep soaking twice per week. Getting this right is one of the most important things you can do for your tree's health and nut production.
I watched my largest walnut tree struggle through a dry July two summers ago. The leaves started curling at the edges first, then turned a dull gray-green color instead of their normal deep green. By early August, the tree was dropping leaves from the lower branches as if fall had come two months early. That drought stress cost me about half my nut crop that year. The tree pulled water and energy away from the nuts just to keep its own leaves alive.
Walnut trees send roots down 6 feet (1.8 meters) or more into the ground. Your walnut tree watering schedule should match this deep root system. Short, frequent bursts of water only wet the top few inches of soil. This trains your tree to grow surface roots that dry out fast in the heat. Deep, slow soaking once or twice a week pushes water down to where the main roots live and builds a tree that handles dry spells much better over time.
Young trees in their first two growing seasons need you to water them twice per week with a slow soak lasting 30 to 45 minutes per session. After those first two years, you can shift to one deep watering per week for mature trees. Each session should put down enough water to soak the soil at least 2 feet deep around the root zone. You can check your depth by pushing a long screwdriver into the ground after watering. It slides in smooth through wet soil and stops at dry dirt.
For your walnut irrigation requirements, drip lines work far better than sprinklers. Set your emitters at 2-foot gaps in a ring around the drip line of your tree. As the canopy grows wider each year, add a new ring of emitters further out to match the expanding root zone beneath the soil. A drip system puts water right where the roots can grab it and wastes far less to wind and sun than a spray head does.
Spread 2 to 3 inches of wood chip mulch in a wide ring from the trunk out to the drip line edge. Keep the mulch pulled back 4 to 6 inches from the bark to stop rot from forming at the base. This layer of mulch holds moisture in the soil between your watering days and keeps the root zone cooler during hot summer months. I cut my watering time by about 25% after I started mulching under all my walnut trees.
Start cutting back on water in late September to help your tree harden off for winter. Reduce your watering to every other week through October and stop by November in most areas. This dry-down period tells your tree to shift energy from growth into cold prep mode. Come spring, ramp your watering back up as new leaves push out and follow your schedule through the growing season once more.
Read the full article: Growing Walnuts: 7 Key Steps
