Supplemental Irrigation for Trees Critical during Dry Weather

Keep your trees healthy for generations to come

June 9, 2021, 8:30 am | Jeffery Iles

AMES, Iowa — Iowans continue to deal with and recover from the impact of the derecho that swept through the state a year ago this August. The massive loss of trees — over 80,000 in Cedar Rapids alone — coupled with the spread of the emerald ash borer across the state, prompted many citizens to plant and now care for freshly planted trees.

But new trees, with smaller, less-extensive root systems, will be challenged in the weeks to come, according to Jeff Iles, professor and chair in horticulture at Iowa State.

Temperatures are expected to ramp into the 90s, with little to no rain forecast for much of the state. Trees, especially those planted over the last several years, will need supplemental irrigation, and now’s the time to make a plan to help them through the summer.

But how much, when and where to deliver water remains a difficult question to answer. In fact, variables such as soil type, tree species, the presence or absence of mulch, and occurrence and timing of natural rainfall all influence watering practices.  With that said, these generalized recommendations will help young and not-quite established trees get through this hot, dry period:

  • Newly-planted trees and shrubs may require supplemental watering one to three times each week during the first few months after planting.
  • Daily irrigation may be required during exceptionally hot and dry weather, and especially for plants that were container-grown.
  • With moderating temperatures and some rainfall, and as plants become established, one supplemental watering each week may be sufficient.
  • Amount?  2-4 gallons of water/inch of trunk is a good estimate. Another useful recommendation is one gallon/sq. ft. of soil surface within and slightly beyond the tree canopy.
  • In subsequent years (and depending upon local weather conditions) frequency of supplemental irrigation can be scaled back.
  • Initially, water should be applied directly over the existing root system. As roots grow and expand their network, the area to be watered must be enlarged as well. After one year of growth in the landscape, roots can easily grow as wide as the developing branch spread/canopy.
  • Applying a layer of organic mulch (2-3 inches deep) over the developing root system will help conserve soil moisture, reduce competition from other vegetation, and keep mowers and other lawn maintenance equipment away from sensitive bark tissue.

"You did a great thing by planting a tree or several trees. But your work isn’t done. Protect your investment with important and much needed supplemental irrigation. Your trees will thank you as will countless, nameless visitors who will visit and enjoy your trees, for years and generations to come," Iles said.

For more information about caring for plants during dry weather, visit the ISU Extension and Outreach Horticulture and Home Pest News site.


Shareable photo: Young trees in dry conditions.

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