Irrigation do’s and don’ts

Editor’s note: This article was originally published in the September 9, 2005 issue of the Landscape Alert.  Check the label of any pesticide referenced to ensure your use is included.

While many homeowners are diligent to keep their lawns watered and green, trees and shrubs often get ignored. Reducing stress to trees and shrubs this time of year is critical. As we go into the fall, trees are shifting internal resources and undergoing physiological changes that will enable them to withstand the rigors of the winter to come. If plants are subjected to severe stresses now, they will be more predisposed to various winter injuries. With this in mind, irrigating trees and shrubs should be on your list of landscape chores. Here are a few “Do’s and Don’ts” to bear in mind as you irrigate your landscape.


Give good, long soakings rather than frequent light waterings. A typical rule of thumb is to provide at least one inch of irrigation per week. How many gallons of water this translates into depends on the size of the tree. If we measure the width of crown spread of a tree we can calculate the area under the drip line. We can then figure the volume of water needed to cover this area with one inch of water. I’ve done this in the table below and converted the volume to gallons. I’ve also calculated the length of time it would take to apply one inch of water assuming a typical garden hose flow rate of 5 to 6.5 gallons per minute.

Increase the irrigation amount as temperature soar
. The one inch per week is a good rough guide but peak evaporative demand can approach two inches per week in Michigan during extremely hot summer weather.

Apply mulch properly.
Mulching is the best way to conserve precious soil moisture in the landscape.

Use irrigation bags on newly established trees.
Gator bags are designed to provide about 15 gallons of water over several hours, providing an easy way to ensure a slow steady watering.

Gallons of water needed to provide 1 inch of irrigation under the dripline of trees of various sizes.

Tree crown spread (ft) Gallons Minutes of watering*
6 20 3 - 4
8 30 5 - 6
10 50 8 - 10
12 70 11 - 14
16 125 20 - 25
20 200 30 - 40

*Assuming 5 to 6.5 gallons per minute from typical garden hose.


Allow water to run-off. Water that runs off is wasted water. If you’re watering by hand and notice water running off, move from tree to tree to allow water to soak in before resuming watering.

Ignore signs of drought stress in landscape plants. Wilting leaves, leaf scorch, dropping leaves and drooping leaders in conifers are your tree’s way of saying, “What’s a guy gotta do to get a drink around here?!”

Water during hot mid-day periods to reduce water loss to evaporation. Some experts argue against watering late in the evening due to possible disease problems associated with wet foliage. Morning is the best time to water – unless you have to be at work.

Dr. Cregg’s work is funded in part by MSU‘s AgBioResearch.

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