The do’s and don’ts of irrigating landscape plants

A hot, dry spring can be tough on newly planted trees and shrubs, so irrigation is important.

Summer has come early to many parts of Michigan, bringing a lot of hot, dry weather. While most homeowners are diligent to water their flower beds and vegetable gardens, it is important to remember to give trees and shrubs a drink, too. Homeowners and landscapers should pay special attention to two groups of plants: trees and shrubs that were planted this spring or last fall and trees and shrubs that suffered late frost damage earlier this spring.

Irrigating newly planted trees and shrubs is important because these plants have not had a chance to become established and extent their roots into their surrounding soil. This is particularly true for container-grown plants, which are often watered daily at the nursery before being planted in the landscape. One of the common symptoms of damage by late frost this spring was leaf dieback or delayed bud break. These trees and shrubs are essentially “playing catch-up,” so it is critical to minimize any further stress to them as to try to rebound and rebuild leaf area.

Here are a few reminders in the form of “do’s and don’ts” to bear in mind as you irrigate your landscape. 

Do

Give good, long soakings rather than frequent, light watering. Some resources recommend more frequent watering, but once a week is a workable and realistic goal to shoot for. A typical rule of thumb is to provide at least 1 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 1 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 1 inch of water, assuming a typical garden hose flow rate of 5 to 6.5 gallons per minute.

Gallons of water needed to provide 1 inch of irrigation
under the drip line 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. 

Increase the irrigation amount as temperatures soar. The 1 inch per week is a good, rough guide, but peak evaporative demand can approach 2 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. Two to 3 inches of mulch is adequate. Research at MSU and elsewhere suggests that the type of organic material used to mulch (e.g., wood chips, pine bark, hardwood bark) is less critical than the fact that plants are mulched.

Don’t

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 usually the best time to water.

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

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