How to Water Trees Through a Dry Montana Summer and Fall

Why do trees need different watering from a lawn?

Because the target is deeper and wider. Missoula Urban Forestry states that trees must be watered deeply to saturate the roots fully, and that a surface application only waters the competing grass and does not benefit the tree. MSU Extension gives the same instruction in fewer words: water slowly and deeply. Its leaf scorch guidance adds that watering more deeply and less often encourages deep root development.

Depth here does not mean several feet. MSU's tree problem MontGuide notes that most mature trees and shrubs concentrate their feeder roots in the upper 10 to 12 inches of soil, and that is the layer to keep moist. A lawn cycle wets only the surface of it. Lawn scheduling is its own subject, covered in our summer watering guide for Montana lawns.

How much water does an established tree need?

Missoula Urban Forestry gives a figure for this climate: mature trees need a minimum of 2 inches of water per week during the growing season, which it defines as May to October. If rain does not supply it, you make up the difference. The city suggests measuring with empty cans set out under the sprinkler. When the average depth in the cans is two inches, the tree has been adequately watered.

To check the result, the city's test is a screwdriver. If it pushes easily into the soil around the tree to a depth of 6 to 8 inches, the tree is probably getting enough. For a tree recovering from damage in very hot, dry weather, MSU Extension Forestry suggests a different delivery: a slow trickle of water for eight hours at night, once every other week. Both methods deliver a lot of water slowly enough to soak in.

Where should the water go?

Out under the branches, not at the trunk. Missoula's guidance is to saturate the soil within the dripline, meaning the area beneath the outer edge of the branches. Colorado State University Extension notes that established large trees have a root spread equal to or greater than the height of the tree, and recommends applying water at many locations under the dripline and beyond where possible.

A soaker hose laid in a loop under the canopy does this well, and Missoula Urban Forestry calls soaker hoses an efficient way to supplement the shallower watering that sprinkler systems provide. If you are adding or adjusting zones, a dedicated tree line on a separate schedule is worth discussing when your irrigation system is serviced.

How should a newly planted tree be watered?

More often, and for longer than most people expect. MSU Extension states that newly planted trees are the most vulnerable to drought because of their underdeveloped root systems, and that trees planted within three years or less are more likely to sustain permanent injury. It calls regular watering for the first year crucial and says woody plants can take up to five years to establish, depending on the size planted.

For an amount, Colorado State University Extension offers a general survival rule of 10 gallons of water for each inch of trunk diameter per watering, measuring the trunk six inches above the ground, so a two-inch tree gets 20 gallons. That is Colorado's figure, offered as a starting point. In Missoula, the city fits new street trees with slow-release watering bags that hold 25 gallons, release it over several days, and need refilling about once a week. Our guide on how to plant a tree in Montana covers the planting side.

What are the signs of drought stress?

MSU Extension lists them: yellowing, leaf scorch, wilting, reduced growth, fewer and smaller leaves, and shorter growth between buds, most apparent in the parts of the crown in full sun. Leaf scorch shows as brown, dead tissue along the leaf margins moving inward, and MSU names aspen, maple, ash, oak, linden, birch and horse chestnut as most susceptible. Scorch is irreversible on the leaves that have it, though better watering may help the tree recover.

Later signs are early fall color and early leaf or needle drop. The hidden cost is the one that matters most. MSU notes that drought-stressed trees go into winter with lower energy reserves and have decreased resistance to pests. On pines in this region that often means bark beetles the next season. If a pine is already off color, our guide to pine needles turning brown helps sort drought from other causes.

Should you keep watering in fall?

Yes, and this is the step most often skipped once the lawn stops growing. MSU Extension's drought guidance is to water trees weekly in the fall until the ground freezes. Its MontGuide refines the timing: ease off in early fall so trees prepare for winter, then, once leaves have begun to turn color and drop, water deeply once each week until the soil freezes. It adds that extreme drought can interfere with a tree's ability to acclimate for winter.

Missoula Urban Forestry gives matching advice for city trees. Reduce the amount in fall but do not quit, water straight through leaf drop, and give one final heavy dose after the leaves are down. In practice that means trees still need a hose after the sprinkler system has been blown out for the season.

Do evergreens need water in winter?

They benefit from it when conditions allow. Evergreens keep their needles and keep losing moisture all winter, which is why MSU calls watering all trees and shrubs before the soil freezes the best line of defense against winter desiccation. The MontGuide adds that winter watering during extended thaws, when the soil is not frozen, is also good practice.

Colorado State University Extension supplies the practical limits: water only when air temperatures are above 40 degrees and there is no snow cover, do it at midday so it soaks in before a night freeze, and plan on about once or twice a month during extended dry periods. A western Montana winter with steady snow cover may offer few such days. An open, dry, windy winter will offer several, and young spruce, fir and pine are the trees to use them on.

How do you water trees without wasting water?

Mulch first. MSU Extension recommends three to four inches of mulch out to the edge of the canopy to keep the fine feeder roots from drying out. Missoula Urban Forestry recommends organic wood mulch 2 to 4 inches thick, spread as wide as the dripline and pulled back at least 4 to 6 inches from the trunk. MSU also points out that removing competing vegetation such as grass from beneath the canopy makes more water available to the tree.

Slow, deep, infrequent watering helps as well, since less is lost to runoff and evaporation than with daily sprinkling. And do not overcorrect. MSU's MontGuide warns that continuously wet soil from excess irrigation damages roots and leaves the tree shallow-rooted and more prone to windthrow. Check the soil before adding more.

Who waters the boulevard tree?

In Missoula, the City's Urban Forestry page says city ordinance requires property owners to care for adjacent street trees, and it asks landlords to remind tenants to water them. That is general information from the city's own page, and its Urban Forestry Division is the place to confirm what applies to your address. Our guide to Missoula tree permits and the right of way explains who can prune or remove those trees.

If a tree is thinning, scorched or dropping leaves early and you are not sure water is the whole story, an assessment will separate drought from root, pest and soil problems. That is part of our tree services, and the Montana Tree Guide has species-by-species detail.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a tree assessment and an ISA Certified Arborist will look at the tree with you.

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