Water Your Trees Before the Ground Freezes

Most people stop watering when the lawn goes dormant. Trees don't stop. Roots keep growing in fall as long as the soil isn't frozen, and evergreens keep losing water through their needles all winter long. A tree that goes into winter with dry roots is set up for damage you won't see until spring: brown needles, dieback, and a slow decline that looks like disease.

Why fall watering matters

• Evergreens lose water all winter. Wind and winter sun pull moisture out of needles and broadleaf evergreen leaves. When the ground is frozen, the roots can't replace it. That's winter burn (also called winter desiccation): the brown, scorched look on the south and west sides of arborvitae, yews and boxwoods in March.
• Young trees have small root systems. Anything planted in the last two or three years depends on you.
• Dry falls happen. Plenty of northern falls are drier than average, and trees go into winter short.

How much and how deep

The goal is moist soil 12 inches deep in the root zone, which extends well beyond the trunk, roughly out to the drip line (the edge of the branches) and beyond.

• A common rule of thumb is about 10 gallons per inch of trunk diameter per watering, measured about knee height on the trunk.
• Water slowly so it soaks in instead of running off: a soaker hose spiraled around the drip line, a garden hose on a slow trickle moved around every 15–20 minutes, or a watering bag on young trees.
• Water when it's dry. If you haven't had about an inch of rain in a couple of weeks, water once every one to two weeks.

Check by pushing a long screwdriver into the soil. It should slide in easily to several inches if the soil is moist.

Which trees need it most

1. Evergreens: arborvitae, yews, boxwood, rhododendron, white pine, spruce.
2. Trees and shrubs planted in the last two to three years.
3. Trees near pavement, walls or driveways where the soil dries fast and reflects heat.
4. Trees on the south and west sides of the house, exposed to winter sun and wind.

When to stop

Keep watering through dry spells until the ground freezes. On a mild winter day when the soil thaws and it's above about 40 degrees, a midday watering can help evergreens in a dry winter. Just don't leave hoses connected outside overnight, and drain them afterward.

Mulch helps

Two to three inches of shredded wood mulch over the root zone holds moisture and evens out soil temperature. Keep it pulled back a few inches from the trunk; mulch piled against the bark (a "mulch volcano") invites rot and rodents.

A good soak in October is cheap insurance for a tree that took years to grow. It's one of the easiest jobs on the fall list, right after the last mow of the season.

Related: Planting Trees in Fall in the Northern US