What is the best technique for watering new trees and shrubs?
Watering is a crucial issue for all new trees or shrubs.
It's important to realize that most trees and shrubs can't be considered established until they have been in the ground for two growing seasons. Until they do become established, watering is a tricky problem.
Trees are planted with their roots in a burlapped "root ball" of soil. To become established, the tree's roots must eventually grow out of the root ball and into the surrounding soil. The soil of the root ball is almost always different from the soil at the planting site. Roots have difficulty passing through this interface, so the process takes time. And since water will generally not move from the surrounding backfill into the soil ball, water in the backfill will not reach the tree. So it's essential to frequently supply the soil ball itself with small amounts of water.
As an example, let's consider the needs of a new, 2-inch-caliper, red maple tree. With a soil ball about 20 inches across and 16 inches deep, the total volume of the soil ball will be equivalent to about 20 gallons of water. If the soil is clay loam, it can hold water up to about 25 percent of the soil volume--some 5 gallons. On a hot, windy July day, the leaves of this transplanted tree can transpire 2-3 gallons of water. As a result, the new tree may suffer severe drought stress in only a few days during hot weather. So the moisture in the soil ball must be consistently replenished.
There are several approaches to watering new trees. You can water by hand with a hose. Or you can allow the hose to trickle at the base of the tree for some time. However, with either of these methods you have no idea how much water has been applied, and there is no guarantee that the water actually went into the root ball--it could have run off into the surrounding soil.
The best approach is some form of drip irrigation atop the soil ball. For the red maple in our example, 2-3 gallons of water should be added to the soil ball every 4-5 days. The simplest way to do this is to punch a few nail holes low on the sides of a 5-gallon bucket, set the bucket next to the trunk, and fill it with water every few days.
Alternatively, 2 or 3 one-gallon-per-hour drip emitters can be fitted into a short length of plastic tubing that can be attached to the end of a hose with a hose fitting. This construction can then be attached to a timer programmed to turn the water off after 1-2 hours of operation. You can also buy drip irrigation kits that can be adapted for watering your new tree or shrub. These kits usually include a hose fitting and a simple pressure regulator that makes the system function properly.
Always use a timer with your drip irrigation system. It's not possible to determine the moisture of the soil ball from the surface wetness. If the soil ball becomes too wet, hot weather will severely stress the tree.
One way to gauge whether the tree is getting enough water is to push a metal rod into the soil ball. If you are unable to easily push the rod more than a couple of inches, the ball is too dry. If, on the other hand, the rod easily slides all the way through the ball, you are overwatering. Drilling an 18-inch hole next to the soil ball will allow you to check for standing water and pump it out with a small, drill-powered siphon pump.
With a little care, you can gradually wean your tree or shrub from its dependence on the soil ball. Once some roots have extended into the surrounding soil, the plant will gradually recover from the shock of having 95 percent of its roots cut off when it was taken from its previous location.
For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service