Different tree species have different water-related tolerances. Some trees will die easily from too much water in their rootzones, while others will die quickly if their rootzones are too dry. The first step in proper plant irrigation is knowing your individual tree.
The following is a guideline for providing adequate water to most trees, divided in categories of newly planted and established.
Newly Planted Trees
- Generally, new plantings need between 15-25 gallons of water per week.
- If site drainage is poor, even proper watering amounts may kill the rootzone.
- Ensure trees are not buried too deep. Root collars should be at or slightly above ground level. A pedestal of undisturbed soil in the middle of the planting hole will ensure that the tree does not settle in too deep after planting. In heavy clay soil, planting holes should also have radial trenches to ensure that drainage is adequate. For backfill, use as much of the native soil as possible. If using amended planting soil, mix at least 50% with native soil- this is especially important when dealing with clay. If new trees are planted too deep in heavy clay conditions, the water will not drain out of the planting hole for long periods of time, and roots will be asphyxiated.
- Proper mulch application (organic wood chips, 3″ thickness across the CRZ, not mounded against or even touching the trunk) will significantly help retain soil moisture.
Established Trees
As a rule of thumb, a mature tree can use 10 gallons of water per inch of tree dbh per week. For example, a 20 inch tree would benefit from 200 gallons of water a week. During drought conditions, this is what you should aim for. Using a soaker hose spread out across the CRZ is the best way to achieve this, as water will seep into the ground slowly instead of just running off the surface. Having a layer of mulch or other organic material (like dead leaves, pine straw, etc) is extremely helpful in preventing water runoff, as well as retaining soil moisture for much longer terms. As a rough guideline for soaker hoses, a typical 50 foot hose with average water pressure will give between 25-50 gallons of water every 1 hr. A 100′ hose should double this amount. Using the example of the 20″ tree again, a 100′ soaker hose left on for 2-4 hrs would provide optimal irrigation, as long as the water is not running off.
Signs of Over- and Under-Watering
Knowing how much water to apply is only half the equation—it also helps to recognize when a tree is getting too much or too little. The two problems can look surprisingly similar, since both stress the rootzone, so it is worth checking the soil rather than guessing from the leaves alone.
Common signs a tree may be over-watered include:
- Soil that stays soggy or waterlogged for long periods between watering
- Leaves that turn yellow, especially across the whole canopy rather than at the edges
- New growth that appears wilted even though the soil is wet
- A general musty or sour smell around the rootzone, which can point to poor drainage
Common signs a tree may be under-watered include:
- Soil that is dry several inches below the surface
- Leaves that wilt, curl, scorch at the edges, or drop early
- Slowed or stunted new growth
- Premature fall coloring during the heat of summer
When in doubt, check the soil moisture a few inches down before adding more water, and remember that drainage and planting depth (covered above) play just as large a role as the amount of water applied. If a tree continues to decline despite correct watering, it is worth consulting a certified arborist to rule out other rootzone problems.
