> Next: Installing Automatic Irrigation
Watering Landscape Plantings of Trees and Shrubs
Knowing how often and how much to water trees or shrubs is vital to their health and beauty. We’ve got the answers to all of your irrigation questions right here.
Water Needs Vary Over Time
For the first year or two after being planted, trees or shrubs benefit from frequent watering. A steady supply of moisture helps them establish strong root systems, with healthy roots reaching out and down in the soil.
It’s important to distinguish deep-rooted trees with tap roots that grow to great depths into the soil from those that are surface-rooted species.
Each type requires its own watering technique. Failure to provide the right method of irrigation may result in the tree dying, roots rotting, or failure and toppling during windstorms. Learn more about this important difference next and special irrigation needs by reading on.
Later, after becoming established, they’ll go for increasingly longer periods without supplemental watering. Each tree or shrub’s own roots have grown so they gather moisture from their surrounding soil, not just near their trunk.
Deep-Rooted and Shallow Rooted Trees
Some trees and a few shrub species have tap roots and few surface roots. They evolved to seek deep groundwater, making them immune to most droughts. Conifers such as pine are among these species, and roots of mature trees may tap underground water aquifers 250 ft. (76 m) or deeper.
These trees perform best when water is applied 2–3 ft. (60–90 cm) out from their trunks while young. Avoid wetting the soil immediately around the tree, water deeply and infrequently, every 7–10 days during dry periods. This watering method saturates the soil, and water penetrates in a plume beneath the tree, while the surface soil around the trunk remains damp but not wet, preventing root and trunk rot. Over time with consistent watering, sub-surface soil in the plume will be saturated in many feet deep.
The second category of tree species requiring special watering methods are surface-rooted varieties, mostly broadleaf deciduous trees. These trees may extend roots at or just below the soil surface many feet away from the trunk in search of water.
Satisfy their water-seeking roots by watering them in a donut-shaped pattern, with dry soil in the middle extending in a circle almost to the width of their foliage and a circular wet zone 2–3 ft. (60–90 cm) wide all the way around the tree, with dry soil beyond.
All the fine, water-gathering hair roots of surface-rooted trees are located in the middle circle, at or within 12 in. (30 cm) of the soil surface. By keeping the soil dry beyond the irrigation area, they won’t continue to grow and search for water, limiting their impact on structure foundations, paving, or other plants in your landscape.
Watered consistently every 5–8 days in this pattern, they too will have a shallow plume of water from which to draw that will sustain them during dry spells or even extended droughts.
The main roots, woody and corky tissue, will remain dry, conveying the water from the hair roots to the tree’s main trunk, limbs, and foliage. This irrigation method prevents the most common reason for failure of shallow-rooted trees: root rot.
Providing Supplemental Watering
By 2–3 years after they were planted, most trees and shrubs can withstand occasional dry periods without watering without damage. By 5 years, only long droughts will make supplemental watering of trees necessary, but shrubs will always require regular irrigation.
Amount to Water
The amount of water that newly planted trees or shrubs will need depends on the species and the type of soil in which each is growing.
Soils have varying water-holding properties. Clay soils typically require less irrigation than sandy soils. The structure—texture or “graininess”— of soils also affects water and air holding capacity.
Finely grained soils often store water better than sandy and rocky soils, while those with lots of decayed organic material require less water.
As a rule of thumb, give trees and shrubs about 1 in. (25 mm) of water per week during their first growing season. Picture the areas beneath the tree or shrub with a puddle 1-in. (25-mm) deep standing on it; that’s how much water to apply to the surface each week.
Depending on your yard’s slope and soil, it may not be possible to apply all of that water at once. Two, three, or four waterings may be needed for all the water to be absorbed into the soil without running off.
After settling in, most trees and shrubs do best with infrequent, deep waterings that soak the soil until it is wet down at least 1 foot (30 cm) deep.
Most of the smallest roots that absorb water are on the tree or shrub’s drip line, an imaginary circular area straight down from the tips of its outermost branches. It’s where rain would drip from the tree after a storm.
Avoid frequent, shallow waterings, since such irrigation encourages root growth near the surface. Shallow waterings make trees and shrubs shallow-rooted and more vulnerable during periods of drought. In some cases, surface roots can invade nearby patios or walkways, causing them to crack.
When to Water
Water at the correct time of day to help keep landscapes healthy. Wet foliage often is an invitation to disease.
The guiding principle is to limit the amount of time tree foliage and shrub leaves stay wet. Watering in the morning before 10 a.m. is ideal, so foliage can dry before temperatures cool in the evening.
The first few times out, you can easily determine when you need to water.
With a shovel or soil-sampling tube, dig down into the soil about 8 inches (20 cm). If the soil at that depth is dry, then it’s time to water. If you find the soil is moist near the surface but dry below, that’s a sign that you need to irrigate more deeply—longer—so that more water penetrates deep into the soil.
One way to ensure that water will make its way to a young tree’s roots is to install deep-watering tubes.
Get your newly planted trees and shrubs off to a good start by watering them regularly whenever natural precipitation falls short or drought conditions occur.