This summer is the second summer in a row, with a relatively dry winter in between, that our landscape plants have been subjected to extreme drought and high temperatures. Last year we saw many landscape plants struggling, and this year has been no different, with some trees and shrubs giving up completely.
Many of us have done our best to keep plants alive this summer, but we aren't done yet; the fall and winter are equally important to a plant's health, especially our trees, which represent a long-term investment and significant value to the overall landscape.
Tree roots continue to grow throughout the winter and need moisture to survive. Dry, cold soils can be damaging to a plant's roots. Moist soils hold more heat than dry soils; so the potential of damage to plants' roots during the winter increases if the soil is dry. To avoid further damage to already stressed plants, it is important to send our plants into the fall and winter with good soil moisture.
Before we talk about how to water trees, it is important to understand how tree roots grow. Basically trees produce two types of roots, large perennial roots and smaller, short-lived absorbing roots. The large perennial roots provide anchorage for the tree, water and mineral conduction, and food and water storage. Perennial roots are woody and increase in size and grow horizontally. In fact, 90 percent of the tree's roots are located in the top 12 inches of soil.
The smaller, absorbing roots are only about 1/16 inch in diameter, make up the major portion of the roots surface area and are responsible for the absorption of water and minerals. These roots grow outward and predominantly upward from the large perennial roots toward the surface where minerals, water, and oxygen are generally abundant.
Both the larger roots and small roots occupy a large area consisting of at least the area under and out to the drip line (the outer edges of the tree's branches) and often well beyond that, up to two to four times the height of the tree.
Knowing how tree roots grow helps us determine the best method to water our trees. First, because we know that most of the tree's roots are within the top 12 inches of soil, deep watering to a depth of 12 inches below the soil surface is recommended. Saturate the soil around the tree at and within the “drip line” to disperse water down toward the roots; watering only at the base of a large, mature tree is not adequate. The objective is to water slowly, dispersing the flow of water to get the water deep down to the trees roots. Watering for short periods of time only encourages shallow rooting which can lead to more drought damage. Using irrigation methods that distribute water faster than it can be absorbed by the soil is only wasteful. Watering at ground level to avoid throwing water in the air is more efficient.