STILLWATER, Okla. – With two summers in a row of brutally hot temperatures and extreme drought conditions, Oklahoma gardeners have been struggling to keep their shrubs and trees alive.

Gardeners have done their best to keep plants alive during this extreme weather, said David Hillock, Oklahoma State University Cooperative Extension consumer horticulture specialist.

“We’ll still have some hot days during the next month or so, but it’s important to remember proper care is just as important in the fall and winter to ensure healthy trees and shrubs,” Hillock said. “A vital landscaping represents a long term investment and significant value to the homeowner.”

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, therefore, the potential of damage to plants’ roots during the winter increases if the soil is dry and cold. To avoid further damage to already stressed plants, it is important to send landscape plants into the fall and winter with good soil moisture.

Hillock said understanding the root system of trees is important for homeowners in their effort to properly care for them.

“Trees basically have two types of roots – large perennial roots and smaller, short-lived absorbing roots,” he said. “The larger roots have several purposes, including providing stability for the tree, water and mineral conduction and food and water storage. Perennial roots are woody, increase in size and grow horizontally. Many people may not realize that 90 percent of a tree’s roots are located in the top 12 inches of soil.”

The smaller, absorbing roots are about 1/16 of an inch in diameter, make up the major portion of the roots surface area and are responsible for the absorption of water and minerals. Hillock said these roots grow outwards 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 you determine the best method to water your trees. Because you know 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,” he said. “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. It’s wasteful to distribute water faster than it can be absorbed. Watering at ground level to avoid throwing water in the air is more efficient.”

Another way to significantly increase a tree’s chance of survival is to mulch around the tree with 2 inches to 4 inches of organic mulch in an effort to reduce moisture loss. In addition to deep watering, do not fertilize a tree that is under drought stress. The salts in the fertilizer may burn roots when there is not sufficient water. If fertilizer is needed and soil moisture is adequate, avoid high nitrogen fertilizers late in the season. Hillock said fertilizers should not be applied much after early September to avoid a flush of leaf growth that may not acclimate before freezing weather.

Take advantage of the time of drought to properly prune trees and shrubs to improve structure, limb stability and remove dead and weakened branches.

“Not pruning trees can further weaken it during drought and set up the tree for deadly secondary insect and disease problems,” he said. “Following these guidelines will help preserve your trees, the most valuable assets to your landscape.


Oklahoma State University, U.S. Department of Agriculture, State and Local Governments Cooperating: The Oklahoma Cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, gender, age, disability, or status as a veteran, and is an equal opportunity employer.

Trisha Gedon
Communications Specialist
Agricultural Communications Services
136 Agriculture North
Oklahoma State University
Stillwater, OK  74078
405-744-3625 (phone)
405-744-5739 (fax
[email protected]

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