Dallas Urban Forest Advisory Committee
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Caring for My Tree

Water
Providing supplemental water is very important in order to get new trees established. Trees need a half-inch to an inch of water every week, with the greater amount being needed in the hot summer months. When natural rainfall drops below that amount, start giving the tree some water. A good rule of thumb to follow is 5 gallons of water per week for every inch of stem caliper (trunk diameter measured 6 inches from the ground on young trees). Don’t just rely on lawn sprinkling to provide adequate water to your young trees - it usually does not water deeply enough.

In Dallas, heavy clay soil is very common. It is possible to over-water trees and other plants growing in clay soil. Before giving water, dig down into the soil three or four inches to see if it is dry.

Water quality is another concern. Water that is high in sodium can burn the foliage, and minerals can build up in the soil if there has been little rain. Water your trees deeply, but less frequently than you would water turf. Avoid spraying water on the leaves if your water contains excessive amounts of sodium.

Mulch
Keeping a layer of organic mulch over the root zone of your tree is at least as important as providing water. Mulch will prevent competition between your tree’s roots and the roots of grass or weeds. Mulch prevents summer soil temperatures from becoming lethal to tree roots. Nutrients leach from the mulch into the soil.

Experts recommend a layer of organic mulch three to four inches thick over the entire root zone of the tree. Mulch should be pulled away from the trunk so that it does not cause the tree’s bark to decay. Wood chips, composted yard waste, or shredded bark all work very well.

Weed Control
Lawn mowers and string trimmers are the leading cause of death for young trees. Lawn equipment will cut through soft bark in an instant, destroying the tree supply lines. Trunk wounds are easily invaded by decay, and the tree quickly loses its ability to feed itself. Minor wounds may eventually close, but repeated damage will surely kill the tree.

Mulch will inhibit some weed growth, but weeds and grass will have to be controlled to keep mowers and trimmers away. Contact herbicides (like Roundup or Kleen-up) work well, but one must be very careful to avoid spraying the tree’s leaves or soft bark tissue. These herbicides have no soil activity, so they will not negatively affect tree roots. Other options for controlling weeds include pulling them by hand and/or using landscape fabric under the mulch

 

Fertilization
There is conflicting information on the benefits of fertilizing shade trees, and on the type of fertilizer that should be used. The only way to know for sure what is needed in your soil is to take a soil sample to a lab for testing. Texas A&M University has a soil lab that will perform this service. Your county extension office can provide more information.

The most current research on fertilizing shade trees indicates that nitrogen is most often the limiting factor. If you choose to fertilize without a soil test, look for a slow-release fertilizer that is high in nitrogen. The fertilizer can be broadcast on the ground surface in late spring or early summer and watered-in. If you have adequately mulched the tree, you probably do not need to add fertilizer..

Be aware that mature post oaks do not seem to tolerate intensively maintained lawn situations. They are sensitive to construction, soil compaction, drainage changes, excessive water, turf competition, etc. These stress factors and others cause the tree to decline and set it up for disease. Do not provide extra water and nutrients to mature post oaks. The more you leave them alone, the better they like it! (Texas Forest Service).

Some general rules from research suggest the following:

  1. Injection of fertilizer into the stem of the tree is no more effective than other methods and in some cases is detrimental to tree health because of wounding.

  2. Deep root treatments are much less effective than surface or shallow soil injection treatments because over 90% of the roots capable of absorbing nutrients are located in the top 18 inches of soil. Deeper roots are structural support.

  3. Most trees do not need fertilizer, especially native species. Fertilizer applications may cause some trees to be more prone to insect and disease problems. Mulch can eliminate the need for fertilizer and provide many other benefits.

  4. More expensive treatments and application of “complete” fertilizers often do not help the tree any more than an inexpensive application of only the needed elements.

  5. Fertilizer is not medicine, it may cause additional stress to trees that already have been impacted.

  6. Applications of mychorrizae, aeration, mulch, compost, and organic matter may increase nutrient uptake and improve root health. These applications may be preferred to applying fertilizer or can be used to reduce the amount of fertilizer needed.

  7. Fertilizer causes accelerated growth in most cases. Make sure your tree does not outgrow its space. Tree roots can expand 2-3 times the width of the canopy.

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