You planted it - now don't just forget about it. Taking care of a young tree is very important if you want it to fill a prominent position in your landscape. Selecting the right tree for the site, choosing a healthy tree at the nursery, and planting it correctly are very important, but your job doesn't stop there.
Providing supplemental water is very important to get new trees established almost anywhere in Texas.
During the hot summer months, trees will need an inch of water every week. 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 central Texas, 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 turf. Avoid spraying water on the leaves if your water contains excessive amounts of sodium.
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, feeding the tree and keeping the soil mellow.
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.
Lawn mowers and string trimmers arc 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 be very careful to avoid spraying the tree's leaves or soft bark tissue. These herbicides have no soil activity, so they will not contact tree roots. Other options for controlling weeds include pulling them by hand and/or using landscape fabric under the mulch.
Judicious pruning is very important to help the tree develop a strong branch structure. Without proper pruning, many trees will develop flaws that make them susceptible to wind or ice damage. If a tree is given adequate care in the first five to ten years, it should become a valued asset in the landscape.
Pruning cuts should be carefully planned to guide the tree's development by removing the least desirable branches. Prune a little every year - never more than one third of the crown at one time. Never "top" a tree because it ruins the health and structure of the tree.
Remove double leaders and branches that cross or rub against another branch. Also remove branches that have a very narrow angle at the point where they attach to the trunk or larger branch. Remove the lower branches, one or two each year, until the crown has been raised to the height you want. Your goal is to create a structure with a dominant trunk and well-spaced lateral branches.
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 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!