In the Urban Forestry Department, we often talk about our Cross Timbers forest type and the vast tree cover that we enjoy here in Edmond. However, it’s also very important to discuss that we deal with greater environmental challenges in caring for our trees than our friends living in the eastern part of the state. Edmond is actually located on the very western edge of the Cross Timbers, and much of our community is naturally made up of a prairie ecosystem. Precipitation, temperatures and wind movement are climatic factors that play a large role in the amount of soil moisture available for tree roots, which as a result determines the height, spacing and species of trees that grow in an area. We see the transition of established Cross Timbers forest into prairie in our area because those climatic factors together do not provide an environment that is as conducive for tree growth as areas even as nearby as Tulsa. This does not mean that we cannot grow trees in Edmond – of course we can! We can grow trees very well, actually. With the recent extreme drought and heat, however, it just means we have to take special care in choosing which species we plant and grow and also take some extra measures to maintain the health of our trees.

Tree Selection

Trees that are native to the Cross Timbers tend to be very drought tolerant and can also endure very high and low temperatures. Many ornamental trees in the landscape today require a lot of moisture and cannot endure dry soil or prolonged direct exposure to Oklahoma’s hot summer sun very well. A lot of people have irrigation systems for this reason, but too much irrigation can stress a tree AND sometimes for those water loving, heat intolerant trees the extremely hot afternoons in August can be too much for them, irrigation or not. Selecting drought and heat tolerant trees that are hardy to zone 7 or higher can help to avoid some of these tree mortality issues that have been encountered. In the very least, strategically place your tree in your yard to give it the best chance for survival. Does it need some shade? Is your planting spot fully exposed in the heat of the day? These are things to consider.

Mulch

Trees growing in a forest will lose their leaves to the ground, where herbaceous plants grow and die and dead trees collapse and decay. All of this dead plant material accumulates and forms what is known as the “litter layer”, which helps the soil to retain moisture, regulates soil temperature, and breaks down to add minerals to the soil for uptake by tree roots.
In the urban environment, we have leaf blowers and rakes. It’s rare to see a litter layer, because we have turf grass on our lawns, so trees lack this protective layer over the soil that keeps it from drying out. This is why we have mulch! A 3-4” layer of mulch across a tree’s root zone (from one edge of the drip line to the other) can significantly increase the amount of absorptive roots that a tree has to take up water and minerals. It also keeps soil moisture from evaporating as quickly as soil without mulch, meaning the tree is less likely to incur drought stress without supplemental water. Be sure when spreading your mulch ring to leave a few inches around the trunk open, since mulch piled against the trunk can lead to disease and pest problems.

Watering

Mature, drought tolerant trees should be able to fend for themselves as far as watering is concerned, but with the past few years of extreme drought even the established trees could use a helping hand. If you have lost a mature silver maple in your yard or even a native oak, there’s a good chance that the drought played a hand in that even if another disease finalized its demise. In addition to your layer of mulch, providing a little bit of supplemental water for your mature trees when we haven’t had rain in a while can help to prevent them from entering decline. In the heat of the summer if it looks like your tree is distressed, apply water just outside the edge of the drip line around the perimeter of the tree. Trees typically need about 5-10 gallons of water per inch of trunk diameter, applied at a slow rate to promote maximum infiltration and little runoff. Any newly planted trees will definitely need regular watering for the first few years after planting, until they become established. “Establishment” for a tree means that it has grown a root system that is extensive enough to access adequate resources on its own, without needing additional irrigation. Before a tree becomes sufficiently established, it is especially susceptible to drought stress. Initially, you should water the root ball and the area at the edge of the planting hole to keep existing roots alive and encourage outward growth. As time goes on, continue watering at or just outside of the tree’s drip line, around the perimeter of the tree. Applying enough water to soak the soil 4 or 5 inches below the surface about once a week (or once every 5 days once we reach the upper 90’s) will provide enough moisture while also encouraging deeper absorptive roots. Shallow, frequent watering encourages roots to grow near the soil surface, which makes them more susceptible to drying in times of drought and extreme heat.

Fertilization

Sometimes when reading about tree stress, fertilization is listed as a preventative and remedial treatment. However, this is only a beneficial recommendation if soil has been tested and shown to be deficient in certain minerals. When nitrogen is added to soil that is not mineral deficient, this triggers growth of shoots and foliage. Growth may seem like a good thing because more food-making foliage is formed, but the tree also has to sustain this new growth with its limited resources. In addition, more energy toward shoot growth means less energy going toward defense and root growth. This could actually result in greater stress and increased susceptibility to pests and disease.

When it comes to fertilization, it’s important to verify that it’s necessary before going forward with treatment. Aside from the potential tree stress issues, excess minerals pollute our waterways when they run off of a lawn. Always perform a soil and/or foliar test and base your amendments on those results. Oklahoma County Extension (405-713-1125) can provide assistance with testing.

Take a look at more topics from the June 2013 Tree Mail message.

Check out last quarter's tree issue topic, "Did Borers Actually Kill Your Tree?"