While the Southern live oak, Quercus virginiana, has been widely planted throughout the warm areas of Arizona, I suspect that few people really know much about the tree.
The good news is that the live oak is well adapted to our desert conditions. Let’s get that settled early. I like this tree for our desert landscapes because it is so hardy, makes a lot of shade and is stingy in its use of water. That is not to say that it will live on rainfall alone. That would be asking too much, but we can classify this tree as a moderate to low water use tree and that makes it a good choice for our yards.
Many people seem to agree. This oak is now quite common around residences and in public areas throughout the desert areas of Arizona. I frequently see it in many of our newer subdivisions. I think one of the reasons that it has become so widely accepted is because of the landscape benefits that it brings to the plant palette. Let’s take a look at some of those benefits.
Because of its dense canopy and single trunk, it makes an excellent shade tree. It also likes our heat and dry climate. Some trees have been in place for many years and display a proven record of success in our environment. It has been reported that some of these trees are growing near now abandoned homes and continue to do quite well on their own. They live with little or no human help.
With all of its benefits, I need to be up front about what some might consider a major flaw. The mature tree puts out a lot of acorns. To me this is not a serious problem. If you do not want them underfoot, just sweep them up as they fall and you are done for the year. However, some may find the acorns a problem. If so, this tree may not be for you.
The live oak is an evergreen tree, meaning that it will carry leaves year round. However, once a year, usually in the spring, it will drop many of its older leaves and replace them with new ones. In this way it keeps renewing its leaf canopy while retaining its ability to conduct photosynthesis at a high level of efficiency throughout the year.
In its native habitat, along the rivers and streams of the South, it can grow up to 60 feet tall and 80 feet wide. Here in the desert, it rarely gets that big. You should, however, plan for it to reach say, 40 feet tall and maybe 40 feet wide. A slow grower, it can become a huge tree, so plan accordingly. Give it plenty of room. The more water it receives, the faster it grows.
The live oak prefers a deep, uniform soil, but will do okay in the presence of calcium carbonate or caliche. It adapts well to rocky, salty or poor draining soil conditions. For best results, though, it should have soil deep enough that the roots can extend down to a depth of at least 3 feet. A moderate soil texture is also helpful because heavier soils can store more water than sandy soils. This means that soils containing a mixture of sand, silt and clay will need to be irrigated less frequently than sandy soils. One of the nice things about this tree is that it can get by with moderate to light water applications. It knows how to be stingy with water, but you can’t just leave it on its own. You have to irrigate at the right time and in the right way.
During an irrigation, the entire root zone must be soaked thoroughly. That means that the whole area underneath the tree canopy, especially out to and slightly beyond the drip line of the tree should be watered slowly enough and long enough that the water will sink in at least 3 feet into the ground. Roots tend to grow into moist soil so correct irrigation is essential to getting the roots down deep. The larger the root system, the more water will be available in the supporting soil profile; and the more water available to the plant, the healthier it will be. If we irrigate correctly, we can get by with an irrigation every couple of weeks to every month or two, depending upon soil and climate conditions.
Can I effectively water an oak tree with a drip system? Yes, you can, if you have enough emitters scattered around underneath the canopy of the tree to wet the entire root zone. One, two or even three emitters are probably not going to get the job done. You will need to add more emitters as the tree grows. Likewise, a 10-minute irrigation is probably not going to get the water down deep enough to fill the entire root zone. It is a good idea to probe the soil with a long barrel screwdriver or soil probe to determine if your set time is correct.
One critical maintenance task that is almost always ignored is pruning, especially during the early formative years of the tree. Oak trees tend to grow branches in all different directions, and some of those directions may not be healthy for the tree. Crossovers, competing branches, water sprouts and malformed branches are all common in the live oak. Early in its life, a young tree should be subjected to a careful training regimen to ensure good branch structure. You may find it necessary from time to time to clean up new problem branches until the tree reaches maturity. Good shape and structure will help extend the life of the tree. Left without this important care the canopy of the tree can become one big, jumbled mess.
Crossovers are branches that cross each others path to the point that they begin to rub on each other as the tree sways in the wind. Rubbing causes wounds in the bark that will allow access to diseases and insect pests. This is not a good thing. To solve this type of problem, it is important to remove the weakest of the two branches and leave, hopefully, a branch that is growing in the correct direction, towards the outside of the canopy.
Sometimes branches arise from point that are too close together. With too little space between the branches they begin to compete with each other for nutrients, sunlight and space to grow. Usually the strongest will live and the weaker will either die or become stunted. It is important to keep about 60 degrees of separation between branches so that they do not interfere with each other. Sixty degrees is about the same space as the distance between twelve o’clock and two o’clock on a clock. Branches growing from the trunk or major limbs with less than this space should be thinned out.
Water sprouts are branches that arise from a major scaffold branch and grow straight up through the canopy. These are bad because they tend to be highly vigorous in their growth. Their vigor tends to draw water and nutrients, often to the detriment of its neighbors, into wood that is weak or in the wrong place. These should be pruned out as soon as possible to force growth out and away from the center of the tree. This will foster better tree strength and branch orientation.
Branches can grow in all kinds of crazy directions. Some grow out like they are supposed to, but some will grow crooked in directions that we simply cannot tolerate. I like to thin these out if I can do it without harming the tree. With that said, you need to know that we should never cut out more than 10 percent of the tree’s total wood at any one time. Heavy pruning, particularly on young trees, can be harmful because it removes key energy reserves stored in the wood and reduces the leaf load of the tree. If we remove too many leaves, the tree cannot feed itself. Balance is what we are trying to achieve.
If you are looking for a good shade tree, consider planting the Southern live oak. It gives good shade and endures well our harsh environmental conditions. With just a little bit of care, this tree will give years of beauty and shade.
If you have questions, you can reach one of the master gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is 520-836-5221, ext. 204.
Rick Gibson is an agricultural extension agent and the director of the Cooperative Extension in Pinal County. His email address is email@example.com.