Saturday, August 6, 2011

Drought tolerance of trees in Southeast Texas

In 2011, we are experiencing the worse drought ever recorded in Texas. In The Woodlands, our forest is our most important asset that cannot be immediately replaced. We need to protect this asset.

Trees often reach deep into the soil to find moisture but some have shallower roots than others. The questions  "Are my trees dying?" and "How can I prevent my trees form dying?" have been raised by many of my readers and friends. Although I have written one other article on this subject, this year is different and we need to address the issue as it is. Here are the answers and some references to put you in the know. I have asked the same questions, so I share my findings and observations. Personally, I have trouble with pines, Magnolias and some hardwoods. I will list trees by name below and give you drought and water tolerance information. A very important thing to remember in the summer - there are trees which hibernate as a defensive mechanism. Unless you are sure the tree is dead, do not remove it until next Spring when it does not bud and produce leaves. If you know the name of the tree, you can use this as a  reference (and other internet sites) to see if it is hibernating, and then rid yourself of it if it is dead. Or you can consult with an arborist.

A tree will have a water content of 200% normally. In a bad drought, it will fall to 100% with the tree living. A large tree needs a lot of water to reach normalcy after it has declined to the severe drought level. At an extreme level, many will die for thirst. You can't allow a tree to reach that stage, because it likely will not recuperate.4

First, do water your trees!! Use a drip irrigation technique or an injection technique. I use a drip irrigation hose and a hand-held nozzle sprayer.  I also use a rotating sprayer if there are plants near the tree that also need to be watered. A friend of mine suggested going to a home supply store and getting a bunch of inexpensive buckets. Put a couple of holes in each one and fill with water to drip irrigate several trees at once. He uses this method for small fruit trees. You can use a triangular placement method. For trees I simply cannot live without, I lay  the hose around the tree in a helical configuration extended to the drip line.  Drip irrigate under low water pressure for one hour. I can see the water dripping, but it is not dripping fast. In 100 degree heat, without any rain, I will do this every 10 days to two weeks. For trees that can stand drought more such as pines, every three weeks. The Texas Forestry Service recommends watering each tree every 10 days 1-4 inches.1 The volume depends on the type of tree. If the roots are shallow and the ground is porous, 1-2 inches is about what I would use for the frequency I recommend.  Both the frequency and volume are dependent on the soil, species and weather. Always water in the evening to the early morning to match the hydration cycle of a tree. 4  If a tree has a low drought  tolerance, you need to pay close attention to it under heat stress and make sure you water the tree in advance of stressing it to the point of wilting. Watering is preventative medicine, not a cure. A nice little article I found searching via Google was in the Lufkin newspaper. 3

Here is a list of the trees I researched. I have most of them in my yard. I have adjusted some of the wording  to include local observations.
+ Bald Cypress: drought tolerant. Defense is to simply shed its leaves and hibernate. Small ones will eventually die if the dry period is for a long time. Tree is beautiful if kept watered. If no access to water, the tree will appear dead by leaves turning a dark brown.
+ Blackjack Oak - drought resistant - self defense is to hibernate and come back next Spring. Leaves turn brown and tree looks dead, but is not. Disease can kill the trees during this hibernation.
 + Bur Oak - very drought tolerant
+ Chinese Tallow: very drought resistant. Tree is hard to kill and invasive. These trees do not belong here. Please do not plant them. Instead, remove them from our ecosystems.
Crepe Myrtle - very drought tolerant. Excellent for cul-de-sac island.
+ Dogwood: low drought tolerance. Needs its water once a week.
+ Live Oak  - very drought tolerant. Also salt resistant. A Texas survivor. Tree likes its water and will grow much more rapidly when adequately watered.
Loblolly Pine: drought tolerant but highly intolerant under an attack by beetles and very vulnerable when small. Doesn't wilt, just dies. Hose configuration helical but from the trunk, not from the drip line.
Long Leaf Pine: very drought tolerant when large, vulnerable when small. Use same helical arrangement as Loblolly. Also highly vulnerable to insect damage and disease when under drought stress.
+ Magnolia: drought resistant - is susceptible to leaf browning and even death if no water at all
Mulberry: drought tolerant
+ Pecan: drought tolerant - needs water in fruit bearing months. Does best near and with water.
+ Redbud: drought tolerant, its leaves wilt easily but tree recovers at night
+ River Birch: drought tolerant - a strong survivor
+ Sawtooth Oak - drought resistant - will defoliate in self-defense against drought. Looks dead but is not.
Shumard Oak - very drought tolerant
Southern Red Oak: drought tolerant. Evidence indicates increased mortality in drought years
Sweetgum: low drought tolerance - watch closely for stress
Sycamore: drought tolerant
+ Texas Persimmon: extremely drought resistant. Survives when many others fail but also vulnerable to insect attacks.
Water Oak: low drought tolerance. Name says it all.
+ Wax Myrtle: drought resistant - definitely has its limits. Entire branches will die without water
White Oak:  very drought tolerant - may lose some limbs in drought
Willow Oak: drought tolerant. Does best where there is water.
+ Winged Elm: very drought resistant.Sheds leaves under duress
Yaupon: extremely drought resistant. Probably the most tolerant of all native trees. Lack of water affects berry production

+ There are many more trees to catalog. Generalizations are made such as "pecan" that is not necessarily true for all species. Breeding for drought tolerance is possible and in the case of food-bearing trees, is often the case. We are also likely to find exceptions to native species. I find articles through Google Search for the tree name and "drought", I can find the needed information on many trees. If you would like any additional name included, please let me know, and I will be glad to research and document the findings.


Scott said...

3 out of my 4 large wax myrtles have completely died. Those are my only casualties so far. Pines, Water Oaks, and Elms are doing ok so far.

jpmiller said...

I cannot immagine what you mean by "A tree will have a water content of 200% normally. In a bad drought, it will fall to 100% with the tree living." If a tree were to have 100% water content there would be nothing but water in the shape of the tree and it would come showering down on you. 200% water content is even more nonsense.

I like your bucket watering idea for my newly planted pecans, walnuts and grapes. How much water do you use? One filling or several per session?

Tim said...

I appreciate your observations. They are very helpful. Do you know how drought tolerant the Chinkapin Oak is? I would assume it would be very similar to the Bur Oak.