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Trees get thirsty, too

Posted: July 12, 2012 6:40 p.m.
Updated: July 13, 2012 5:00 a.m.

Weekly watering of a Japanese stewartia using a 5-gallon bucket with drainage holes drilled in the bottom.

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Summer is officially here and we now have the temperatures to prove it! Along with high temperatures come afternoon showers and sometimes downright gully-washer rains. These rain events can be quite dramatic and appear like many inches have fallen from the sky. Unfortunately, most times this is not the case. Even in the rare occasions when we do actually get close to 1 inch of rain, it falls so hard and fast that the ground has little time to absorb it.

We are constantly reminded during the summer how important it is for us to keep cool by staying hydrated as our bodies loose water through sweating. Trees also keep cool by losing water. It happens through “pores” on the underside of their leaves, a process called evapo-transpiration.

So when it comes to keeping your trees cool, alive, healthy and thriving through the grueling heat, water is the top, No. 1, primary key to ensuring their longevity in your landscape. After all, what’s summer if you don’t have the shade of a tree to sit under? Not only that, but a well-placed large-maturing shade tree can help cool your home and lower your air conditioning costs as well.

There are many factors which influence how much water is needed for a tree. Some of those factors include: the type of soil, the size of the tree, the species of tree, temperature, humidity, wind and length of sunshine. For example, a large tree in a sandy yard -- on a hot, sunny, low humidity, breezy day -- will lose a lot more water faster than in a yard with clay soil on a hot, cloudy, thick-with-humidity calm day.  

Here are some general rules-of-thumb for providing trees a drink or two. Since mature trees roots are likely to inhabit a large area of the landscape (wherever there is space, oxygen, nutrients and water) there is generally more open area for the roots to capture and absorb water. Under normal summertime conditions, about 1 inch of rain per week is enough to keep them going. During periods of no rainfall (less than 1 inch per week) it’s best to provide supplemental water to an existing tree. An in-ground irrigation system is beneficial during these times, but nothing beats a good ol’ fashion oscillating sprinkler set at the base of the tree for a good soaking.

Under abnormal or extreme summertime conditions, such as consecutive days over 100 degrees, everything in your landscape should be provided with ample water to thoroughly moisten the soil. In fact, sometimes hydrating the soil and watering the tree doesn’t even help when it gets over 100 degrees as many trees close their pores to conserve water. Unfortunately, when this happens, the leaf tissue gets so hot it burns. This is called leaf scorch, which perhaps you have recently seen on some trees that just couldn’t withstand the heat.  Leaves that have partial scorching will normally not affect tree health or growth. If the entire tree canopy is scorched, turns brown and all of the leaves fall off shortly thereafter, the tree should be OK. New leaves should re-sprout provided it gets water, but growth and health will be impacted as the tree will have to use energy to set new leaf buds for next spring. If the entire tree canopy turns brown and all of the leaves hang on for a week, then the tree is mostly likely dead.

If you happen to have a newly planted tree, one that has been in the ground for five years or less, the supplemental and direct watering is absolutely crucial during high temperature days. Whether the tree was transplanted from the nursery field (loosing 90 to 95 percent of its roots) or transplanted from a container, it takes time for it to establish in the surrounding landscape. During this time, the tree is under transplant shock. Studies on trees have shown that it takes about one year per 1 inch of tree diameter for the tree to replace the lost root system. This is based on the premise that the tree was transported, handled, installed, watered and mulched properly. During this time of acclimation in the landscape, a minimum of 5 gallons per 1 inch of diameter per week (on average) is needed to help the tree establish into its new home.  

Here at the city, we are watering our new trees for the first two growing seasons. Watering starts from the day they were planted through the time that the leaves fall off. During the second year, we start watering once the buds start to swell and continue watering all the way until the leaves drop off in the fall. Each and every week the trees receive an average of 10 gallons of water unless we get at least 2.5 to 3 inches of rain within a 24-hour period.

You may be thinking, “What’s so special about 2½ to 3 inches of rain?” Well, according to calculations by Sam Davis, the city’s deputy public works director, 1 inch of rain falling onto a 3-foot diameter area of ground (this is the size of the tree planting area) equates to 4.2 gallons of water. This amount is way less than the 10 gallons of water we provide each week. In addition, these calculations do not account for the rain having to soak through the 2-inch layer of hardwood mulch before reaching the root ball soil. In order to get 10 gallons of rain water, we need at least 3 inches of rain within a 24-hour period to adequately water the tree and allow us to skip an entire week of watering until the following week.

If you are thinking, “Liz, I’ve got an in-ground irrigation system and you said it’s good enough to water the trees.” It’s good enough to water the mature trees that have been established in the landscape for a long time. It’s not good enough to direct the needed quantity of water for the new tree’s root ball.

Keep in mind that most trees reach heights of 30 to 100 feet tall and up to 60 feet wide. They are big and take a lot of water to keep them going. City staff are diligent or some may even say fanatical about watering the new trees for several reasons: 1) these trees are an investment of public funds; we need to ensure their growth and survival for decades to come and 2) they are on the job 24 hours a day, 365 days a year providing us with a variety of benefits (wildlife habitat, aesthetics and air purifiers) and actual economic value (storm water retention, carbon sequestration and enhancement of property values).

Thus, the next time you are out and about in your yard sweating and wishing for a tall cool drink, remember that your tree is probably thirsty as well, so be sure to provide it with a drink of water, it will thank you.


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