It’s here. The “D” word has arrived and every day it is becoming more and more part of California’s vocabulary – drought. Already several municipalities within the San Francisco Bay Area are talking about rationing water this year, and if you have lived here for awhile you may already know the drill and have plans for cutting back to help conserve. What you may not know is how cutting back will affect your mature tree. 

Mature Trees Are Important
If you are lucky enough to have a mature, large tree (or several of them) throughout your yard, then you are aware of the benefits they provide: shade for those hot summer days, a place for wildlife to live, and added real estate value to your property. Therefore, it is important to make sure they survive. 

How do I know if my tree needs water?
Your tree will tell you if you know where to look. Look at the top of your tree. If there are wispy dead twigs at the very top, or the leaves are undersized and/or turning brown, but the rest of the tree looks relatively good, your tree is not getting enough water. There is not enough water to reach to the very tippy-top of the tree. This is one of the first signs of drought stress. Mature trees vary widely in their need for water depending on size, age, soil composition, slope and, most importantly, species. The best way to tell if your tree needs water is to probe the soil. Find a 12″ long screwdriver (or similar type of probe) and insert it into the soil at the drip line of your tree (See the diagram indicating where the drip line is). If it slides in easily and has tiny clumps of moist soil attached to it, then the soil is moist and doesn’t need watering. But if the soil is rock hard and powdery dry, time to water! 

“But we NEVER water our mature tree!”
Some of you may be thinking, “We never water our large Valley oak (or whatever large tree you may have) and it is doing fine.” Yes, your tree may appear to not be getting any water during the summer months but if they are planted on very edges of your landscape they are accustomed to regular summer watering. Remember roots can grow to be twice the distance as the tree is tall to find water. Also, you are not accounting for the decrease in water during the winter months. Your large tree can probably handle a couple of winters with less than average rainfall but even native landscape trees (such as oaks or redwoods) will require some water during a drought. The best thing to do is to put the tree on your watch list and monitor how it is doing. If it looks stressed, watering it might pull it back from the brink. 

What is the best method for watering my mature tree?
The goal is to get water down to the absorption roots which tend to grow in the top 8-10″ of the soil (See the diagram above). If your tree is on flat ground an overhead sprinkler works best. Set it up to cover the area from the trunk to the drip line. Be careful not to have the sprinkler hit the base of the tree. Having your irrigation blasts the tree trunk can cause other problems, plus you are wasting water as there are no absorption roots at the base of the tree. Set an empty tuna fish can within the area of the sprinkler and turn on the water. When the can fills up (about 2 inches deep) turn off the water. Your tree has received 2 inches of water. You may need to move the hose several times to cover the area. On your first try, set a timer and check on the system periodically to make sure the water isn’t running off into the gutter. If it is, you will need to turn off the sprinkler and wait for the soil to absorb the water. Once the area from the trunk to the dripline has received 2 inches of water, check it once a week and only water again when the soil is dry. 

What about using a soaker hose? 
A soaker hose is an acceptable way to water your mature tree, especially if it is located on a slope. Wrap the soaker hose round the tree two thirds of the way out from the drip line. (See diagram for ideal watering zone) You can zig-zag the hose within the watering zone to better cover the area. Generally speaking, with the heavy clay soil that tends to be prominent within Contra Costa County, it will take an hour for the water from a soaker hose to penetrate the soil one inch. Therefore, you may have to leave the hose on for six to eight hours for the water to reach the absorption roots. However, if your trees is growing in sandy soil it will take less time to reach the roots. Again every situation is different and the first time you use the soaker hose, use your 12″ soil probe to monitor the rate of water and make adjustments. 

What about laser tubing to water my tree?  
Laser tubing was designed as an upgrade to the old fashioned drip irrigation with holes in the tubing (not an add on emitter) designed not to clog. You can purchase laser tubing with holes spaced 6″ to 24″ apart. Laser tubing doesn’t allow for maximum coverage like you get with overhead sprinklers but laser tubing can be very effective if you carefully calculate the amount of water that is being applied. The biggest problem with any drip irrigation system is that it is usually installed when the tree is young and if it is not expanded to account for the growth of the tree it will under water the tree. And if it is hooked up to a clock to come on automatically, be sure to periodically check it make sure is still working properly. 

The goal is moist, not soggy
You want to avoid over watering your tree and creating a quagmire of soggy mud. Tree roots need to breath and if the soil is too saturated the roots can suffocate and die. And to confuse matters, the symptom of an over watered tree looks the same as a drought stressed tree. That’s why it’s important to monitor the watering process. If the soil is soggy, stop watering and let the soil dry out. 

Continue to water your mature tree until the first rain
Of course the most critical time to monitor your mature tree’s water needs is during summer when the days are long and the sun is hot. Depending on your tree you may need to deep soak twice a month or more. But once the days shorten and the weather begins to cool, you can cut back to once a month until the winter rains take over. 

Warning! Watering mature trees in a drought will cost you.
During the last drought we recommended a client deep soak their mature tree twice a month. They followed our recommendation and two months later called to say that their tree was looking better but it had cost them an additional $200 on their water bill. A $200 spike in your water bill may be a little disconcerting however, what is your mature tree worth to you? It can take decades for a tree to grow to maturity therefore, a couple of above average water bills can be very cost effective. And compared to the cost of removing the tree when it’s dead, it’s a no brainer.

How do you decide if your tree is worth saving?
The best way is to ask someone who knows! Any one of our certified arborists can evaluate the health and condition of your tree and let you know whether it is worthy of preservation. If it is a non-native tree like a birch that likes lots of water and is already on its way out, it may not be worth saving. 

Watering mature trees is not an exact science.  
Arborists wish there was a one-size-fits-all watering instructions for trees. That would definitely make our job easier! But that is not to be because every situation is different. This article could go on forever with hundreds different scenarios for different species of trees. That’s why having an experienced certified arborist consult with you about your trees is the best way to determine how to water them.