The peak heat of August in our Salt Lake City summer brings with it some of the most uncomfortable and stressful weeks for our gardens and the gardeners that inhabit them. While one may catch a bit of reprieve from the hostile sun in the shade under the canopy of a tree, the tree itself often bears the brute force of that sun all day.
An established tree relies on water banked deep in the soil from our seasonal rains, a savings account that by now is nearly exhausted. And until we see a summer monsoon (which we may very well not get again this year), the urban forest needs a drink.
Giving your trees the right amount of water at the right time assures the needs of the plant are met, but also conserves precious water in our arid steppe climate. Overwatering can promote disease by depriving the soil of oxygen, and underwatering won’t get the water deep enough to reach the roots that need them most. To decide whether or not you need to water, dig down to a depth of 4 inches; if the soil is dry at this depth, it is time to water. It is important water deeply, rather than frequently.
How much water do my trees need?
The boss move in this whole scenario is to measure and meter the water you are applying. This is easily done with one of my favorite low tech tools, the five gallon bucket. Most friends of mine water their trees with the hose on a slow trickle, and move the hose around to various points to deep water. I prefer a 50-ft. length of “sweating” soaker hose, the type made from recycled tires. Whatever your method, measure your flow by timing how long it takes to fill a five gallon bucket, and run some simple math. With my 50-ft. length of soaker hose, it takes three minutes to fill a 5 gallon bucket.
Newly planted trees require frequent weekly watering, around 15-20 gallons per week. New trees may take one to two years to become fully established, depending on variety.
Established trees require less frequent watering. A proper watering may last from 10 days to four weeks, depending on soil type. In my area of the Sugarhood, we have a light clay/ clay loam soil, which holds water quite well, and my trees require watering every two to three weeks depending on temperature. As a general rule of thumb, plan on 10 gallons of water per 1 inch of tree diameter. “Ask a 4th Grader” math moment: Diameter is how wide a tree is, not its circumference, which is how big around it is.
Example: My beautiful park strip tree is 24 inches in diameter, so it requires 240 gallons of water. My soaker hose puts out five gallons of water in three minutes. 240 (gallons of water) divided by a five gallon bucket is 48 buckets, 48 buckets x three minutes is 144 minutes, so leaving my soaker hose on for two and a half hours gives my tree the right amount of water.
Where do I water?
Avoid watering at the base of the trunk. Instead, notice the tree’s dripline—the outer edge of the canopy of the tree. Identify the midpoint between the trunk and the drip line. Then water from that midpoint to the dripline and beyond. (Feel free to re-read that last part, I had to when writing it to make sure it made sense.) Water as evenly as possible in this zone.
I’m good at a lot of things, but remembering to turn off the hose isn’t one of them. Set a timer on your phone to remind you when you are finished, or when to rotate the hose. There also exists a handy little dial timer valve that works like a kitchen timer. Rotate the dial to the desired time, and when it reaches the end if turns off the water. Brilliant.
Set a calendar reminder to notify you when to dig down to that four-inch depth to check the moisture content of the soil, and possibly water again.
James Loomis is the Green Team farm manager for Wasatch Community Gardens.