When it rains, I often hear people say that now they don’t have to water their trees and shrub, because Mother Nature is taking care of them. This is a myth. Our local region is classified as semi-arid, with extraordinarily little atmospheric humidity.
The annual moisture your home receives can be very different from an area just five or ten miles away. It’s a function not just of rain but of the water that comes in the winter snow — and that varies greatly, too. Early fall snow & spring snow can have ten times more water in it than the colder snow that falls from December through February. Boulder and the Helena Valley, for example, get about 9” to 12” of annual moisture. But our location here at Tizer Botanical Gardens & Arboretum averages about twice that amount, because we get much more snow — between nine to 12 feet annually.
Nothing is completely black and white in plant care, and no magic guideline on how much water a tree or shrub needs to get well established. Many factors need to be considered. The species of the tree is one of them. Even very drought tolerant plants may need more water than you think. The amount of sun a plant gets is also important.
Soil is a major factor. In our local region, much of the soil is decomposed granite or rocks and sand. This type of soil does not hold moisture. Another factor is wind. If you are in a very windy location your plant will lose way more moisture than in a milder windy location. So, a tree growing in decomposed granite in a windy location may need 40 or more gallons a week to get established.
The slope of your property is a factor that many people overlook. If your property is on a gentle south-facing slope the soil temperature will be warmer than it would be on a gentle north-facing slope. On a steeper south-facing slope the soil temperature could be 10 to 15 degrees warmer than a small, level area just a few feet from the slope. This is due to the angle of the sun’s rays hitting the ground surface. The steeper the slope on a south or west-facing slope the faster the soil will dry out.
A newly planted 1” caliper tree may need about 10 to 20 gallons of water a week to get established in reasonably good soil. In decomposed granite and a windy site it may take 30 to 40 gallons a week. And a 500-pound B&B root ball tree 2” to 3” in caliper planted in poor soil may need 50 to 60 gallons a week.
I simply cannot publish an accurate guide as there are too many variables. But the bottom line for most of us living in the Rocky Mountain States is, you will almost never over-water a newly planted tree or shrub.
One thing that isn’t important is the method of watering: By hand with a hose, sprinklers or drips all can get the job done. Just realize that you need to do the math and understand how many gallons are needed on a weekly basis.
The average impulse sprinkler, for example, will only put out about 16 ounces of water an hour in the diameter of an average 1” caliper trees’ root area. And that’s only if the tree is in the range of where at least 80% of the sprinkler is hitting the ground.
Too many times, people bring back a dead tree and say they watered it daily very well. Just this week one was brought back. The customer was told the tree needed 30 gallons a week to get established. They said it was on an automatic sprinkler system and was watered well every day. Their sprinkler ran 15 minutes per day. That tree probably received about four or five ounces of daily water, or about 35 ounces a week — far from 30 gallons.
Once established for two or three years, that tree might do fine on just five or 10 gallons a week plus what Mother Nature gives it. It’s easy to do the math for drip systems if you know how many gallons per hour the system emits. If you water by hand with a hose, the math is pretty straightforward, as well: How many minutes does it take to fill a five-gallon bucket? Then you know exactly how much water the plant is getting per week.
We often have seen professional landscaping and irrigation businesses way underestimate how much a tree needs to get established. Go by the what the business who sold you the tree recommended for success. I can’t express enough how important this is.
Richard Krott is co-founder of Tizer Botanical Gardens & Arboretum.
