How Much Water Does Your Tree Need?

Not too Much, Not too Little

It’s that time of year when you sit out on your patio and smell the neighbor’s sprinklers as they rotate back and forth across the grass, hitting the fence and sometimes the street. Irrigation systems are an enormous timesaver and are often the make it or break it element to thriving landscapes in Denver. But are they delivering enough water to your trees? And if you don’t have one, how much water should you be giving those trees? Remember, every tree in Denver needs supplemental water year round for its entire life.

Before getting into it, it might seem counterintuitive to be using water in a drought-prone area. But, trees on private properties have been shown to decrease private property water usage overall. So, you can think of watering your trees as an investment into water conservation. The shade trees provide decreases water loss to evaporation, more so than the amount of water required to keep them healthy. Plus, if you're willing to go the extra mile, trees can be watered with recycled or already used water. One of the easiest ways is to have a bucket in the shower to catch runoff or extra water, just be sure to not get shampoo, soap, etc. in the bucket.

Okay! Now back to the trees. This is about to get a little complicated. But, as long as you rely on your senses, you'll be on the right track. Dry, crunchy leaves mean water more and soft, yellow leaves mean dial it back a bit. You can always test soil moisture by sticking your finger in there and seeing if it comes out dry and dusty or wet and muddy, too.

And now…the down and dirty details of how to calculate your trees water needs….

The basic rule of thumb is that your tree needs 10 gallons of water for every inch of caliper* at every watering. For most landscapes, you’ll need to water your trees every other week in summer and fall and every week throughout the summer months.

*Caliper is the diameter at breast height, or about 4’ up, of the tree. You can measure this roughly, no need to break out the geometry equations (Circumference = (pi)*D anyone?)

If you’re watering with a hose or leaky bucket your math is easy since bucket size is measured in gallons. Get yourself a 5 gallon bucket from a hardware store, poke some holes in the bottom, and fill it with the hose or kitchen sink as many times as you need to meet your trees’ needs. This is where you can add the recycled or re-used water sources, too.

Those of you with drip irrigation systems have some somewhat simple math, too. If you take a look at your system, you should have a black pipe with small tubing (spaghetti line) coming out of it and drip emitters located either at the connection to the pipe or the end of the spaghetti line. Those emitters are color coded by how many gallons per hour they emit. There are some weird colors out there but usually they come in red (2 gal/hr), black (1 gal/hr) and blue (0.5 gal/hr). If your tree needs 10 gallons of water every other week and you have 3 red emitters around the base of your tree, each watering for 30 min, 2 x week, how much water are you giving your tree? 

3*2gal/hr*0.5hr/watering*2x/wk= 6 gal per week. Your tree is getting about 2 extra gallons/wk. You can adjust your system up and down to make the math work as the temperatures fluctuate.

If you have spray irrigation (a sprinkler system), the math gets a little more complicated since irrigation is measured in inches, like rainfall. Start by calculating the area to be watered, which is the entire width of the tree multiplied by 3. Say you have a 3’ wide tree, your rough area to be watered would be 9-10 square feet. Now you need to see how many inches of water fall in this area. Collect a few tins of wet cat food, tuna fish, or canned beans and set them in your yard, around the base of your trees. After the irrigation runs, see how much water has collected. You can eyeball wet and dry spots from here and maybe troubleshoot how to even that out. Take the average of the amount of water in each can, not the total combined amount. Time to get out the calculator (if you haven’t already). Multiply that number by 0.623.

Example: a newly planted Denver Digs Tree is 1” caliper and 3’ wide. It needs 10 gallons of water about every other week. Sara’s sprinkler system delivers .5” of water to the base of the tree each time it runs and it runs 3 times a week. How many gallons of water is Sara’s tree getting? Area to be watered: 3’*3=9sf | Water in gallons: 9 sf*0.5”water*0.623 =2.8 gallons each time the sprinklers run | Water per week: 2.8gal * 3x per week = 8.4 gal/wk | Water deficit: 10-8.4= 1.6 gallons

Sara’s tree needs 1.6 extra gallons of water per week. As the tree grows, it will need more. If Sara’s tree has grass growing under it, that grass is using most of the water before it gets to the roots of the tree. We recommend replacing the grass beneath your tree with mulch. Increasing the duration or frequency of watering the lawn in order to meet the tree’s needs can lead to water waste and problems including increased Japanese beetle population. Some good options would be to supplement with either of the two above irrigation methods - a drip system or a leaky bucket. 

Remember, all the math aside - you and your tree should have a long life together. Spending time in your yard observing and getting to know your outdoor space can bring health benefits to you and improve the health of your tree as well. Have fun!