The Front Range of Colorado and the Denver Metro area is a semi-desert climate. Rainfall averages 12" per year. Yet somehow we grow trees here... lots of trees! Supplemental irrigation is the key to our urban forest success story. The trees we plant have to be watered by people. There simply is not enough natural rain and snowfall moisture to support the types of tree we plant.
How much water is needed then?
That depends on many factors: Tree species, soil type, exposure, available soil surface area of the planting site, and ultimate size of the tree are just a few things to be considered. "Know it before you grow it"!
After you have planted your new tree and have watered it in thoroughly wait a week before you water it again. Check the soil around the root ball for moisture. Dig a small hole in the soil backfill next to the root ball about 6" deep. Grab a handful of soil from the bottom of the hole and feel it for moisture. If it is wet don't water the tree. Check the soil again in a few days, if the soil is still moist give it another day or two then check the soil again. If the soil is dry set the hose on top of the root ball near the trunk and let in run at a slow trickle until the soil is wet down to a depth of 8" to 12". Be sure to check the root ball itself for moisture as well.
The key here is to check the soil before you water! Watering on a set schedule may be too much or too little water. More new trees die from over watering than under watering.
to support a large established tree. When trees are planted in turf, generally they get enough water to sustain themselves if the grass is receiving proper amounts of water. However, it is suggested to test the soil for moisture, as described above at least every two to three weeks to make certain the water is penetrating at least 8" to 12" deep.
Tree roots are located in the top 8" to 12" of the soil and can grow 3 to 5 times beyond the drip line of the tree in search of water and nutrients. The critical zone to keep irrigated is under the drip line of the tree canopy. The drip line of a tree extends from branch tip to branch tip on the other side of the tree. This applies to trees in turf or in bed areas. Feel free to water outside the critical zone as well.
General rules of thumb for watering: