There is no magical formula or exact rule for watering. Watering is based on the site condition, time of year, type of tree and, most importantly, soil type. Clay soils, while taking longer to wet, will retain moisture longer. Sandy soils, which easily absorb water also, dry faster.
KNOW YOUR SOIL TYPE!. Texas A&M Forest Services indicated that local environmental conditions affect how much water is lost from a tree and soil due to evapotranspiration. Trees will use and require more water during hot, windy, low humidity days versus cooler, calm, high humidity days. It benefits your trees if you track your local weather conditions and replace or replenish soil moisture as needed. Watering wisely is the best way to conserve our precious water resource and to benefit your trees.
Newly planted trees: Fill the water basin, allow the water to soak into the ground and fill the basin again. Depending on your site, soils and environmental conditions you may need to water as little as once per week or as often as three times per week. Two to three days after the initial watering, check the moisture of the backfill soil and the root ball soil (they will differ in texture). Get on your hands and knees, dig into the soil, grab a handful and feel it. If the soil is moist, check back in a couple of days. Repeat this process until you have determined the best watering schedule for your tree. If the soil stays squishy wet, you have a drainage problem or you are over watering. If the soil is crumbly, dry and hard, you’re not watering enough. The goal is to keep the soil evenly moist to a depth of 12 to 18 inches. Your watering schedule should change throughout the year. If it does not, most likely you will either be over or under watering your trees.
Older established trees: Water the entire root zone under and beyond the tree canopy. Apply enough water to wet the soil from a minimum of 12 inches deep to a maximum of 18 inches. This can be anywhere from 1 to 3 inches of water as it covers the ground area. Using some type of soil probe will help you determine how deep the water is percolating into the soil. Most mature trees only need to be watered on a bi-monthly to monthly schedule, depending on the type of tree and time of year.