How Often Should I Water My Purple Plum Tree?

Watering your tree correctly goes a long way to ensuring its health. Purple plums (Prunus x blireana), which grow in U.S. Department of Agriculture plant hardiness zones 6 through 8, tend to do best in cooler climates. But even in the right climate, how much to water will vary. How much water your plum tree needs depends on the soil type, tree size and environmental conditions. A plum tree will show signs of too much, too little or incorrectly applied water.

Watering Frequency

Plum trees need the soil to be moist down to 24 inches below the surface. You should be watering your tree enough to maintain moisture to this depth. In summer, using a sprinkler or flooding the area every 14 to 21 days may be enough to keep the tree healthy.

Water Loss Due to Evapotranspiration

Every day, water is lost from your soil through evapotranspiration. The rate of this process can help you calculate a more accurate watering schedule for your plum tree. This rate depends on the tree's size and the weather conditions, and it is important to know because it will help you determine how many gallons of water to apply to your plum when you next water it. Temperature and humidity help determine the evapotranspiration rate.

In the coolest parts of the growing season, in early spring and late fall, or when it is foggy, the plant loses an average of 0.1 inches of water per day. This rate increases to 0.2 inches per day when there is some fog or the weather has warmed slightly closer to summer. Warm summer days without fog may cause the tree to lose 0.25 inches of water each day, but if it is above 100 degrees Fahrenheit or hot and windy, the rate is 0.3 inches per day.

Size Matters

Older, larger plum trees need more water than first-year trees with little growth. The amount also depends on the evapotranspiration rate. Most purple leaf plums grow to cover a 20 by 20 foot area. This translates to a fully grown plum requiring more than 50 gallons of water on a 100 F, windy summer day or as little as 18 to 20 gallons in the cool of the early spring or fall. To find out how large your tree is, measure the canopy space all the way to the drip line. The square footage of that space on the ground is the size of your tree you need to use for determining watering quantities.

Water Stress

The symptoms are similar whether you water a tree too much or too little. Both problems may cause leaf drop and browning, but too little water also can cause the leaves to wilt. To determine if you really are over- or under-watering, test how the soil feels in your hand. Take a sample from 4 to 8 inches below the surface and try to form a ball with it in your hand. If it forms a compact ball without any water leaking out, your soil is moist enough. If water is also squeezed out when you use loose pressure to create the ball, wait a couple of days because the soil is soggy. If you cannot form a ball, it is time to water again.