How much should I water? Lawn and tree watering guidelines for summer

Kari Lewis, MSU Extension - Glacier county

We had some hot, windy weather the end of May that added some stress to our crops and trees.  While we’re dependent upon Mother Nature when it comes to watering our dryland crops, there are a few guidelines for watering lawns and trees that I wanted to share to keep them vibrant and healthy.

When watering trees, we need to remember that most of the small feeder roots that are responsible for water uptake are located in the top 12 to 18 inches of soil.  Trees and shrubs should be watered when the top 6 to 9 inches of soil in the root zone are dry.  A relatively easy way to check this is to take a long screwdriver and push it down into the soil near the tree.  You can also take a shovel or hand trowel and dig a small hole under the plant canopy and feel the soil.  If the soil is cool and moist 6 to 9 inches below the surface, no water is needed.  If the soil is dry, or its very hard to push the screwdriver down into the soil, it’s definitely time to water. 

During the spring and summer, trees should be watered about three to four times a month, approximately two inches per watering.  Trees should be watered not at the base of the trunk of the tree, but instead at the dripline.  The dripline is the area at the edge of the tree’s canopy, which is the most effective way to water the tree’s feeder roots which are responsible for water uptake. 

To determine how much water a tree needs, you can track how long it takes for a watering session to moisten those top 9 to 12 inches of the soil.  Also, the trunk diameter of the tree at chest height is correlated to how much water the tree needs.  For example, if the tree is four inches in diameter, at your chest height, that tree needs approximately four gallons of water each week. 

Regular watering throughout the spring and summer helps the tree grow and flower, transport nutrients, cool the tree, and defend against pests and other stresses.  During August, watering should be decreased to allow the tree to harden off for winter.  Once the leaves fall off the deciduous trees, trees should be watered well again prior to going into winter.  If there’s no snow during the winter, trees should be watered twice/month as well.  When watering trees, do not water the foliage of fruit or deciduous trees, that encourages blight, rust and mildews. 

Lawns should receive at least one inch of water per week, which is best applied in a heavy soaking, versus numerous, shallow waterings.  One inch of water equates to the amount of a tuna fish can, so you can set tuna fish cans on your lawn and then time how long it takes to fill the cans to allow you to determine how often to water to ensure the lawn receives one inch of water/week.  If the weather is hot and dry, the lawn will likely need more than one inch per week. 

For both lawns and trees, early morning is the best time to water, as the temperature is rising.  It provides for the most efficient use of water.  Watering in the evening can lead to disease issues as the leaves are wet going into the cool, dark night.