You may have been given watering instructions like this: “During the growing season, make sure your plant gets an inch of water each week. What does this measure mean? What is it based on? How do you know you have provided the necessary water?
“Recommended inches of water” refers to the amount of water (from precipitation and irrigation) that must be applied to the soil surface to 1) penetrate the soil to the level of the roots of a plant and 2) meet the water requirements of the plant. Water penetration to the depth of the roots is desirable because the roots absorb water intended for use by the plant. Watering to the expected depth of a plant’s roots also encourages deeper and stronger root growth, which makes the plant more drought tolerant.
• The depth of the roots depends largely on the type of plant and its age.
• The roots of small plants such as ground covers and annuals are usually found within 1 foot of the soil surface.
• The roots of medium-sized plants such as shrubs are usually found within 2 feet of the soil surface.
• The roots of large plants such as trees are usually found within 3 feet of the soil surface.
There are exceptions. Some annuals (like tomatoes and pumpkins) have very deep roots, reaching 3 feet or more in the ground. Some trees (like maples) and shrubs (like blueberries) have relatively shallow roots that are concentrated in the top 12 to 18 inches of the soil.
How much water is needed to reach the root zone? One inch of water applied evenly to the soil surface without runoff will, on average, reach a soil depth of 6 to 8 inches. Therefore, the small to medium plant will need about 1 inch of water per week; the average to medium-sized plant will need 1 to 2 inches per week; and the large medium plant will need 2-3 inches per week.
Once you know how many inches of water a plant needs, how do you know it got so many in a week? When measuring precipitation (or water from an overhead sprinkler that is distributed slowly and evenly over a planting area), you can use a rain gauge or a small, straight-sided container (such as a tin of tuna or food for cat) placed on the ground surface. The depth of the water in the gauge or container indicates the amount of water that has been applied to the soil surface.
What if you apply water with a watering can or drip system? A little math will help. One gallon of water is 231 cubic inches. To apply one inch of water to one square foot of soil (1 inch by 12 inches by 12 inches), you will need 144 cubic inches of water or about two-thirds of a gallon of water.
Using this conversion, you will see that to apply 1 inch of water to one square meter (1 inch by 36 inches by 36 inches or 1,296 cubic inches) – the area covered by a small bush – you will need to apply approximately 5 gallons of water. ‘water. To apply 1 inch of water to an area 5 feet by 5 feet (1 inch by 60 inches by 60 inches or 3600 cubic inches) – the area covered by a dwarf fruit tree – you will need to apply approximately 15 gallons of water .
If you are using a drip system with emitters that deliver a certain amount of water per unit of time (eg ½ gallon per hour, 1 gallon per hour), you can estimate the amount of water applied to a plant. (in gallons) by multiplying the combined flow per hour of transmitters around a plant by the run time.
Regardless of the watering method, you can check if enough water has been applied by digging to the depth of the expected root zone with a trowel or shovel. If the soil is wet at this level, you’ve done your job. Otherwise, more water is needed.
Other Factors Affecting Watering Practices
In addition to the depth of the roots, other factors affect the desired amount of irrigation applied each week:
Sandy soil retains less water than clay soil and dries out more quickly; more irrigation will be needed.
In rainy conditions, the amount of water applied by irrigation should be reduced by the amount of rain received. If it’s hot and dry or windy, plants (and the surrounding soil) lose water more quickly; more irrigation will be needed.
Soil on slopes often drains (and dries up) more quickly than soil on level ground and probably requires more irrigation. Likewise, soil in raised beds and containers drains faster requiring more frequent irrigation than in earth plantings.
A thick layer of mulch minimizes water evaporation and improves moisture retention in the soil. Plantations with a 2-3 inch layer of mulch require less frequent irrigation than those without mulch.
Remember that the goal is to bring water to the root zone of the plant. Do not water lightly each day to reach the total amount of water for the week; light watering results in a moist soil surface and a dry root zone. Instead, apply the recommended water slowly over one to three applications so that the water penetrates deep into the soil and runoff is minimized.
For more watering advice, see Washington State University’s “Watering Home Gardens and Landscape Plants” (EB1090), available free at pubs.cahnrs.wsu.edu/publications/pubs/eb1090.
Jeanette Stehr-Green is a WSU Certified County Clallam Master Gardener.