Water management in a raised bed garden is a bit more challenging than in a traditional garden. Because drainage is assured, plant stress caused by overwatering is minimized, but problems associated with underwatering are magnified, especially in high-profile (deep) beds. Plant stress caused by either insufficient or overabundant water can delay maturity and reduce yields.
To overcome the water-holding limitation inherent in raised beds, apply water in frequent, even, daily small doses. This practice not only ensures a consistent supply of moisture in the bed profile, but also reduces the amount of moisture lost through seepage from the base of the bed. Drip irrigation is the ideal application method because it delivers a low-volume, steady supply of moisture in frequent doses. The amount and frequency of watering depend on the water-holding capacity of the soil, weather conditions and requirements of your plants.
Soil in the plant root zone acts as a reservoir for water. Soil texture (particle-size composition) is the primary factor influencing the amount of water a soil reservoir can store. Fine-textured soils such as loams are able to hold more water than sandy, coarse-textured soils. Some loams are capable of storing as much as 2 1/2-inches of water per foot of soil; sandy loams, only 1 1/2. All other factors being equal, a sandy soil will dry and require irrigation sooner than a loam soil.
Crop water demand ranges from 0 to 3 inches per week, depending on growth stage and weather conditions. Seedling plants growing in beds covered with plastic mulch might require only a fraction of an inch of water during a cool, cloudy week, whereas a fully grown tomato plant loaded with fruit can use up to half an inch in a single hot, windy summer day.
By having a rough estimate of the daily water requirements of your plants, you can calculate the length of time to irrigate, keeping in mind your system's delivery rate. For example, a mature tomato crop uses roughly 1/2-inch of water per day, the equivalent of 30 gallons evenly distributed over 100 square feet. If your system delivers 30 gallons per 100 square feet per hour, you will need to irrigate one hour each day.
Using stage of growth and weather conditions to schedule irrigation is a calculated guess at best. A better method is to evaluate the soil moisture condition by either sampling with a soil probe or short piece of thin-walled pipe or installing a soil-moisture-measuring instrument. The probe should be inserted into the bed 6 to 12 inches from the base of the plant, where roots are actively growing, or for closely spaced crops, in the plant row. To avoid cutting drip emitter lines, determine their locations before inserting the probe.
Nearly all garden crops grown under irrigation extract water from the top 2 feet of the soil profile. In fact, up to 95 percent of the roots are in the top 12 inches, so it isn't necessary to sample below a 12-inch depth when you use a soil probe.
You can estimate available soil water by appearance and touch. To estimate moisture content, remove a handful of soil from the probe and squeeze very firmly with your fingers. If the soil ball is squeezed immediately following an irrigation or heavy rain, water will drip from it. If this high level of moisture persists, plant health and performance could suffer, but if you're growing plants in raised beds, excessive moisture shouldn't be a problem. The ideal soil moisture content is indicated when, upon being squeezed, the soil ball emits no free water and only a wet outline of the ball remains on your hand. If there is no outline, the soil is dry and should be irrigated. Although measuring soil water by appearance and feel is not precise, with experience and judgment, you should be able to schedule irrigation with a reasonable degree of accuracy.
A more accurate method of measuring soil moisture to schedule irrigation involves the use of a tensiometer, a device that measures how tightly water is held in the soil which is an indirect indication of soil moisture content. A tensiometer consists of a sealed water-filled tube equipped with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. As the soil around the tensiometer dries, water is drawn from the tube through the ceramic tip, creating in the tube a vacuum or tension that can be read on the gauge. When the soil water content is increased through rainfall or irrigation, water enters the tube through the porous tip, lowering the gauge reading.
Soil tension levels used to schedule irrigation vary with soil texture. In a sandy loam, irrigation should begin when soil tension reaches 20 centibars (a unit of pressure) and cease when it falls to 10 centibars. A soil tension reading of 0 indicates complete saturation. In a finer-textured soil such as silt loam, there is no need to irrigate until soil tension reaches 30 centibars.
Insert the tensiometer as you would a probe, 6 to 12 inches from the base of the plant or in the row if crops are closely spaced.
Expect to spend extra time at first to learn how the tensiometer reacts to various weather and cropping situations. With time, you will be able to schedule irrigations to achieve the desired soil moisture response.
Tensiometers come in various lengths. The 6-inch model is ideal for use in raised beds. Although not required, a 12-inch model placed alongside the 6-inch tensiometer helps determine the depth of water penetration so that water won't be wasted.
Because of their cost ($50 to $60 for a 6-inch model), it is impractical to place a tensiometer in every bed or crop. If you have only one tensiometer, locate it among plants having the greatest water demand (tomatoes would be the candidate in most gardens). Adjust irrigation to match water requirements of other crops accordingly.
For information on the use and care of your tensiometer, refer to the owner's manual. With proper care, a tensiometer can provide many years of useful service. For a list of tensiometer (also called an irrometer) merchandisers, refer to the appendix.
Following these guidelines will help you manage your water resource. Proper scheduling of irrigation will help ensure maximum production with minimum watering.