A soil probe being inserted into the bed

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.

Drip irrigation with its high frequency/low volume watering keeps a much more ideal soil moisture level than does a sprinkler.

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.