MSU Extension

Irrigation is vital for maintaining high yields in Michigan blueberries. Blueberries grow best in moist soils. Many Michigan blueberry plantings are located in areas with a high water table so the bushes have ready access to water located close to the surface. But, blueberries are shallow-rooted and sensitive to drought stress, and most Michigan plantings are on sandy soils that hold very little water. Drought prior to harvest reduces berry size and yield, but drought stress anytime in the summer or fall also reduces bud set for the following year. Severe drought events reduce returns for several years.

Most fruit in Michigan is not irrigated, but 70 percent of the Michigan blueberries are irrigated. Irrigation is particularly critical for young plants that have limited root systems. A consistent supply of soil moisture optimizes vegetative growth on young plants by reducing moisture stress during the growing season. For established plants, the goal is to optimize fruit production in the current and future seasons. Irrigate to prevent moisture stress during the June-July fruit development period, when drought stress reduces berry size and yields. Drought in August and September reduces bloom next year, so avoid stress after harvest when fruit buds are formed. Occasionally, soils are dry as blueberries enter winter, and fall irrigation may help prepare plants for winter conditions.

Irrigation systems

Overhead sprinklers are best where the water supply is adequate and spring frosts are likely. Trickle systems are best if your water supply is limited. Trickle systems apply water directly in the row with little evaporation loss. Traveling guns are economical, but can apply excessive water rates and have poor uniformity.

Overhear sprinkler irrigation is one of the most popular options since it can be used to
reduce spring freeze injury during bloom.

The soil water reservoir depends on soil texture and plant rooting depth (Table 1). Assume the rooting depth is 12 inches for young plants and 18 to 24 inches for older, established fields. Sandy soils may hold less than 1 inch of available water in the root zone, and half of this can be lost in two hot, summer days. Many blueberry fields have slightly elevated areas that dry out quicker than other areas. Hardpan or a shallow water table may limit rooting in other areas of fields. These variable characteristics complicate scheduling. As a rule, irrigate to maintain the most drought-prone areas of your field.

Table 1. Available water in a blueberry root zone as
affected by soil texture and rooting depth.

Evapotranspiration (ET) is the amount of water lost to the atmosphere and includes the evaporation from the soil plus water lost by the plant (transpiration). Blueberry fields in southwest Michigan lose 0.18 to 0.24 inches per day during the summer. Daily potential ET values are available on Enviro-weather. After selecting the station nearest your farm, select Fruit from the Tools ribbon along the top of the page. After selecting Fruit, you can go to the bottom of the center column and at the bottom of the page you will find Irrigation Tools. There are three different tools available. Irrigation Scheduling is a web based tool allowing you to set up your own farm fields to use Enviro-weather data to schedule your irrigation. It requires you to set up your own User ID and track and report precipitation and irrigation on your fields. Potential ET is the potential ET calculated from daily data at the MAWN Enviro-weather sites. These MAWN Potential ET values are the best estimates of water use in Michigan blueberry fields. The Potential ET data begins on April 1 and gives the temperatures and rainfall for each date and calculates the ET and the cumulative water use. The ET will over-estimate water use early in the year before the canopy is fully leafed out and under-estimate water use when the canopy is fully leafed out and conditions are hot and dry. Maximum water use during the preharvest fruit growth stage is probably 0.20 to 0.25 inches.

Allowable soil moisture depletion in blueberries is generally considered to be 50 percent, so irrigate when half of the available water is used. This means that irrigation should be applied before 0.2 to 0.6 inches water is lost from sands and loamy sands, or 0.8 to 1.5 inches are lost on sandy loam or loam soils.

Irrigation scheduling allows water to be applied when it is needed. This reduces costs, the amount of water used and loss of nutrients. You need to know how much water the soil can hold. You should irrigate when half the available soil water has been used. If you know how much water the plants are using (ET), you can irrigate when half the available water is used. For example, a root zone of 18 inches on a loamy sand soil (0.07 inches water per inch of depth) holds 1.3 inches of available water:

(18 inches) x (0.07 inches water/inch) = 1.3 inches water

If the root zone were depleted by 50 percent, you would need to apply 0.65 inches:

(0.5 depletion) x (1.3 inches) = 0.65 inches to apply

If the ET for the last several days was 0.25 inches, you would need to irrigate every two days; for 0.2 inches every three days and if the plants were only using 0.1 inches then you would need to irrigate every six days.

The evapotranspiration rate varies during the year depending on the amount of leaves on the plant and the weather condition (temperatures, relative humidity, wind). The temperature is the most important factor; increased heat increases ET much more than increasing humidity decreases ET.

Sprinkler systems

The amount of water applied by sprinkler systems is determined by the size of the nozzle and the water pressure at the nozzle. For example, a 9/64-inch nozzle at 45 psi will deliver about 0.15 inches an hour. If the system delivers 0.15 inches water per hour, 0.6 inches would be pumped in four hours. However, about 20 to 30 percent of water from overhead sprinklers may be lost to evaporation, so increase the operating time accordingly. Also, irrigation systems are not completely uniform; they apply more water in some areas than others. The uniformity of sprinkler systems can be measured (Ley, 1994b), but are usually only 70 percent uniform. This means that to recharge all areas of the field, 30 percent more water than calculated would need to be applied. In our example, operating time should be increased by 20 percent to account for evaporation losses, plus 30 percent due to non-uniformity. So, increase operating time of four hours by 50 percent to six hours to ensure all areas receive 0.6 inches.

Trickle irrigation

The application rate for lower volume trickle systems (48-inch spacing, 0.42 gph emitters) is about 0.17 inches per hour. The more common moderate flow systems (24-inch spacing, 0.42 gph emitters) deliver about 0.3 inches hour. Since evaporation and uniformity are not significant in trickle systems, we do not need to increase the application time. We would need to run the lower volume system twice as long to apply the same amount of water. These systems should be run at one to two hours every day to replace the water used by the plants.

There are several rules of thumb for trickle irrigation systems.

• For young plants, apply 20 gallons/day per 100 feet of row.
• For mature plants, apply 35 gallons/day per 100 feet of row.
• Ontario, Canada estimates that peak daily demand for mature blueberries is about 4.5 gallons/day (18 liters/day).

Dr. Hanson's work is funded in part by MSU's AgBioResearch.