Using irrigation to maintain high yields of blueberries

Eric Hanson and Mark Longstroth, Michigan State University Extension, Department of Horticulture - June 17, 2008

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Irrigation is vital for maintaining high yields in commercial blueberries in Michigan. 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 plantings. Because roots are shallow (12-inches), more frequent irrigation is needed than for older plants. Good soil moisture levels optimize vegetative growth on young plants by reducing or avoiding moisture stress during the growing season. For established plants, the goal is to optimize fruit production for current and subsequent seasons. Irrigate to prevent moisture stress during the June-July fruit development period, when drought stress will reduce berry size and yields. Drought in August-September reduces bloom during the next year, so avoid stress after harvest when fruit buds are formed.

Comparison of irrigation systems options for Michigan blueberries.

System	Percent of Michigan acreage1	Advantages	Disadvantages
Overhead sprinklers	36	High uniformity and rates Frost protection	High installation/operation costs Large water supply needed
Drip-trickle	16	Low cost to install and operate High uniformity Small water supply	Rates too low to catch up Can’t frost-protect Limited root zone coverage
Traveling gun	14	Moderate initial cost	Poor uniformity Can’t protect from frost High operating costs
Sub-irrigation	4	Variable initial cost Low operating costs Canopy is dry	Not suited for all sites Can’t protect from frost
No irrigation	30	No cost	High risk

1Michigan Department of Agriculture; Michigan Fruit Inventory 2006-2007

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 because they apply water directly in the row with little evaporation. Traveling guns are economical, but may apply excessive water rates and have poor uniformity.

The soil water reservoir depends on texture and rooting depth (Table 1). Assume the rooting depth is 12-inches for young plants and 18-inches for older plants or excavate beside bushes to determine exact depths. Sandy soils may hold less than one-inch of available water in the root zone, and half of this can be lost in two warm 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 drought-prone areas of your field.

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

Soil texture	Available water (inches)
Soil texture	Per inch of depth	In root zone (12-18 inch depth)
Sands	0.03	0.4 - 0.6
Loamy sand	0.07	0.8 - 1.3
Sandy loam	0.13	1.6 – 2.3
Loam	0.17	2.0 – 3.1

Evapo-transpiration (ET) is the evaporation from the field, plus the amount of 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 the Michigan Automated Weather Network (MAWN) (http://www.agweather.geo.msu.edu/mawn/). The potential ET is part of the report given for daily data. MAWN sites include Grand Junction, Fennville, South Haven, and West Olive. These MAWN Potential ET values are the best estimates of water use in Michigan blueberry fields. The ET will over-estimate water use early in the year before the canopy is fully leafed out and underestimate 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 of 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 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, you can irrigate when the plant has used half the available water. For example, a root zone of 18-inches on a loamy sand soil (0.07-inches of 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

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

Estimated blueberry water use in Michigan blueberries in inches

Month	Monthly use	Weekly use	Daily use
May	0.48	0.12	0.02
June	2.87	0.72	0.10
July	5.09	1.26	0.17
August	2.13	0.53	0.07

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-inches spacing, 0.42 gph emitters) deliver about 0.3 inches per 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 least one to two hours every day to replace the water used by the plants.

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

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