MSU Extension

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.

Most blueberries grown commercially in Michigan should be irrigated. Soils are often sandy and hold little water, and blueberries are shallow-rooted and relatively sensitive to moisture stress.

Every 10 to 20 years, droughts occur that are severe enough to kill Michigan blueberries or cause so much injury that plants require multiple years to recover. Severe events occurred in 1988 and 2005. The cumulative water deficit in the figures below illustrates the difference between rainfall and evapotranspiration in Grand Junction and West Olive, Michigan during the 2005 season. By the end of the season, non-irrigated blueberry fields had deficits of 7 to 10 inches. Although this was extreme, deficits of 1 or 2 inches occur at some point during most years. Drought prior to harvest reduces berry size and yield, but stress anytime in the late summer or fall also reduces bud set for the following year.

Irrigation is particularly critical for young plantings. Young plants with shallow roots (12 inches) need more frequent irrigation than older plants with deeper roots. The goal for young plantings is to optimize vegetative growth so that plants reach full production quickly. Non-irrigated new plantings usually require two or more seasons to reach maturity, so returns on investments are delayed. Severe drought during establishment can delay maturity and returns for many years.

Irrigation systems

Various irrigation systems are used in Michigan blueberries and each has strengths and limitations (Table 1). Overhead sprinklers are the most common and are preferred for locations prone to spring frosts that have a sufficient water supply. Trickle systems are preferred where frost potential is low and/or water supply is inadequate for sprinklers. A limitation of trickle systems is that they may not have the capacity to replenish water during prolonged drought periods. Sub-irrigation should be investigated for fields that require tile drainage. Subterranean irrigation is accomplished controlling drainage and pumping water into underground tile systems. It can be effective but requires specific soil and topography characteristics.

Table 1. Comparison of irrigation systems options for Michigan blueberries.

Optimizing irrigation scheduling

Water holding capacity of soils varies depending on texture and rooting depth (Table 2). Blueberry soils are sandy and variable, which complicate irrigation scheduling. These sandy soils may hold less than 1 inch of available water in the root zone, and half of this can be lost in two warm summer days. Many blueberry soils are complexes (e.g., Au Gres-Saugatuck, Pipestone-Kingsville) with slightly elevated areas knolls and ridges that dry much more rapidly than lower areas. Hardpan may limit rooting depth in specific areas of fields. Many areas have a shallow water table that also limits rooting depth. Plants may benefit from capillary ascent of water from saturated zones, but the volume of water supplied by capillarity is not understood. These variable characteristics all add difficulty to scheduling.

Rooting depth varies considerably. Where the summer water table is shallow, roots may be confined to the top 12 inches. In well-drained soils without physical obstructions, such as hardpan, blueberries may root to 24 inches or deeper. For irrigation purposes, assume a root depth 12 inches for new plants and 18 inches for established plants, or excavate beside bushes to observe depths in your fields.

Table 2. Available water in a blueberry rootzone as affected by soil texture and rooting depth.

Allowable soil moisture depletion is generally assumed to be 50% of available water, but this has not been studied adequately for blueberries. This means that irrigation should be applied before 0.2 to 0.6 inches of ET losses on sands and loamy sands, or 0.8 to 1.5 inch losses on sandy loam or loam soils.

Evapo-transpiration (ET) is 0.18 to 0.24 inches per day during the summer in the blueberry region of Michigan. Daily Potential ET values for Michigan blueberry areas ( Grand Junction, Fennville, West Olive) are available on the Michigan Automated Weather Network (MAWN) (http://www.agweather.geo.msu.edu/mawn/). Use the MAWN Potential ET values directly as estimates of water use from blueberry fields. More research is needed to develop crop coefficients (Kc’s) to adjust Potential ET values for Michigan blueberries specifically.

How much water to apply? Irrigate enough to recharge the root zone. For example, if established plants with an 18 inch-deep root zone, on a loamy sand soil (0.07 inches water per inch of depth) was depleted by 50% of available water, 0.6 inches water would need to be applied:

(18 inches) (0.07 inches water/inch) (0.5 depletion) = 0.6 inches

If your irrigation system delivers 0.15 inches water per hour, 4 hours of operation would theoretically deliver 0.6 inches. However, 20 to 30 % of applied water may be lost to evaporation, so operating time may need to be increased because of this inefficiency. In addition, most irrigation systems do not approach uniformity; they apply more water in some areas than others. Overhead sprinkler systems often are only 70% uniform. This means that order to recharge all areas of the field, 30% more water than calculated would need to be applied.

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