Maintaining irrigation on Michigan blueberries

Strategies for irrigating under hot, dry conditions and limited water supplies.

Much of southwest Michigan blueberries have received almost no rain since the beginning of June. In fact, we have needed over 9 inches of water since June 1 to satisfy the demand for many crops. Blueberries grow best in moist soil and suffer in dry soils. Blueberry roots are shallow and sensitive to drought. Severe drought symptoms have been showing up in several unirrigated blueberry fields. About 75 percent of Michigan’s 22,000 acres of blueberries are irrigated, but even growers with irrigation are struggling to keep up with the very high temperatures and water demand by the plants and the ripening fruit.

It is important to give the plants water before they totally deplete the soil. Blueberries show stress when the available water reaches 50 percent. Allowing soils to dry beyond 50 percent can cause severe stress and reduce berry size. Under these hot conditions, soil water is depleted more quickly, so more frequent irrigations are needed. For sandy soils, smaller more frequent irrigations are the correct way to irrigate. Growers lucky enough to have heavy soils can apply more water less frequently.

Unirrigated blueberries
This unirrigated blueberry field shows where the soil water in a light
sandy soil has been depleted as opposed to the wetter parts of the field
that either have a water table close to the surface, or a heavier soil that
retains more water.

A full-sized blueberry bush probably needs 3 to 4 gallons of water a day. It will survive with less. The first thing to suffer is fruit size with less water to pump up the berries. As the soil dries out, the leaves will burn when they get too hot and the leaf tissue dies and dries out. Also, the berries of this year’s crop will shrivel. When the soils become very dry, this year’s green stems shrivel. At this point, next year’s crop is destroyed. When the green stems shrivel, that stem is dead.

Once a field has been harvested you can reduce the amount of water applied. Without the fruit there is less demand for water. I think you can reduce irrigation by a quarter, but keep a close eye on the bushes and if you see leaves begin to burn, you need to increase the amount of water. To ensure flowers for next year’s crop, you need to maintain the bushes with as little stress as possible through September.

Available soil water in most unirrigated fields is now gone. Our blueberries began to leaf out in April and were fully leafed out in mid-May. A field of mature blueberries with a cover crop uses water at a rate close to the predicted evapotranspiration (ET). Temperature is an important factor; heat increases ET much more than humidity decreases ET.

I have graphed the daily demand and rainfall for one site in southwest Michigan (Bainbridge, Mich., northern Berrien County) in Figure 1. Under the hot, dry conditions in June and July, blueberry fields have lost 0.18 to 0.24 inches per day. Maximum water use during the preharvest fruit growth stage is generally 0.20 to 0.25 inches per day, so our water use is not unusual. What is unusual is the lack of rain. The rains received this year have only been enough to replace a few days of demand. I have graphed the cumulative demand in Figure 2. You can see rainy days as days when the slope of the line dips. These graphs illustrate clearly that the soil water is totally depleted and the only water blueberries have is applied by irrigating.

Figure 1. Daily demand and rainfall for one site in southwest Michigan.
Daily demand and rainfall

Figure 2. Cumulative water demand.
Cumulative water demand

Many experienced blueberry growers have been irrigating continuously since the beginning of June. These same growers report that young plants are dying and berries are shriveling in irrigated fields. One of the problems is that blueberries have a small shallow root system for the size of the plant and it is almost impossible to suck enough water out of the soil to supply the leaves. As the soil dries, roots have harder and harder time extracting water. We are losing over an inch of water a week on soils that only hold 1.2 inches per foot of soil. Growers should be irrigating before 0.6 inches water is lost (three days of 0.20 inches ET) from sands and loamy sands, or 0.8 to 1.5 inches (four to seven days) are lost on sandy loam or loam soils.

Many growers are now being forced to choose which fields to irrigate since they cannot irrigate all their fields fast enough given their pumping capacity. If the water supply is adequate and the limitation is how much acreage can be irrigated at one time, try to cycle through the acreage more rapidly by irrigating each field for a slightly shorter time.

Other growers are running out of water and need to use limited supplies as efficiently as possible. Irrigate at night when evaporation losses are lower so more water gets into the soil. Try to irrigate long enough to replenish the top foot of soil. Although this will require you to extend irrigation intervals (irrigate less often), water is used more efficiently. Watering with small amounts more frequently wets only the top couple inches and more of this is lost to evaporation.

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