Water Management for Blueberry Fields
Blueberries have most of their effective root systems in the upper 18 inches of soil. Therefore, because they are shallow-rooted, blueberries are subject to drought injury. A uniform and adequate supply of moisture is essential for optimum growth. As a guide, assume that blueberries require from 1 inch to one and one-half inches of water per week. If this is not supplied by natural soil water or rainfall, irrigation is necessary.
In most areas of western Washington and Oregon, irrigation is required to maintain adequate soil moisture from mid-June to mid-September. The demand for moisture is greatest from the time of fruit expansion until harvest. July and August are the lowest rainfall months and this is the period when the developing fruit makes the greatest demand. This is also the period when floral initiation for next year’s crop begins. If soil moisture and nutrients are lacking, a reduced set of buds will occur.
Some of the newer cultivars are resistant to fruit cracking (see Blueberry Cultivar Selection). However, with a continuous supply of moisture, the fruit skin remains elastic and cracking is less likely to occur. Cracking often occurs after a period of drought. Fruit growth is slowed and the skin becomes less elastic. Then, when precipitation or a period of high humidity occurs, the fruit flesh swells faster than the skin can accommodate it and the skin splits. Fruit may also shrivel under conditions of water stress.
Growers should have an adequate and dependable supply of good quality water throughout the growing season. Refer to Fertilizer Guide 9, Irrigation Water Quality (see Bibliography), for more information.
Several factors need to be considered when scheduling irrigations. The rate at which soil moisture is removed by the plant depends on the air temperature, amount of solar radiation, humidity, wind speed, plant age, stage of plant development, etc. Water will also be lost from the soil surface by evaporation, and cover crops also use available soil moisture. Soils differ in water holding capacity.
Available water holding capacity of soils based on soil texture class (1)
|Soil Texture Class||Available Water Holding Capacity
(inches water/inch soil)
(1)Soil is saturated.
The following example shows how to determine the water holding capacity of the rooting volume, assuming that blueberries are planted on a sandy loam. At a rooting depth of 18 inches, the total water holding capacity of the rooting volume is 18 inches of soil times 0.11 inch of available water/inch of soil depth, which equals 2.00 inches of total water holding capacity. The total water available in the rooting volume should not drop below 35 to 50 percent of the total water holding capacity. This assures easy access to water by roots and prevention of drought stress.
Continuing the example above, the total amount of water available should not fall below 1 inch, half of the 2-inch total water holding capacity. A blueberry plant growing vigorously in summer can evapotranspire up to 0.25 inches per day. With 1.00 inches of water available in the rooting volume, and approximately 0.25 inches per day being used, it only takes 4 days for the blueberry plant to use all available soil moisture before it is stressed. Thus irrigation will be highly desirable for this soil under peak use conditions. In general, blueberries grown in light soils with low water holding capacities will benefit from irrigation during most years.
Although in the northern United States a vigorously growing blueberry plant in the summer is estimated to evapo-transpire up to 0.25 inches per day, we do not have values for daily water use rates of blueberry plants in the Pacific Northwest. Values here depend on cultural conditions, soil, weather, etc., and may be anywhere from 0.25 to 0.5 inches of water per day in warm, dry weather. It is evident that irrigation would be required during the summer months to prevent drought stress.
Various methods can be used to schedule irrigations. Some are more reliable than others. For example, when plants show visible signs of water deficit, such as wilting of foliage or shriveling of berries, plant growth has already been stressed. Irrigation at this point may save the crop, but production has already been limited.
A somewhat better method is the “feel” method, in which the soil’s appearance after being squeezed by hand is used to estimate water content. After much experience, this method may be quite reliable, and charts are available to describe how different soils should look and feel at different moisture contents. However, a common mistake is to feel the surface soil layers rather than soil around the root tips where most moisture is taken up.
Evaporation pan method
In strawberries and raspberries, evaporative pan data can be used to schedule irrigations. The ratio of consumptive water use (CUc) to evaporation (E) is known as the proportionality factor (Kc). For raspberries and strawberries, the Kc value has been calculated for the Pacific Northwest. This value is necessary to schedule irrigations.
For highbush blueberries, a Kc factor has yet to be determined for the Pacific Northwest. In the absence of pan evaporation data, in-field soil monitoring methods will need to be used to schedule irrigations.
Soil moisture monitoring devices
Tensiometers and electrical resistance blocks (gypsum blocks) are the two most commonly used methods to monitor soil moisture. Both of these types of sensors need to be placed at a depth of one-third to two-thirds of the active rooting depth of the crop at 6 inches and 12 inches deep for blueberries.
Depending on the soil type, tensiometer readings approaching the 60 to 65 centibar level at the 12-inch depth indicate the need for supplemental water. A reading of 10 centibars or less indicates the soil is at, or near, field capacity.
Blueberry growers in the Pacific Northwest use either sprinkler or drip irrigation systems. Sprinkler irrigation can also be used for frost control (see Winter Acclimation and Cold Hardiness ). Of course, the system has to be set, and enough water and pressure has to be available, to irrigate the entire field at once until the risk of frost is over.
For small fields with a limited amount of water, a drip system with initially one emitter per plant, followed by the addition of a second emitter as the plants mature, is commonly used. Two emitters per plant, one on each side, are recommended for mature plantings.
With only one emitter per plant, it is important that it not be placed near the center of the crown as this may lead to problems of phytophthora root rot. Also, in fields newly planted with container stock, the emitter should be placed near the edge of the medium so that the lighter soil of the potting medium wets before water moves into the heavier surrounding soil. In general, sprinkler irrigation is best for young plantings.
Blueberry plants cannot translocate (move within the plant) water, or most nutrients, laterally. Therefore, if only half the root system gets water or fertilizer, only half of the top portion benefits. It is important for drip irrigated fields to have a uniform water supply.
Once the emitter system has been installed, it should not be disturbed, as roots will grow predominantly in the wetted zone. The main advantage of drip irrigation systems is in maintaining even soil moisture levels, especially when a mulch is used.
Compared to overhead sprinkler irrigation systems, drip irrigation is more efficient, with less water loss; fertilizers may be applied through the drip; and plants can be irrigated during harvest. The risers of sprinklers may also interfere with machines on those fields that are machine harvested. The negative aspects of drip irrigation are the facts that dust cannot be washed off fruit, plants cannot be cooled by irrigation on hot summer days, frost protection is not possible, a cover crop cannot be irrigated, and maintenance to prevent emitters from clogging is more time-consuming and more difficult to monitor than with overhead sprinklers.
For maximum efficiency of a drip irrigation system, growers should seek professional advice on water purification, equipment selection, and system layout to ensure uniform water distribution. Note that in hot, dry summers, many growers in the Pacific Northwest have not been able to supply drip irrigated blueberry fields with sufficient water for adequate renewal growth and maximum fruit size. Overhead irrigation works best under these conditions. Some growers supplement drip irrigation with overhead systems.
This fact sheet is adapted from Oregon State University Extension Publication PNW215, Highbush Blueberry Production. The authors of Highbush Blueberry Production are – Oregon State University: Bernadine Strik, Glenn Fisher, John Hart, Russ Ingham, Diane Kaufman, Ross Penhallegon, Jay Pscheidt and Ray William; Washington State University: Charles Brun, M. Ahmedullah, Art Antonelli, Leonard Askham, Peter Bristow, Dyvon Havens, Bill Scheer, and Carl Shanks; University of Idaho: Dan Barney. PNW215, Highbush Blueberry Production can be purchased from the Department of Extension & Experiment Station Communications, Oregon State University. How to Order