Research on Most Effective Way to Keep Your Blueberries Hydrated
A major goal for blueberry growers during the summer months is to produce strong, healthy vegetative growth flushes which will produce most of the fruiting wood for next year’s crop. Among many things, sufficient water available for plant uptake is needed to accomplish this goal.
Blueberry is a shallow-rooted species and is susceptible to drought stress, especially under the growing conditions commonly found in Florida. Most Florida blueberry production is in sandy soils amended with pine bark, or in pine bark beds on top of well-drained sandy soil. Both situations have relatively low water holding capacities making irrigation management particularly challenging under conditions of high evaporative demand. Pine bark water holding capacity varies with particle size, age, and level of decomposition. Other things being equal, large particle size pine bark generally has less water holding capacity than smaller particle sizes and “new” bark retains less water than “aged” or partially decomposed bark. At field capacity, aged pine bark may hold up to 21% water (by volume) compared to only 12% (by volume) for new pine bark (Smith, Krewer and Ruter, 2012).
Water distribution from drip emitters in pine bark can be limited. Rapid water infiltration rates with limited lateral movement (channeling) often occurs in bark beds resulting in inadequate root zone coverage. Increasing irrigation run times will apply more water below the root zone where it is unavailable for plant uptake but will do little to increase root zone coverage. To improve root zone coverage, many growers use two drip lines per row and/or supplement drip irrigation with overhead irrigation during prolonged droughts. Properly designed micro-sprinklers provide excellent coverage of blueberry root zones but they must be properly maintained. As bark ages, lateral water movement and root zone coverage from drip emitters generally improves. However, new plantings with fresh bark can be very difficult to irrigate efficiently.
When establishing a new planting with fresh pine bark, the bark can be applied ahead of planting along with nitrogen fertilizer and periodic overhead irrigation to begin conditioning the pine bark for better irrigation management after planting. Depending on the bark composition and the amount of nitrogen applied, this should be done one to three months before planting (Smith, Krewer and Ruter, 2012).
Figure 1 shows the average daily evapotranspiration of mature ‘Emerald’ blueberry plants in north-central Florida for each month throughout the year. Water use was relatively low during winter but increased rapidly during spring and peaked during late summer. The long, hot days and large plant canopies contributed to the high water demand of blueberry plants during mid to late summer. In this case, the average daily water use per plant approached, or slightly exceeded, 2 gallons during mid to late summer (July, August, and September) (Fig. 1). Individual daily water use varied from the monthly averages and was significantly greater than the monthly averages under conditions of high evaporative demand. Given the high water demand during the summer months, the shallow blueberry root system, and the limited water holding capacities of most soils in commercial Florida blueberry fields, efficient use of irrigation to minimize stress and support rapid vegetative growth and plant health presents challenges.
Maximizing root zone coverage as much as possible is important since there is a tendency for water to move vertically, not horizontally, in pine bark, or in pine bark amended sands. When using drip irrigation, during extended dry weather it may be necessary to occasionally supplement your normal irrigation with overhead irrigation. Because of the limited water holding capacities of these soils and the shallow rooting depth of blueberry, multiple irrigations per day of short durations are more efficient than long irrigation run times which allow much of the water to move below the root system. The objective is to bring the wetted portion of the root zone to field capacity without applying excess water and fertilizer below the root zone where it is unavailable for plant uptake. When pulse irrigating, it is important that the wetted zone does not remain excessively wet but is allowed to partially dry between irrigations. There are various soil moisture monitoring devices but not all are effective in pine bark and sand/bark mixtures. Moreover, special calibration may be needed for highly porous soils such as pine bark or pine bark amended soils.
By Jeff Williamson, IFAS, University of Florida, Gainesville, FL
Literature Cited and Further Reading
Smith E., G. W. Krewer and J.M. Ruter. 2012. Fertilizing Blueberries in Pine Bark Beds. University of Georgia. http://extension.uga.edu/publications/detail.html?number=B1291&title=Fertilizing%20Blueberries%20in%20Pine%20Bark%20Beds
Williamson, J.G., L. Mejia, B. Ferguson, P. Miller and D.Z. Haman. 2015. Seasonal Water Use of Southern Highbush Blueberry Plants in a Subtropical Climate. HortTechnology. 25:185-191. http://horttech.ashspublications.org/content/25/2/185.full
Munoz-Carpena, R. Field Devices For Monitoring Soil Water Content. Bulletin 343. University of Florida http://edis.ifas.ufl.edu/pdffiles/AE/AE26600.pdf