Commercially acceptable growth rates of woody ornamental nursery stock can be achieved using managed allowed deficits, i.e., deficit irrigations, of 20% to 40% plant-available water (PAW) before initiating irrigation (Beeson, 2006; Welsh and Zajicek, 1993). Deficit irrigation can reduce irrigation frequency (Beeson, 2006), thereby reducing nursery water runoff and energy consumption. Using deficit irrigation for container production of nursery stock is especially valuable for water conservation when rain occurs, because rainfall can account for part or all of a plant's daily evapotranspiration. Rainfall is more effective in penetrating plant canopies than overhead sprinkler irrigation (Beeson and Yeager, 2003), being nearly 100% efficient and more uniform than overhead irrigation. Therefore, less rainfall is required to return a container to capacity than overhead irrigation. However, to apply deficit irrigation strategies, or determine how much rainfall is sufficient and when additional supplemental irrigation is required, knowledge of PAW is necessary. This is especially critical during the early stages of plant production, when roots have not completely colonized a container's volume. At this stage, the lower substrate may be at or near 100% container capacity, whereas PAW may be critically low within the actual root ball.
PAW can be found by first recording the mass of a plant/container with a saturated substrate from which excess water has been allowed to drain. The plant is then allowed to transpire until wilted. PAW is calculated as the difference in mass between saturated and wilted conditions (Beardsell et al., 1979b). For a container with a full root ball, total PAW (PAWTotal) can be consistent across species if the container size and substrate are identical (Beeson, 2006).
Although a similar approach can be applied to plants growing in a container in which roots have not yet filled the container volume, there are drawbacks. Stressing a plant to physical wilt can cause leaf drop in some species and may stunt shoot growth after stress is relieved. Because roots are growing into uncolonized substrate, volume of PAW continues to increase, delaying shoot wilt and therefore skewing the estimate of PAW. In situations in which root colonization of a substrate is not complete, PAW can be termed effective PAW (PAWEffec). This skewing is especially prominent when evaporative demand is low during the drying period. For evergreen species that are dormant or that have rigid leaves and stems, determining the point of plant wilt can be difficult. Applying supplemental irrigation that exceeds the PAWEffec is a waste of water and can lead to excessive leaching of nutrients. Overestimating PAWEffec would result in underirrigation and perhaps commercially unacceptable growth rates. Precision irrigation of container plants requires an accurate estimate of PAWEffec.
An alternative for plants that have not colonized a container's volume is to determine root ball volume relative to total container volume. Volume of a developing root system can be easily determined by water displacement. One could then calculate a percentage of container volume and infer PAWEffec if PAWTotal is known. However, water distribution in container substrates varies by substrate composition (Beardsell et al., 1979a; Bilderback et al., 1982; Richards et al., 1986), container size (Bunt, 1976), and container configuration (Bilderback and Fonteno, 1987; Davis et al., 1964; Spooner, 1974; Williams, 1978). In general, hydraulic gradients are established by opposing forces of capillary rise from water interacting with pores within a substrate and gravity pulling water to the bottom of a container (Bilderback and Fonteno, 1987). Although gradients can be determined, they vary with substrate composition and root ball height (Bilderback and Fonteno, 1987). Thus, simple determination of percent root ball volume of a developing root system in a container cannot accurately predict PAWEffec, even if PAWTotal is known.
With increasing demands being placed on landscape nurseries to reduce ground or watershed water withdraws for irrigation, and to reduce nutrients in runoff water, nurseries should begin practicing precision irrigation, applying only as much water as needed, when needed. To do so, knowledge of PAW accessible at a given stage of growth is needed to accurately apply deficit irrigation strategies. Previously, there has been no research in this area for container-grown nursery crops. Objectives of the research presented here were to: 1) verify a nondestructive method for determining PAW in containers in situ, 2) verify the method was valid across plant species, and 3) to examine relationships between PAW and canopy variables among species. To demonstrate the use of this method, a partial data set from an experiment to quantify water use of Viburnum odoratissimum was used to calculate PAWEffec and PAWTotal during production from rooted cuttings to market-sized plants.
Beeson R.C. Jr 1992 Restricting overhead irrigation to dawn limits growth in container-grown woody ornamentals HortScience. 27 996 999
Beeson R.C. Jr 2006 Relationship of plant growth and actual evapotranspiration to irrigation frequency based on managed allowable deficits for container nursery stock J. Amer. Soc. Hort. Sci. 131 140 148
Beeson R.C. Jr & Yeager, T.H. 2003 Plant canopy affects sprinkler application efficiency of container-grown ornamentals HortScience 38 1373 1377
Bilderback, T.E. & Fonteno, W.C. 1987 Effects of container geometry and media physical properties on air and water volumes in containers J. Environ. Hort. 5 180 182
Bilderback, T.E., Fonteno, W.C. & Johnson, D.R. 1982 Physical properties of media composed of peanut hulls, pine bark, and peatmoss and their effects on azalea growth J. Amer. Soc. Hort. Sci. 107 522 525
Jones, J.W., Allen, L.H., Shih, S.F., Rogers, J.S., Hammond, L.C., Smajstrala, A.G. & Martsolf, J.D. 1984 Estimated and measured evapotranspiration for Florida climate, crops and soils Ag. Exp. Station, Instit. Food and Agric Sci. Univ. of Florida. Bulletin 840
Gaskalla, R. Director Division of Plant Industry 1998 Grades and standards for nursery plants, 2nd ed. Florida Dept. of Agr. and Consumer Services Tallahassee, FL. p. 237
Richards, D., Lane, M. & Beardsell, D.V. 1986 The influence of particle-size distribution in pinebark:sand:brown coal potting mixes on water supply, aeration and plant growth Sci. Hort. 29 1 14
Tyree, M.T. & Hammel, H.T. 1972 The measurement of turgor pressure and the water relations of plants by the pressure-bomb technique J. Expt. Bot. 23 267 282
Welsh, D.F. & Zajicek, J.M. 1993 A model for irrigation scheduling in container-grown nursery crops utilizing management allowed deficit (MAD) J. Environ. Hort. 11 115 118