Abstract
Two studies were conducted to evaluate different irrigation regimes for production of container-grown sawtooth oak. Three irrigation treatments (single, 3 times daily, and 6 times daily) and 2 substrate treatments (100% pinebark and 4:1 (v:v) pinebark:coir) were evaluated to determine their effects on irrigation application efficiency and growth of sawtooth oak (Quercus acutissima Carruthers) in a pot-in-pot production system. Irrigation application efficiency increased with cyclic treatments compared to a single application and was increased with the pinebark:coir substrate compared to pinebark alone in the single application treatment. Growth was greater when irrigation was applied in 6 cycles than in 1 single application. Trees grown in pinebark:coir substrate were larger than those grown in the pinebark substrate.
This research project was designed to evaluate alternatives to traditional production practices of shade trees for use in the urban landscape. Sawtooth oak (Quercus acutissima Carruthers) is native to eastern Asia, from the Himalayas through China, Korea, and Japan. Sawtooth oak was introduced into the eastern United States in 1862 (Rehder 1940) and adapts well to climates from northern Florida west to eastern Texas and Oklahoma, northward through Missouri, and eastward to New York and southern New England (Gilbert and Henry 1988).
The quality and quantity of water used, along with the leachate leaving container nurseries, is of great concern for nurseries in the United States (McWilliams et al. 1991). With increasing emphasis on water quality, commercial nurseries are being targeted as potential sources of ground and surface water contamination (Evans and Stamps 1996). Although overhead irrigation is inefficient, many container-grown landscape plants are irrigated with overhead sprinklers, especially larger plants (Beeson and Knox 1991). Overhead irrigation may apply 374,000 L/ha daily (40,000 gal/ac), with losses from 40% to 90% through evaporation during application and runoff (Bir 1988).
An alternative to the standard practice of overhead irrigation is cyclic irrigation through a spray stake in each individual container (Martin 1989; Lamack 1993). With cyclic irrigation, a plant’s daily water allotment is subdivided into 2 or more applications with prescribed intervals between applications. This contrasts with conventional irrigation practices whereby the daily water allotment is applied in a single application (Karam et al. 1994). Cyclic irrigation may improve application efficiency by allowing water to move through the container substrate (Karam and Niemiera 1994). Irrigation application efficiency improves up to 38% with cyclic irrigation over single applications (Tyler et al. 1996). In addition to reducing water loss, growers using cyclic irrigation can expect greater plant utilization of applied nitrogen (N) as well as reduced nutrient loss from containers (Karam and Niemiera 1994).
Two studies were conducted to determine the effects of cyclic micro-irrigation and pinebark substrate amended with the peat substitute coconut coir on growth of sawtooth oak as well as irrigation application efficiency in a pot-in-pot production system. Coconut coir is produced from the mesocarp tissue, or husk, of the coconut (Cocus nucifera) fruit. Coir-based substrates have greater water-holding capacities than comparable peat-based substrates (Evans and Stamps 1996). With pot-in-pot production, introduced around 1990 (Parkerson 1990), a “socket” pot is permanently placed in the ground. The container holding the plant is then placed inside the “socket” pot.
Materials and Methods
Experiment 1
Ninety-six uniform bare-root liners, 46 to 60 cm (18 to 24 in.), of sawtooth oak were planted in #15 [56 L (15 gal)] “GripLip” containers (Nursery Supplies, Fairless Hills, PA) in April 1996 in a randomized complete block design with 8 blocks. All treatments (2 substrates × 2 fertilizer × 3 irrigation) were assigned to each block. The substrates were 1) 100% pinebark and 2) 4:1 (v:v) pinebark:coconut coir. Substrate physical properties (Table 1) were determined using the North Carolina State University Porometer (Fonteno et al. 1995). Both substrates were amended with 3.5 kg/m3 (6 lb/yd3) of dolomitic limestone. Trees were topdressed with either 179 or 358 g (6.3 or 12.6 oz) of an 8- to 9-month controlled release fertilizer (Sierra 17-6-10 plus minors, O.M. Scotts Co. Inc., Marysville, OH).
Airspace, water-holding capacity, total porosity, and bulk density of container substrate.
The 3 irrigation treatments were 1) 2,160 mL (72.9 oz) of water in a single application at 10:00 A.M.; 2) 2,160 mL of water applied divided into 3 applications, at 10:30 A.M., 1:00 P.M., and 3:30 P.M.; and 3) 2,160 mL of water divided into 6 applications, at 8:00 A.M., 9:30 A.M., 11:00 A.M., 12:30 P.M. 2:00 P.M., and 3:30 P.M. Irrigation was applied through maxi-jet spray stakes attached to a Bosmith pressure compensating emitter (Acuff Irrigation Company, Cottondale, FL) at a rate of 400 mL (13.5 oz) per minute. Multiple irrigation lines down each block allowed for irrigation treatments to be randomized within each block. Trees were watered every other day until June 3 then daily thereafter. Initial height and trunk diameter measurements were taken following planting in April 1996. Final measurements were taken in September 1996.
Experiment 2
To simulate a pot-in-pot environment, plywood boxes were built and insulated with styrofoam insulation board, 2.54 cm (1 in.) thick, with a hole cut in the top of the box for container placement inside the box. An access door allowed for daily leachate collection. Six trees representing each irrigation and substrate treatment from Experiment 1 were placed in the above-ground pots.
Leachate volumes were recorded from the aboveground pots for each irrigation event. Leachates from three 2-week periods (June 5–18, July 12–25, and August 21–September 3, 1996) were evaluated separately with days used as replications. Soluble salts and pH were determined for each experimental unit monthly using the Virginia Tech Extraction Method (Yeager et al. 1983). Because pH was not affected by any treatments, data will not be presented. The General Linear Model procedure of the Statistical Analysis System (SAS Institute) was used in all analyses of variance.
Results and Discussion
Total airspace was 33% greater for 100% pinebark, while water-holding capacity was 12% greater for the pinebark:coir. Total porosity and bulk density were similar for both substrates (Table 1). Substrate physical properties were within acceptable ranges as reported by others (Bilderback 1980).
Experiment 1
Tree height and trunk diameter were affected by substrates and irrigation treatment, and there was no significant interaction (P = 0.05) (Table 2). Tree height increase was about 23% greater with plants grown in the pinebark:coir substrate compared to the pinebark substrate. With trunk diameter, plants grown in pinebark:coir had a 50% greater increase than plants grown in pinebark. Tree height and trunk diameter were also affected by irrigation treatment (Table 2). Plants grown with the 6-cycle treatment had a 21% greater height increase than plants irrigated with 1 cycle. Trunk diameter followed a similar trend with respect to irrigation treatments. These results support previous work showing an increase in growth with cyclic compared to a single application (Ruter 1997). There were no treatment effects on substrate pH and no fertilizer effects on growth (data not shown). Electrical conductivity was greatest for the cyclic irrigation treatments in August and September among irrigation treatments, and greatest for the 358 g (12.6 oz) fertilizer treatment in June and August among fertilizer treatments (Table 3).
Effects of irrigation and substrate on height and diameter increase of Quercus acutissima in a pot-in-pot production system.
Experiment 2
Irrigation application efficiency was affected by an irrigation × substrate interaction (Table 4). Irrigation application efficiency was greater for the 3- and 6-cycle compared to the single application for all periods. These results are consistent with prior research showing increased irrigation application efficiency with cyclic irrigation (Tyler 1996; Ruter 1997). Irrigation application efficiency was greater for pinebark:coir compared to pinebark for all periods for the single application irrigation treatment. Irrigation application efficiency appeared to increase for the continuous treatment as the season progressed (data not shown), most likely as a response to increased plant needs. Irrigation application efficiency was 100% for the July 12–25 and August 21–September 3 periods for the cyclic treatments for both substrates. These data also suggest that maximum benefits from cyclic irrigation occurs early in the spring or when plants are recently repotted.
Conclusions
Cyclic irrigation increased irrigation application efficiency by reducing leachate volume. The addition of coir to pinebark substrate can increase irrigation application efficiency when a single irrigation event is used. Cyclic irrigation resulted in increased growth of Sawtooth oak compared to a single irrigation event (Table 2). With increasing emphasis on water quality and quantity used, growers might consider changing irrigation practices to improve irrigation application efficiency of container-grown trees. Nurseries need to especially be concerned early in the season when plants are not yet established. It is this time of year when there is less demand for water and fewer roots to take up nutrients. Considering most nurseries fertilize early in the season, it is this time of year that poor irrigation application efficiency can result in increased water quality problems from leaching of nutrients. Many growers of large container plants can apply cyclic irrigation methods without major changes in existing equipment.
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