Glenn Fain, Ken Tilt, and Jeff Sibley

Production of nursery crops has changed dramatically in the last 40 years, and pressures are mounting to more efficiently use water and nutrients in nursery settings. Recent AAES-sponsored research in the Department of Horticulture at Auburn University has shown that using a water-retentive plant media and cyclical irrigation can reduce water needs and nutrient runoff problems.

The most significant changes in the nursery crop industry have occurred in three areas: a shift from primarily field-grown to primarily container-grown plant production; development of soilless media components (such as pinebark, ricehulls, and peat products); and improvements in irrigation practices. Progress in the container production area has brought about the pot-in-pot production system, which was evaluated in this study. Pot-in-pot, introduced around 1990, is a nursery production method that combines some of the benefits of both field and container production. In this system, a “socket” pot is permanently placed in the ground and a container plant is then placed inside the “socket” pot. Growers continue to evaluate various moisture holding components to blend with pinebark, the primary soilless media used by southeastern growers. In the media portion of this study, pinebark was amended with coconut coir or peat to determine if one of these blends would reduce potential container run-off from entering groundwater. Nursery irrigation is still in the early stages of development. Many growers still remember and some still practice opening gate valves (water spigots) timed by wrist watches. Automatic systems are now in use in many nurseries. Auburn research recently refined automation to show that cyclic micro-irrigation is a more efficient alternative to the standard practice of overhead irrigation. Micro-irrigation applies water through individual tubes to each container. With cyclic irrigation, a plant’s daily water needs are subdivided into more than one application with a small volume applied at specified intervals. Conventional irrigation practices apply the daily water volume in a single application resulting in high run-off through the porous medium.

An AAES study determined the effects of cyclic micro-irrigation on growth of red maple (Acer rubrum), cultivar Franksred (Red Sunset^TM), grown in three different blends of coconut coir, peat, and pinebark. Seventy-two bare root tree transplants, five to six feet in height, of cultivar Franksred were planted in 15-gallon containers in April 1997 and grown in full sun. Three different media were used: 100% pinebark; pinebark:peat (4:1 by volume); and pinebark:coconut coir (4:1 by volume). Each media was amended with 7.7 pounds per cubic yard of dolomitic limestone and, after planting, trees were topdressed with 11.8 ounces of Osmocote 15-9-11. Initial height and trunk diameter were taken after trees were planted, and final growth measurements were taken at the end of the study on September 23, 1997.

For each media, three irrigation treatments were compared: application of a given volume in a single application at 10 a.m.; the same volume divided into three equal applications at 10:30 a.m., 1 p.m., and 3:30 p.m.; or the same volume divided into six equal applications beginning at 8 a.m. with 90 minutes between cycles. Initial irrigation volume from April to mid-June was 2.5 quarts per tree; from mid-June to mid-July the volume was increased to 4.5 quarts per tree; and from mid-July until harvest the volume was increased to 5.8 quarts per tree. Irrigation was applied through maxi-jet spray stakes with a Bowsmith model HPC6 pressure compensating emitter at a rate of 13.5 ounces per minute. Spray stakes deliver water in a 360-degree even pattern across the media surface.

Of the three media blends tested, water-holding capacity was greatest for pinebark:peat and pinebark: coir. Tree growth was affected by media and irrigation treatments, as shown in the table. Shoot dry weight was about 8% greater with trees grown in pinebark:peat compared to trees grown in pinebark alone. Trees grown in pinebark:peat were 7.09 inches higher than those grown in pinebark:coir and 5.51 inches higher than those grown in pinebark alone (see the table).

Trees grown with cyclic irrigation had the greatest shoot dry weight, with trees receiving the three-cycle and six-cycle irrigation treatments having 23% and 17% greater shoot dry weight respectively than trees grown with a single irrigation application (see the table). Trunk diameter of trees receiving three-cycle and six-cycle irrigation treatments were 23% and 26% greater, respectively, than that of trees grown with a single application irrigation. Tree height also was affected by irrigation treatment. Trees grown with three-cycle irrigation had a 16% greater height than trees grown with a single irrigation application.

Cyclic irrigation reduced the total amount of nitrogen (N) leached by a minimum of 89% in June and August when compared to a single irrigation application. While N concentration was generally higher in cyclic treatments, reduced leachate volume (i.e. greater irrigation application efficiency) resulted in less total N leached. For example, with the six-cycle irrigation in August the N concentration was 34.6 ppm; however, total N leached per pot was 0.2 mg per pot. This is a 99% reduction compared to the single irrigation application. This study indicates greater retention of N with cyclic versus a single irrigation application. Leachate N concentration was greatest for 100% pinebark in June at 9.3 parts per million (p.p.m.) compared to 5.5 and 6.0 p.p.m. for pinebark:peat and pinebark:coir, respectively.

Cyclic irrigation appears to improve irrigation application efficiency by allowing time for water to move slowly through the micropore system of a container media. Growers using cyclic irrigation can expect greater plant utilization of applied N and reduced water and nutrient loss from containers. Earlier work at Auburn has shown a 47% reduction in N leached from nursery pots irrigated with cyclic treatments compared to a single overhead application.

With increasing emphasis on water quality as well as quantity used, growers should consider changing management practices to improve irrigation application efficiency of container-grown trees. Cyclic irrigation and peat or coir amended pinebark media are proactive methods to improve water quality by reducing runoff and nutrient loss from containers. Reduced leachate volume and increased N retention in the media may allow for more effective use of controlled-release fertilizer and thereby reduce potential negative impacts on the environment.

These results show that both three- and six-cycle irrigation may lead to increased growth in production of red maple and other trees when compared to current single application practices. The pinebark:peat blend (4:1 by volume) produced increased shoot dry weight over 100% pinebark and pinebark:coir (4:1 by volume). Furthermore, many growers of large container plants can apply cyclic micro-irrigation methods without major changes in existing equipment.