Pulp or Plastic?
Research comparing plastic to pulp containers shows there's little difference in plants' growth performance, but in some regions and under some conditions, container type may affect mortality.
Current container nursery crop production in the U.S. consumes a substantial amount of plastic and water, raising environmental concerns. Decreasing consumption of plastic and reducing water use will assist the nursery industry in achieving greater sustainability and protecting natural resources. The research reported here examined the use of biocontainers and a sustainable irrigation system to determine how nursery producers can most practically - and profitably - adopt sustainable practices. Preliminary results suggest there is no effect of container type on growth. In some locations, however, research showed that container type may affect mortality.
In pot-in-pot trials at Crystal Springs, Miss., plants gained approximately 20 cm in height regardless of container type.
Plastics and alternatives
In growing operations across the country, above-ground nursery production and pot-in-pot production rely almost exclusively on plastic containers. In 1993, approximately 240 million pounds of plastic were generated by the nursery industry in the manufacture of high-density polyethylene and polypropylene nursery containers; that accounts for 58.8 percent of total plastics consumed by the nursery and floriculture industries. Just 1 percent of horticulture plastics are recycled, in spite of the fact that plastic pots and trays are recyclable. Non-plastic containers are slowly being adopted by a select number of businesses, but there remain several lingering concerns, including durability during plant production and shipping, biodegradability in the landscape, and plant growth during and post-production. Despite the challenges for growers, market demand is increasing: Research shows that consumers may not only desire biodegradable containers but may be willing to pay more for them.
Plants grown in biocontainers used in above-ground research showed little difference in growth compared to those grown in plastic pots, but mortality rate was significantly lower.
Water is essential to container nursery crop production. Because the nursery industry has shifted from primarily field-produced crops to container-produced crops, the need for irrigation water is increasing. Over 75 percent of nursery crops in 17 of the major nursery producing states are currently grown in containers. A container nursery with 70 percent of the land in production under overhead irrigation could use between 14,000 to 19,000 gallons of water per acre per day during the peak growing season.
While demand is unlikely to decrease, reliable supplies of water may do so. Scientists and industry leaders anticipate less water available for production in the future. U.S. municipalities in all or part of California, Florida, North Carolina, Texas and Oregon have already responded to competition for water and/or concerns regarding water quality and runoff with container nursery irrigation restrictions.
The objectives of this study were to test pulp-based containers for water use, plant performance and container strength in above-ground production plots and pot-in-pot production, and to examine a conservative irrigation regime based on water consumption for above-ground container production.
Above-ground container experiment
In mid-May 2011, rooted Euonymus fortunei 'Roemertwo' Gold Splash® cuttings were potted with 85 percent pine bark:15 percent peat (vol/vol) into 1-gallon conventional plastic pots or one of two pulp-based biocontainers. The pot sizes were 3.8 L for plastic and 3.9 L for both biocontainers. Plants were fertilized with 8 g of 19.0N-2.2P-7.5K per container or comparable fertilization. Irrigation application volume replaced 100 percent of the water used since the previous substrate moisture measurement. Dielectric probes connected to a datalogger were used to measure volumetric water content (two probes per irrigation zone replicate). The datalogger program calculated evapotranspiration and then opened solenoid valves for the appropriate time to apply what was lost through evapotranspiration. Plants were irrigated twice daily.
Figure 1. Euonymus fortunei Gold Splash® mortality grown above-ground in three container types (El Paso, Tex.).
Irrigation was applied through four overlapping shrub spray sprinklers per irrigation zone. Emitters were mounted on 1.3-cm diameter risers at a height of 66 cm. Irrigation zones were 10 square feet. A single solenoid valve provided irrigation for each treatment replicate. The experiment was a randomized complete block design. There were three replicate zones per treatment and 15 plants per zone. Sixteen border plants were included in each zone but not utilized for data collection.
Above-ground production experiments were conducted in Illinois, Kentucky, Michigan, Mississippi, West Virginia and Texas. Mortality, height and growth index were recorded monthly from June through October; mortality was reported as the cumulative mortality for the season.
Preliminary results are presented from El Paso, Tex., and East Lansing, Mich. In Texas, there were no differences in plant height or growth index. Plants gained approximately 7 cm in height over the course of the experiment, and the growth index increased from approximately 11 at the beginning of the experiment to 18 at the termination of the experiment.
Plant mortality was significantly greater for the plastic pots than either biocontainer: 60 percent mortality versus 20 percent in each of the biocontainers (Figure 1). Plastic pots may have had substantially higher temperatures than biocontainers, as plastic container temperatures can exceed 135 °F in the southern U.S. during the growing season. Because fiber containers are porous, plants in them may have benefited from evaporative cooling.
In Michigan, there also was no significant difference in plant height or growth index between pulp-based container and plastic container (control). The average height of plants had increased from 12 cm (June 30, 2011) to 20 cm (Oct. 18, 2011). The growth index ((height + width1 +width2) / 3) had increased by five units during the experimental growing season.
Figure 2. Dry weight of Euonymus fortunei Gold Splash® grown above-ground in three container types (East Lansing, Mich.).
The plant mortality was 13 percent in plastic containers, which was higher than 2 percent in pulp-based containers. Plastic containers generally had greater temperature than pulp-based containers. The biomass (dry weight) of each treatment was measured on Sept. 24, 2011; there was no significant difference in biomass between treatments (Figure 2).
Pot-in-pot production experiment
Betula nigra bare root liners were potted with 85 pine bark:15 peat (vol/vol) into 7-gallon containers in mid-June 2011. The containers were a conventional black plastic pot or one of two pulp-based biocontainers. A 15-gallon plastic container served as the socket pot. A gap existed between the production and socket pot for some container types. This gap was filled with bubble wrap and sealed. Copper-treated fabric was placed between each production and socket pot to prevent roots from escaping the production pot and rooting into the soil.
Container moisture content was determined with a theta probe. Irrigation was applied to replace 100 percent of daily water use. Irrigation was delivered with one emitter per container.
The experiment was a completely randomized design with eight replications, conducted from July through October 2011. Pot-in-pot experiments were conducted in Kentucky, Michigan, Mississippi and Texas.
Preliminary results are reported for Crystal Springs, Miss., and East Lansing, Mich. In Mississippi, there was no effect of container type on plant biomass. Plants gained approximately 20 cm in height regardless of container type. Growth indices increased from 40, based on measurements taken near the beginning of the experiment (July 21, 2011) to 58 on Oct. 3, 2011. Minimal plant mortality occurred in the pot-in-pot experiment and was independent of container type.
In Michigan, there was also no effect of container type on plant height and growth index. The plant growth index had increased from an average of 23 cm (June 30, 2011) to 67 cm (Oct. 3, 2011). The plant height generally increased by 40 cm regardless of treatment. There was no plant mortality during the growing season.
What did we learn?
These preliminary data suggest that pulp-based containers do not have negative consequences on plant growth in above-ground or pot-in-pot production. In fact, plant stress may be reduced and survival may be greater in biocontainers compared to conventional black plastic containers when used in above-ground container production. Future research will investigate water consumption and temperature differences between biocontainers and conventional black plastic containers, strength of biocontainers during and after production, and landscape plant performance.
Results of this research were presented at the 2012 SNA Research Conference. For information about the Southern Nursery Association and the 2013 SNA Research Conference, visit www.sna.org.
Lead author Amy Fulcher, Ph.D., is an assistant professor for sustainable ornamental plant production and landscape management in the department of plant sciences at the University of Tennessee, Knoxville. She can be reached at email@example.com.
Genhua Niu, Ph.D., is an associate professor of environmental horticulture at the Texas AgriLife Resarch Center of Texas A&M University in El Paso.
Guihong Bi, Ph.D., is an associate research professor of ornamental horticulture at Mississippi State University's Central Mississippi Research and Extension Center in Crystal Springs.
Michael Evans, Ph.D., is a professor in the department of horticulture at the University of Arkansas, Fayetteville.
Tom Fernandez, Ph.D., is an associate professor in the department of horticulture at Michigan State University, East Lansing.
Robert Geneve, Ph.D., is a professor in the department of horticulture at the University of Kentucky, Lexington.
Andrew Koeser is a research assistant in the department of crop sciences at the University of Illinois, Urbana.
Susmitha Nambuthiri is a researcher in the department of horticulture at the University of Kentucky, Lexington.
Nick Pershey is a former graduate research assistant in the department of horticulture at Michigan State University, East Lansing.
J. Ryan Stewart, Ph.D., is an associate professor in the department of plant and wildlife sciences at Brigham Young University, Provo, Utah.
Sven Verlinden, Ph.D., is an associate professor of horticulture in the division of plant and soil sciences at West Virginia University, Morgantown.
Xueni (Vickie) Wang, Ph.D., is a graduate research assistant in the department of horticulture at Michigan State University, East Lansing.
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