Woody Plants in Alternative Media

As growers face limited supplies of pine bark-the industry standard for container plant production-alternative substrates are filling in. But woody plants grown in these media must prove their performance in the landscape as well as in production, and research has shown that several varieties fare well.
By S. Christopher Marble, Glenn B. Fain and Charles H. Gilliam

Pine bark is the most common nursery substrate used for horticulture container crop production in the Southeastern United States. In recent years, pine bark supplies for horticultural production have begun to decline due to reduced domestic forestry production and increased use of pine bark as a fuel. In many areas in the Southeast, pine bark suppliers are unable to fill orders for container nursery producers with limited supplies, leading to possible price increases. So it's important to develop alternative substrates for use in container production of horticultural crops.

Container substrates composed primarily of wood and wood-based products have been heavily investigated in recent years. The use of wood fiber substrates has been successful in production of vegetables, annuals and perennials and woody ornamentals. For alternatives to be considered suitable substrate replacements, they must not only have desirable characteristics as a container substrate - adequate drainage, inert, pathogen free, and so on - but must also cause no negative fertility effects (for example, nitrogen immobilization) following planting in the landscape. Before widespread adoption of these materials can take place, landscape performance of plants produced in these substrates must be established.

In a study conducted in Alabama in the mid- to late 2000s, researchers evaluated the landscape performance of three commonly grown woody ornamentals - crape myrtle, magnolia and oak - following container production in alternative wood-based substrates (WholeTree [WT] and Clean Chip Residual [CCR]). Results showed that all species performed similarly following planting into the landscape when grown in WT or CCR compared to the pine bark industry standard.

The study showed that there is potential in the use of WT and CCR as alternatives to pine bark. WholeTree substrate consists of entire pine trees harvested from plantations at the thinning stage (about 10 to 15 years) and hammer milled through specific screen sizes, depending on crop needs. WholeTree is composed of the entire shoot portion of the pine tree (wood, limbs, needles and cones) and contains approximately 80 percent wood fiber. Clean Chip Residual is also a by-product of the forestry industry. Mobile equipment is now being used for in-field tree harvesting operations that process pine trees into "clean chips" for pulp mills. The remaining material, CCR, is then sold as boiler fuel or spread across the harvest area. Clean Chip Residual contains approximately 50 percent wood, 40 percent bark and 10 percent needles.

It works for annuals

Research suggests WT and CCR can be used successfully to produce a wide variety of container grown landscape plants; however, no studies have focused on the post-transplant landscape performance and survival of woody plants grown in either substrate. Previous research has shown plants grown in a wood fiber substrate may require additional nitrogen (N) applications to produce growth similar to plants grown in pine bark or peat moss. Further, wood particles incorporated into the soil or when used as a landscape mulch have also been shown to cause N immobilization. A review of the literature identified only one study in which landscape performance of plants previously grown in an alternative wood fiber substrate was evaluated. However, in this case, annual bedding plants were evaluated for survival and growth, and as annual bedding plants were evaluated, these studies only lasted several months. The annuals investigated were: begonia (Begonia × semperflorens-cultorum) 'Cocktail Vodka' and 'Cocktail Whiskey'; coleus (Solenostemen scutellarioides) 'Kings-wood Torch'; impatiens (Impatiens walleriana) 'Dazzler White'; marigold (Tagetes erecta) 'Bonanza Yellow' and 'Inca Gold'; petunia (Petunia × hybrida) 'Wave Purple'; salvia (Salvia splendens) 'Red Hot Sally'; and vinca (Catharanthus roseus) 'Cooler Pink'. All species had been previously grown in a pine tree substrate made from Pinus taeda or pine bark prior to being planted into the landscape at three different fertilizer rates. Results indicated that, while N immobilization occurred with no N addition, growth and performance of annuals in the landscape were similar for pine tree substrate and pine bark under fertilized conditions.

Although bedding plants have shown acceptable landscape performance following container production in a high wood fiber substrate, no research has yet focused on landscape performance of woody plants, which have a much longer lifespan in the landscape. Nitrogen deficiency from incorporation of wood particles from container substrates with high wood content - WT and CCR - could be problematic for the landscape industry if growers shift to using alternative substrates for container plant production. Therefore, our objective was to evaluate the performance of three woody ornamentals - crape myrtle, magnolia and oak - originally grown in WT, CCR or pine bark following planting into the landscape.

The study

Plants used in this study were container grown in pine bark, WT or CCR for an entire growing season prior to being planted in the landscape. They then were allowed to establish for two growing seasons.

On March 25, 2008, three species of woody ornamentals including crape myrtle (Lagerstroemia indica × faurei 'Acoma'), magnolia (Magnolia grandiflora 'D.D. Blanchard') and oak (Quercus shumardii) were transplanted from 3-inch, 4-inch and No. 1 liners, respectively, into No. 3 containers containing WT, CCR or pine bark. The CCR was obtained from a 10-year-old loblolly pine (Pinus taeda L.) plantation in Atmore, Ala., that was being thinned using a total tree harvester.

Following harvest, trees were processed through a horizontal grinder with a 4-inch screen to produce the CCR. The CCR was delivered to Auburn University's E.V. Smith Research Station in Tallassee, Ala., on March 29, 2007, and was stored in an uncovered pile exposed to ambient climate (not aerated or turned) for about one year.

The CCR was then further processed through a swinging hammer mill to pass through a one-half-inch screen on March 11, 2008. The WT was obtained from a pine plantation (about 10 years) in Georgetown, Ga., with pine trees harvested at ground level and the entire shoot portion chipped. The WT was delivered to the E.V. Smith Research Station on Jan. 18, 2007, aged for 432 days in a similar manner to CCR, and then further processed using the swinging hammer mill to pass through a one-quarter-inch screen on March 3, 2008. Aged pine bark was of unknown age.

Each substrate treatment was mixed with sand on a 6:1 (v:v) basis. Each substrate was pre-plant incorporated with 18N-2.6P-9.9K (18-6-12) Polyon® (8- to 9-month formulation) at 14 lb<0x00B7>yd - 3, or approximately 84 grams of product per tree; 5 lb<0x00B7>yd - 3 dolomitic limestone; and 1.5 lb<0x00B7>yd - 3 Micromax®. All amendments were incorporated into each substrate treatment on the day of potting. Following transplanting, plants were placed outdoors on a gravel container pad and overhead irrigated twice daily, delivering one-half inch in total.

Plants were arranged by species in a randomized complete block design with 20 single pot replications per treatment, and were grown in containers for nine months. In December 2008, six plants from each substrate treatment were chosen to be planted out into the field by selecting plants with a similar growth index [(plant height + plant width1 + plant width2) / 3] based on Tukey's Mean Separation Test (P <0x2264> 0.05). Plants were transplanted by species into a clay-loam soil (pH 6.2) at the Old Agronomy Farm, Auburn University, Ala. Oaks were planted into two rows with 12 feet in between rows; each plant was spaced 8 feet apart. Crape myrtles and magnolias were planted into three rows (three separate rows for each species) spaced 10 feet apart with 5 feet in between each plant. Plants within each species were arranged in a randomized block design with pairs of plots for each substrate randomized within each of three blocks.

Although plants were watered in by hand following transplanting, they received only rainfall thereafter. All plants were mulched at the time of transplanting with pine straw (2-inch thickness) and again on June 30, 2010. They were fertilized on June 25, 2009, by broadcasting Polyon® (8- to 9-month formulation) 13N-5.6P-10.9K (13-13-13) at a rate of 1 pound of product per 1,000 square feet. Weed control was conducted by hand weeding and applying directed applications of RoundUp Pro® herbicide at a 2 percent spray solution.

Caliper measurements were taken on Nov. 1, 2010, by measuring trunk circumference at 6 inches above the soil line; plant height was also measured at this time. Landscape marketability ratings were taken on the same date, and were measured on a scale of 0 to 5: 0 = completely non-marketable (chlorotic or yellow foliage, sparse canopy); 3 = acceptable for landscape planting (green foliage, good canopy); 5 = very marketable (dark green foliage, dense, lush canopy).

On Nov. 3, 2010, all plants were destructively harvested. Plant shoots were cut at 6 inches above the soil line. Roots were extracted by connecting a clamp to the stump just below the soil surface, connecting the clamp to a hydraulic cylinder mounted on the front of a small tractor, and raising the cylinder mount until the taproot and lateral roots were loosened from the soil. Coarse roots were removed with the hydraulic cylinder, with additional loose roots collected by hand. It is likely that the extraction method did not recover all fine fibrous roots for each tree; however, as the same technique was used for all three species in all plots, the relative amount of roots removed should be comparable. Following destructive harvest, shoots and roots were dried in a forced air oven at 131°F for 14 days, at which time dry weights were recorded. Data were subjected to analysis of variance (ANOVA) with means separation by Tukey's Studentized Range Test (P ? 0.05) using the Proc GLM feature of SAS

How the woody plants fared

Height and caliper measurements show that no differences in plant growth occurred regardless of substrate used during container production in any of the three species evaluated (Table 1). While height and caliper measurements were similar among treatments in each species, differences were observed when comparing shoot dry weight of crape myrtle (Table 2). Crape myrtles previously grown in CCR had higher shoot dry weights than plants previously grown in WT. However, both alternative substrates (WT and CCR) had similar shoot dry weights to the pine bark standard. Magnolia and oak shoot dry weights revealed no difference among treatments. Root dry weight indicates that each species had similar root growth regardless of substrate treatment (Table 2).

Regardless of container substrate used during plant production, all three species evaluated performed similarly in terms of growth following planting in the landscape. In addition, visual marketability ratings taken before harvest indicate that all plants would be considered marketable in a home or commercial landscape setting (Table 3).

Previous work has demonstrated that when wood byproducts are used as container substrate, N immobilization may occur, requiring additional fertilizer applications for optimal plant growth. Soil N content (along with plant visual ratings and growth) has also been shown to decrease as organic mulch depth increased. Others have shown that decomposing mulch material is readily available to soil microorganisms, thereby reducing plant available N as soil microorganisms outcompete plants for nutrients.

It should be noted that fresh wood-based materials were used in many of the above cases. However, it has been shown that using fully composted or aged materials may reduce N immobilization and alleviate the need for additional N fertilizer applications in containers. Further, aged materials are considered a more desirable container substrate due to smaller particle size and better water holding capacity. Previous work showed that growth and flowering of petunias (Petunia × hybrida 'Dreams White') and marigolds (Tagetes patula 'Little Hero Yellow') was greater when grown in aged WT substrate when compared to fresh WT. Substrates in the current study were aged before use; it is possible this may have alleviated any potential N immobilization since all plants displayed similar growth.

As container nursery growers continue to seek alternatives to pine bark, suitable substrates must not only provide acceptable growth during container production, but must also demonstrate no negative effects (for example, N immobilization) following transplanting. The woody ornamental species evaluated in this study performed similarly in a landscape setting with the same amount of fertilizer after previously being grown in properly aged, alternative wood-based WT or CCR substrates, compared to the industry standard pine bark. Whether fresh material of WT or CCR can be used with similar results requires further research.

S. Christopher Marble is a graduate research assistant, Glenn B. Fain is an associate professor and Charles H. Gilliam is a professor in the Department of Horticulture at Auburn University, Auburn, Ala. They can be reached at marblsc@auburn.edu, gbf0002@auburn.edu and gillic1@auburn.edu, respectively. Additional authors include G. Brett Runion, a plant pathologist; Stephen A. Prior, a plant physiologist; and H. Allen Torbert, a soil scientist and research leader at the USDA-ARS National Soil Dynamics Lab in Auburn, Ala. Daniel E. Wells is a graduate student at the Louisiana State University School of Plant, Environmental, and Soil Sciences in Baton Rouge.

Research was supported by the Horticultural Research Institute and the USDA-ARS Floriculture Nursery Research Initiative, and a version of this report was published in the Journal of Environmental Horticulture, March 2012.

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