Nursery producers are under increasing pressure to maintain production efficiency and grow high-quality woody plants in a cost-effective and environmentally conscious manner. The industry standard for years has been the black plastic (BP) container. A number of companies have introduced alternatives to black plastic containers in recent years, with the idea of growing healthier plants and also to provide sustainable and recyclable products to the industry. Black plastic containers are lightweight, durable, familiar to growers and well-suited for mechanization. However, the disadvantages of growing trees in black plastic containers are numerous: Smooth sides lead to circling roots (which can lead to girdling roots in the landscape after planting); substrate temperatures often exceed lethal limits for root growth due to solar absorption of the container; and while these containers have the potential to be recycled, this is seemingly a rare practice.
One alternative to BP containers is the use of fabric types. Fabric containers have been available for over 20 years, and there are many brands—as well as sizes and shapes—available. Growers do not appear to be convinced that the benefits outweigh the negatives, though, as plastic containers are still the most common container type used in the nursery industry.
One advantage of fabric containers is the ability to continually “air prune” roots. Air pruning occurs when the root tip reaches a pocket of air, which causes the tip to desiccate and forces the root to branch. Due to the rigidity of the sidewalls, woody plants grown in plastic containers often have more circling roots on the outer periphery of the root ball, while fabric containers may allow for a “more natural” root system because of the effects of air root pruning. However, this effect depends on the length of time the plants are grown in the container.
Roots deflected in plastic containers grow in many directions, including up, down or around the root ball; some roots may “kink” 180 degrees and go back in the direction from which they grew, causing constrictions and circling roots. Research has found that fabric containers have fewer circling/girdling roots compared to those grown in other smooth-sided containers. Other studies have found that malformed roots that begin with container production can lead to later tree instability and possible failure.
One issue with growing woody plants in containers is the additional labor required to consolidate and overwinter these crops in northern zones. If they’re too low, winter substrate temperatures can cause root death or slowed growth in spring. These temperatures can often be moderated by overwintering stock in polyhouses, or consolidating plant material, covering them with mulch, straw and/or poly during fall. This is laborious and expensive to producers.
Research to the rescue
Research at Colorado State University, Fort Collins, examined the effects of container type and overwintering methods on the growth and health of Chanticleer® pear over two growing seasons.
Two-year-old, lightly branched bare root whips were planted into three container types in May 2010. The three containers were all #15 in size: black plastic (Lerio Corp., Mobile, Ala.), Root Pouch® (RP) (Averna & Associates, Hillsboro, Ore.) and Smart Pot® (SP) (High Caliper Growing- Root Control, Inc., Oklahoma City, Okla.). Whips were lightly root pruned prior to planting; average caliper measured 0.7 inches with a height of 5.3 feet. Containers were randomly placed on a black woven cloth and spaced 3 feet between containers and 6 feet between rows. Trees were attached to a wire trellis with a 6-foot tall bamboo stake to prevent them from tipping over. Irrigation was applied by drip; each tree received 1.5 gallons every other day in 2010; this was increased to 1.5 gallons each day in 2011.
Throughout the growing season, height and caliper (measured at 6 inches from container surface) were measured monthly. In September 2010 and 2011, 30 trees were destructively harvested to obtain leaf, shoot and root dry weights. After these trees were removed from their containers, the root ball was evaluated for circling roots, matted roots and overall root ball quality.
In November 2010, the remaining trees were moved into two overwintering treatments – either left “lined out” or consolidated into a block. Substrate temperatures, measured by thermocouples, were collected in both winter of 2010-2011 and during the 2011 growing season.
Circling roots on the outer periphery of a black plastic container root ball; this photograph was taken 18 months after planting.
The overwintering treatment significantly affected substrate temperature; on average, the lined out containers (regardless of container type) were about 1°F warmer than consolidated containers. We attribute this to the fact that the lined out containers were more exposed to solar radiation. Container type also affected winter substrate temperatures: Trees grown in BP containers were consistently warmer than RP or SP containers (1.9°F and 2.3°F, respectively). We hypothesized that BP containers heat up and cool down more rapidly during winter months, which could negatively affect root and top growth. We observed the substrate for trees growing in BP containers warmed up more quickly in spring, pushed leaf growth and induced earlier flowering compared to RP and SP; this growth was subsequently injured by a late spring frost in 2011.
Summer substrate temperatures, measured after trees were re-spaced, also resulted in warmer temperatures for BP-grown trees, though there were minor statistically significant container effects. Average summer substrate temperatures in all container types remained in the optimal temperature range for root growth, between 59 and 80°F.
Layout of the container tree production study.
What we found
Container type did affect tree growth, both in 2010 and 2011. While trees planted in BP containers displayed greater height and caliper growth in 2010 than those grown in fabric containers, the opposite happened in 2011 (see Figures 1 and 2). In 2011, the trees grown in BP containers increased in caliper by only 12.9 percent, compared to RP (27.9 percent) and SP (26.0 percent). Similarly, the height of trees grown in BP containers increased by only 11.6 percent in 2011, compared to 25.0 percent for RP and 26.3 percent for SP. We suspect that the BP container actually enhanced growth the first year because the substrate warmed up more quickly from solar absorption, which stimulated root and shoot growth. We also suspect that in 2011 the BP-grown trees broke dormancy earlier, which resulted in spring frost injury; RP- and SP-grown trees remained dormant and didn’t suffer growth setbacks from late spring frost.
An example of the prolific bottom root matting that occurs in black plastic containers; photo taken six months after planting.
Following the winter of 2010-2011, there were effects on the trees based on overwintering treatment. Consolidated trees had greater leader growth (26.7 inches) compared to lined-out trees (11.6 inches) as well as greater average twig growth (14.1 inches for consolidated trees; 9.0 inches for lined out trees). Consolidated trees also had greater leaf dry weight and shoot dry weight (see Figure 3).
Note lack of root matting of Chanticleer® pear grown in a Smart Pot® container six months after planting.
Perhaps the most interesting finding was the overwintering effect on root production and dry weight. Consolidated trees produced 35.3 percent greater dry root ball weight (2.9 pounds) compared to lined-out trees (2.2 pounds). We found root balls from BP containers in the lined out treatment to be significantly less stable (“root ball integrity”) compared to trees grown in BP in the consolidated group. Root ball integrity was not affected for RP and SP trees.
Slight root matting with Chanticleer® pear grown in Root Pouch® container six months after planting.
The greatest differences in tree growth were likely the result of the overwintering treatments; trees placed in the consolidated group had significantly greater leader growth, leaf and root dry weight and average twig growth. However, container type also affected tree growth. When comparing container type, we found that trees in BP containers had smaller leaf area compared to trees in RP or SP. Again, we attribute this to the early spring growth and resulting in deacclimation of the trees grown in BP – which made them more susceptible to injury or death by a late spring frost.
Overall root ball quality (frequency of circling roots) and integrity was significantly poorer for trees grown in BP compared to those grown in RP and SP. Trees grown in BP containers had greater circling and kinked roots. We found few, if any, circling roots on the outer periphery and less bottom root matting on the root balls for SP- and RP-grown trees.
What we conclude from our research is that fabric containers likely have a place in the nursery industry, and trees produced in these container types may have better developed root systems with fewer circling roots. We cannot speculate whether fabric containers will replace the majority of black plastic containers, as this depends on the industry adoption of these alternative types. In addition, our research suggests that nursery producers in northern climates must continue to consolidate plant material – trees left lined out were smaller and experienced greater winter injury, which decreased overall plant growth.
Figure 1. Height of Chanticleer® pear grown in three containers over three growing seasons. Means within a year with the same letter are not significantly different (Pr ≥ F 0.05).
One option growers could consider is that since trees in BP containers grew more quickly than those in fabric containers during the first growing season, growers could start whips in BP containers and then transplant to fabric containers the second season to finish trees to size. Should a grower consider this option, corrective root pruning would be necessary, because we found circling roots and bottom matting to be twice as likely to occur in BP containers during the first year of production (compared to RP- and SP-grown trees).
While there are numerous advantages to growing trees in fabric containers, we observed some drawbacks, including ease of mobility (the containers are soft-sided and root ball damage may occur during transport) and degradation of the material (tearing and handle breakage). In addition, planting into fabric containers may require more labor, and mechanization may not be available.
Figure 2. Caliper of Chanticleer® pear grown in three containers over three growing seasons. Means within a year with the same letter are not significantly different (Pr ≥ F 0.05).
Cost among the three container types is similar, but depends on quantity purchased and product availability. However, as when adopting the use of other technologies and strategies, nursery producers can account for these factors and achieve success. Our research suggests that the benefits of using fabric containers in the nursery may outweigh potential downsides, as trees grown in fabric containers may display better growth and experience fewer problems with their root systems. It’s important to note that our research only focused on one species; other plant taxa may respond differently to production in fabric containers.
A second study that we recently completed, which examined the effects of the three container types on tree landscape establishment, is being analyzed. In addition, we studied evaporative water use among the three container types to attempt to answer questions regarding water use differences between fabric and black plastic containers. We plan to share the results of these studies in a future American Nurseryman publication.
Figure 3. Dry shoot and root weight of Chanticleer® pear under two overwintering treatments (consolidated or lined out). *** = significantly different at Pr ≥ F 0.001.
Alison O’Connor is a Ph.D. candidate; James E. Klett is Professor and Extension Specialist; and Tony Koski is Professor and Extension Specialist in the Colorado State University Department of Horticulture and Landscape Architecture. O’Connor can be reached at email@example.com.
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