Container Production Strategies Influence Root Ball Morphology

MATERIALS AND METHODS

On February 11, 2009, in Loxahatchee, Florida (USDA hardiness zone 10a), mahogany seeds were placed into propagation (liner) containers in substrate consisting of 45% super fine pine bark, 20% Florida peat, 10% horticultural perlite, 15% Allgro compost, and 10% coarse sand. àr ee propagation container types tested were: 1) Bottomless Ellepot (EP) constructed of paper 50 mm diameter × 90 mm tall, with a volume of 137 cm³ (Ellegaard, Esbjerg, Denmark, Ellepot paper made by Ahlstrom Stalldalen AB, Stalldalen Sweden from spruce, pine, and polyester long fibers, 27g/m², 190 microns thick, 1320 N/m dry tensile strength in machine direction, 2.0 N tear strength), arranged 10 mm apart in a plastic tray (27 cm × 53 cm), which exposed 100% of the paper sides to air and rested on a plastic ring (8 mm wide) as part of the holder tray; 2) EP with same dimensions placed in a tray of smooth (EPS) black plastic cells (60 mm tall × 50 mm wide), spaced about 5 mm apart; and 3) a tray of smooth-sided (SM) black plastic containers 40 mm top diameter × 90 mm tall (volume 105 cm³) with a slightly tapered cone and a single drainage hole at the bottom. Trays (each with 40 to 55 containers) were arranged in a randomized fashion on wire mesh benches 80 cm from the ground in full sun in a non-climate controlled, open-sided greenhouse.

RESULTS

Three-way interactions were mostly insignificant, so they are not described in this analysis. Mahogany propagated in SM had slightly smaller trunk diameter and were shorter than trees in EP when harvested from the propagation container (Table 1). Trees in EP had greater root branching and root length in the top half of liner root balls, fewer deflected tap roots and lateral roots, and actively growing roots more evenly distributed vertically when compared to SM and EPS (Table 1; Figure 1).

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Figure 1.

Root systems after 5 months in three propagation containers. The large-diameter lateral woody roots emerging from the tap root in EP are lacking on the other two.

Mahogany harvested from both 3.8 L container types that were propagated in EPS had a much larger percentage of the trunk circled at the liner position (78%), produced more trees graded as root culls (79%), and the imprint on the root system imposed by the propagation container was highly visible (rating = 4.6) when compared to seedlings grown in SM and EP (Table 2). Trees propagated in EP had the least deflected (lower % trunk circled, % culls, imprint rating) root systems, and those from SM had shallower roots than EPS.

Mean root depth was greater in both 3.8 L container types measured just beyond the position of the liner root ball when trees were retained in propagation containers 12 months (87 mm) compared to 5 months (50 mm, data not shown). Response to retention time depended on the propagation container type for four measured root attributes (Table 3). In contrast to SM propagation containers, increasing retention time in EP containers had no impact on % trunk circled, % root culls, and number of straight roots in 3.8 L containers. When held five months, propagation container type had no impact on number of roots deflected at the position of the container; however, when held 12 months, fewer roots deflected in EP than in SM containers.

Root pruning SM liners by shaving (pruning) 5 mm from the periphery reduced by a factor of 4 or 5 the % trees in both 3.8 L containers graded as culls and % trunk circled, respectively (Table 4). Root pruning EP liners had no impact on 3.8 L trees (Table 4) because there were few roots deflected by the EP periphery (Table 1). Root pruning SM also increased the % of total root (>3 mm diameter) number (56%, root pruned; 42%, not root pruned; P < 0.05) that grew to the periphery of both 9.5 L containers (data not shown).

Percentage of total-tree root cross-sectional area (CSA) in the top 2 cm measured at the periphery of the 3.8 L root ball was larger for trees grown in PC1 than in SC1 containers for both retention times from both propagation containers (Table 5). Both the number of horizontal roots (those growing 0 to 45 degrees from the surface) and CSA of the five largest horizontal roots were approximately two to three times larger for trees in PC1 than SM1 containers. à e ratio of diameter in the five largest horizontal to diameter in the five largest descending roots (those growing 45 to 90 degrees from surface) was eight and three times greater for PC1 than SC1 for 6 and 13 months retention time, respectively. Growing trees in SC1 containers resulted in a greater arc without roots (>1 mm diameter) than growing in PC1 after 6 months in 3.8 L containers; there was no difference at 13 months. Root depth for trees from both propagation container types was not affected by 3.8 L container type 6 months after shifting but was significantly greater in SC1 than PC1 13 months after shifting (Table 5).

Impact from growing mahogany trees in 3.8 L and 9.5 L containers of two types was consistent (i.e., there was no interaction) across propagation container type, retention time in propagation container, and root pruning for 11 measured attributes (Table 6; Figure 2). Trees harvested from SC3 containers had slightly larger trunk diameter and total-tree height (P < 0.05) than trees from PC3. Roots on trees from SC3 had higher values of attributes associated with lower quality, including % trunk circled with roots, 3.8 L container imprint rating, root cull (graded according to Florida Grades and Standards, Anonymous 1998), and total deflected root length. Trees in PC3 containers had about six times the number of straight roots (69% vs. 11% of roots > 3 mm diameter) as those in SC3 containers. Trees in PC3 had 44% of root system CSA deeper than 8 cm at the periphery of the 9.5 L container, whereas 83% was positioned there on trees in SC3 containers. Ratio CSA of five largest horizontal to five largest descending roots was 49 times greater on trees from PC3 than SC3 containers (Table 6; Figure 2).

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Figure 2.

Root systems in 9.5 L containers for six months, grown in four combinations of propagation container and larger (3.8 L and 9.5 L) container.

Impact on growing trees in 3.8 L and 9.5 L containers depended on the propagation container type for four root attributes (Table 7). Growing trees in EP and then shifting to PC1 and PC3 resulted in the least % trunk circled and % trees with roots that touched or crossed within the dimensions of the propagation container. For both propagation container types, growing trees in PC3 resulted in a threefold or more increase in number of horizontal straight roots (those > 3 mm diameter) and % roots that grew to the 9.5 L container periphery compared to trees in SC3. à e longer retention time in both 3.8 L containers was associated with greater root circling and deflection, reduced quality, and slightly greater depth of horizontally oriented roots (Table 8).

DISCUSSION

Retaining trees in containers for different time periods, and root pruning or not when shifting the liner, resulted in few meaningful differences in trunk diameter and tree height at the end of the study when trees were in 9.5 L containers; container type had only a slight effect. Trees in SC containers were larger than those in PC probably due to drier conditions (not measured) in PC containers. ài s was attributable to the porous nature of the container sides and bottom; fabric containers with porous sides have been shown to increase evaporation from the container root ball (Arnold and McDonald 2006). Irrigation management could be adjusted to maintain higher moisture content.

Finished liners in EP had attributes associated with high quality root systems best described as an abundance of horizontal straight roots growing from an aborted tap root (Balisky et al. 1995; Svensen et al. 1995); roots in the other two liners were deflected downward and around the container (Table 1; Figure 1). EP propagation containers that were inserted into smooth-sided liner cells (EPS) produced root systems similar to those in SM (Table 1), which indicated that the paper comprising the sides of EP should be exposed to air, not placed against a solid plastic wall. When finished in 3.8 L containers, root systems from EPS containers had a more prominent liner imprint (Harris et al. 1971) than those propagated in SM (Table 2). à e slim air gap between the plastic sides and the EP paper created an ideal environment for root growth and caused this imprint formed by roots circling, ascending, and descending mostly outside of the paper. Mahogany should not be grown using the EPS system because it encouraged a severe root imprint at the position of the liner. In contrast, trees propagated in EP and finished in either 3.8 L container type had almost no measurable root circling or imprint at the position of the liner (Table 2).

Mahogany root defects at the liner position on trees in 3.8 L containers increased with retention time in SM propagation containers but not for EP containers (Table 3) as in other studies (Salonius et al. 2000; Gilman et al. 2012). However, root pruning SM liners retained 12 months when shifting to 3.8. L containers dramatically reduced defects at the liner position (Table 4) without impacting trunk or height growth (data not shown). ài s enhancement of quality did not occur for trees propagated in EP because there were far fewer defects to remove (Table 1). Mechanical root pruning was also a reliable method of managing roots of other tree species when shifting liners to larger containers (Gilman et al. 2012), or when planting into field soil (Krasowski and Owens 2000). ài s eliminates the imprint imposed on the root system by the container, which reduces the likelihood of stem girdling roots and can enhance anchor-age (Gilman and Wiese 2012).

Propagation container type failed to influence consistently any measured attribute across both 9.5 L container types; i.e., the effect of propagation type depended on which larger container was used when data was averaged across 5 and 12 months retention time in propagation containers and root pruning (Table 7). In contrast, the effect of larger container type (either PC or SC) was consistent for nine root attributes of trees propagated from either propagation type (Table 6). ài s analysis could falsely lead us to conclude that root quality depended more on the 3.8 L and 9.5 L container type, and less on the propagation container type. However, when data was averaged across retention time in 3.8 L containers on trees retained for 5 months in propagation containers, propagation type had a significant effect on root morphology in the 9.5 L root balls. For example, root defects at the SM liner position including % trunk circled (51), % culls (42), and imprint rating (3.7) were much greater (P < 0.01) than the same attributes for trees grown in EP propagation containers (8%, 3%, and 1.7, respectively, data not shown). ài s analysis shows that both propagation container and the larger container impacted root quality.

à e deeper and deflected nature of the root system in finished SM liners (Table 1) likely explains the abundance of root defects at the liner position in both 9.5 L container types (Table 7). Trees did not grow out of that condition created in the propagation liner in either larger container type. à e lack of root deflection in EP propagation containers (Table 1) was responsible for the small imprint at that position and far greater number of roots reaching the side walls (periphery) of the PC 9.5 L container (Table 7; Figure 2). Root tips in EP liners remained in the horizontal position near the liner periphery without deflection, which positioned them for growing horizontally into the PC container. However, in SC 9.5 L containers, root defects on trees propagated in EP mimicked those of trees propagated in SM liners, suggesting that the benefits of growing a high-quality root system in the liner (i.e., in EP) disappeared when shifting into a larger SC container. ài s was attributable to the largest roots from both propagation container types growing downward from the bottom of the liner to the bottom of the 3.8 L and 9.5 L SC containers (Table 4). Once at the bottom, roots deflected and continued to grow along the bottom forming an imprint that remained with the tree in the 9.5 L container (Table 6; Figure 2) as others have found for smaller containers (Selby and Seaby 1982). Aggressive growth at the bottom of the 3.8 L SC containers appeared to inhibit initiation or growth of horizontal roots closer to the substrate surface, and resulted in a vertically oriented and circling root system on finished 9.5 L SC trees (Figure 2). Deflection of structural roots downward in the container forced them to grow parallel and cross one another directly under the trunk (Table 7) causing constrictions and inclusions that can restrict passage of substances through vascular tissue (Lindström and Rune 1999).

In contrast to SC containers, growing trees in PC produced a root system with a more horizontal than vertical orientation (Table 6; Figure 2). ài s has not been reported before for containers of this large size. Vertical root growth was discouraged by the elevated and highly porous bottom that stopped elongation of roots that penetrated it. Vertical roots died back (brown root tips growing through the bottom were visible) once exposed to the dry air beneath the elevated bottom which effectively root pruned them. Air pruning at the bottom appeared similar to that of at least one other container that prunes with air (Gilman et al. 2010). Inhibition of descending vertical roots induced formation of new roots or growth on existing roots close to the soil surface, and promoted growth in horizontal-oriented roots distributed throughout the root ball profile. à e tremendous (49-fold, Table 6) increase in horizontal growth in 9.5 L PC was caused by a combination of 1) continued growth on existing non-deflected horizontal roots in the 3.8 L PC containers (Table 5), and 2) initiation of new horizontal roots at the flare in the 9.5 L container (Table 6). Neither of these phenomena occurred in SC containers. Mahogany trees with horizontal-oriented lateral roots close to the top surface of the root ball develop a different root system in the landscape than those with vertical and circling roots, leading to better anchorage (Gilman and Harchick 2014).

CONCLUSION

Mahogany root systems in a container can be grown with attributes associated with well-anchored landscape trees (i.e., with straight roots, some close to the surface).