Field production

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Irrigation and fertilization: Tree production practices in the field nursery impact transplantability. Some tree species receiving drip irrigation at the base of the trunk for the duration of the production cycle in the nursery often have a concentration of fine roots in the root ball (Gilman et al, 1992; Ponder and Kenworthy, 1976). Drip and micro-spray irrigation appear to provide a similar root response (Gilman et al. 2002). Fine root growth on some species can also be stimulated close to the trunk by making fertilizer applications only on soil that will become the root ball, not outside the root ball (Beeson and Gilman, 1995). Many nurseries fertilize trees in this manner. Click here for three sample field fertilization protocols.

On the other hand, one study showed that after trees received overhead irrigation and broadcast fertilization for two years in the nursery, concentrating the irrigation or fertilizer close to the trunk did not influence fine root growth (Beeson and Gilman, 1995). Apparently, to increase fine-roots near the trunk, irrigation and fertilizer application to the root ball area must begin soon after the tree is planted in the nursery field.

Root pruning: Circling roots and other root defects on liners should be cut and removed before planting into the field. Some field nurseries routinely root prune tangential to the trunk certain species such as oaks one or more times annually to help them survive the transplanting process. Others prune only once several weeks to several months before harvesting (click here for sample root pruning protocol). Some nurseries do not root prune trees during production. Certain trees like holly, crapemyrtle, red maple and others are not routinely root pruned because their root systems are dense and fibrous. Root pruning during production can increase tree survival after digging, but may have little or no impact on growth of surviving trees after transplanting to the landscape (Gilman 1992; Gilman 2001). In warm climates, without frequent irrigation after transplanting, trees pre-dug from a field nursery or those root pruned repeatedly prior to digging them from the nursery are less stressed after planting than freshly-dug trees. (Research in the cooler climates is mostly lacking but conversations with some quality growers reveal that many trees are not root pruned during production.) Trees that are root pruned regularly during production may not need to be hardened-off (see below) for as long as trees that are root pruned less rigorously prior to landscape planting.

On the other hand, if irrigation can be provided frequently (this is rare) until trees are established in the landscape, root pruning prior to transplanting has little impact on survival on trees with a trunk diameter less than 5 inches. Nurseries are usually best prepared to provide this intensive irrigation management required for freshly-dug trees. Freshly-dug trees usually cannot be irrigated properly in a landscape setting. Some deciduous trees in the cooler regions (hardiness zones 2 through 7) are not irrigated regularly after transplanting because they are dug and/or planted at the end of the dormant season. The cool weather and lack of foliage allows the tree to survive unless the region is experiencing drought. Arborists often root prune larger trees prior to moving them with the intention of improving transplant survival. Nursery operators usually like to harvest only root pruned trees in summer, although there are new techniques that could dramatically improve summer survival (Gilman et al. 2002).

Burlap: Trees grown in the field and not harvested bare root are dug with a soil ball. Burlap is secured around the root ball with nails, string or wire. Synthetic burlap is occasionally used on root balls so the nursery operator can dig the tree several months prior to planting it in the landscape. This pre-digging helps the tree harden-off and survive once transplanting into the landscape. However, synthetic burlap should be removed at least from the upper portion of the root ball before backfilling. It is probably best to remove all synthetic burlap. Although there are few published reports of synthetic burlap preventing root regeneration or penetration into the backfill soil, there are verbal accounts from landscapers and researchers of synthetic burlap preventing root growth out of the root ball on certain species.

Recent innovations: A recent innovation should eliminate the need for synthetic burlap in a wire basket. It allows the tree to be held in the nursery for many months after digging without producing large diameter roots out of the root ball. Here is how the system works. Trees are dug and placed in natural burlap then secured with wire or string to hold the root ball tight. The intact root ball is lowered into a pre-dug hole in the nursery that is lined with a sleeve made from black nursery ground cloth (see illustration at left). In several weeks to a month or two depending on season of year and species, small regenerated roots will grow partially through the fabric and hold it onto the outside of the root ball. This will allow the tree to be held in the nursery for many months and then lifted without loosing soil from the ball. The natural burlap may have decomposed but the sleeve on the outside of the ball will be intact and will hold the soil in the root ball. Of course the synthetic sleeve on the outside of the ball must be stripped away from the rootball and removed at the planting site.

Hardening-off trees: Field-grown trees dug several weeks or months prior to shipping to the landscape site are said to be "hardened off". Freshly dug trees are not "hardened off" and are very susceptible to death if not watered appropriately (Gilman 2001). During the "hardening off" period, roots begin to grow within the root ball, and the tree may drop some leaves. There may be other physiological adjustments made by the tree that are not now well understood. Roots often grow through the burlap wrapped around the root ball of a hardened off tree. Some nurseries provide overhead irrigation to the foliage during the hardening off period, especially with summer digging. Purchasing freshly dug trees in the southern US that are not hardened-off is not recommended unless you have assurances that they were regularly root pruned during production.