Bud Unions are Beautiful

On January 13, I attended an excellent workshop, sponsored by the city of St. Peters, MO and the Missouri Department of Conservation (MDC) on the hot topic of "Tree Root Depth." The objective of the session was to promote discussion among professionals who grow, sell and plant trees regarding the fact that an alarmingly high percentage of trees are apparently being planted too deeply.

Figure 1. A budded liner, showing a bud union and a "crow's foot" root system 4 inches below the soil surface.

The issue of tree root depth has generated enough concern in the nursery and landscape industries that a working group has been established to study the problem and propose possible solutions. The group, chaired by Dr. Gary Watson, a well known tree root researcher from Morton Arboretum, has representation from the American Nursery and Landscape Association, the International Society of Arboriculture and several other nationwide industry groups. During the workshop, local moderator Vickie Phillips, with St. Peters Parks and Recreation Services, effectively described and illustrated the problems in St. Peters that led her to seek funding from MDC to conduct the workshop. Dr. Watson served as keynote speaker and did an outstanding job of describing the problem and setting the stage for the lively discussion that ensued among the 100 plus professionals assembled.

One generalization that emerged from the workshop was that there are many factors working together to determine how deeply the roots of a tree get placed when a tree is planted in a landscape. Many landscape trees begin, for example, as seedlings grown in outdoor seedbeds. When small seedlings are transplanted, the grower may cut the tips of the roots to promote root branching. Often, most of the lateral roots produced by the transplant arise within one inch of the pruning cut, resulting in a seedling with most of the roots in a "crows foot" several inches below the original soil line (Figure 1).

If such a tree is replanted at the same depth as it was during its first season, all lateral roots may be three to six inches below the soil surface. If a tree is grown as a cultivar (clone), such as "autumn purple" ash, it will bud during the second season, with the bud placed one to two inches above the soil surface. The seedling understock is cut back to just above the bud to force the autumn purple top to grow from the bud. When the budded tree is harvested after another growing season as a "budded liner," it may have the same submerged crow’s foot root system that it had at the end of its second season with an additional one- to two- inch "shank" of understock stem above the soil line. Just above this shank, there is a curved piece of stem where the bud grows outward and then upward. The junction between the seedling understock and the budded cultivar, know as the "bud union," may be obvious for a number of years following budding. When a budded liner is sent to a Midwest tree grower to be grown to a landscape-sized tree, it represents a key factor in the root depth equation.

There are two incentives for a tree grower to plant a budded liner at a depth that will cover the bud union. First, if the tree is planted deeply, it may not need to be staked to keep it from blowing over in the field. Secondly, bud unions are considered unsightly by many retail purchasers of trees. If the bud union of a tree is covered with soil, the tree may sell more easily to wholesale tree buyers who are trying to please their retail customers. However, if the grower decides to plant at a depth that will hide the bud union, the first main lateral roots may be up to six inches below the soil surface.

Once a budded liner is growing in the nursery field, depth of roots may increase as soil is thrown up around the trunks during mechanical weed cultivation. If the liner was planted with the bud union covered, when the tree is harvested with a tree spade, the spade may sit on top of three to six inches of soil that contains no roots and many roots may be cut off deeper in the soil. The resulting rootball may contain very few roots and those that are present may be found near the bottom of the ball.

Landscape planting technique is the final factor in determining how deep the roots of a tree end up. Given the worst case scenario, the soil ball in a two-inch caliper (stem diameter) tree may have no roots in the top six inches and those at the base of the ball may have been severely truncated. If such a tree is planted at the same depth that it was growing in the nursery, the majority of its abbreviated roots may be placed 12 to 18 inches below the soil surface. In many urban and suburban soils, such a tree has a negligible chance of survival since there is insufficient oxygen to allow roots to develop from buried root system. To avoid this situation, many tree experts recommend probing the soil ball of a tree with a metal rod to assure that there are large lateral roots within three inches of the soil surface.

The moral of this tree root story is that trees should be "planted not buried" (Gary Watson). There are numerous factors from seed to landscape planting that influence the morphology of a tree root system. Growers, sellers and planters of trees are becoming increasingly aware of this issue and are beginning to develop procedures they can follow to minimize the chances of planting too deeply. However, it is important for retail tree buyers to understand it is normal for the base of a budded tree to look crooked and/or bumpy for a number of years. In fact, an obvious bud union should provide the peace of mind that goes with knowing that roots are probably up near the soil surface where they are less likely to suffocate.

Christopher Starbuck, Ag Ext-Plant Sciences, University of Missouri 573-882-9630