Landscape Architect Articles

LCN Tree Care August, 200408-01-04 | News

The Obsolete Taproot

By Carl E. Whitcomb, Ph.D.

Figure 1.
A naturally grown taproot plunges deep into the earth to secure water and nutrients. Large taproots, however, can make transplanting difficult.

Root Development in Nature

When a seed germinates in the wild, a strong primary or taproot plunges downward. The tip of the taproot has a strong dominance that suppresses secondary root branching in the same manner as the tip of a new shoot suppresses production of side branches. The objective of the taproot in the wild is to extend deeply to anchor the new plant and access moisture to avoid dehydration. The objective of the new shoot is to reach sufficient vertical height to access light to support leaf functions and to avoid being overshadowed by competing vegetation. A typical tree seedling top response is to develop few, if any, side branches until the leaves on the main stem are positioned in sunlight. Likewise, a typical response with the taproot is to produce few, if any, branch roots until the taproot has extended considerable distance, often three feet or more, and provisions for the plant have been secured. Since there are limited energy resources stored in the seed, the young plant proceeds most efficiently. Only after the taproot is secured and is providing water and nutrients and the new leaves are producing energy does appreciable secondary branching begin to occur both above and below ground.

A burr oak that was trained to grow a more even root system suitable for replanting. The tree was one of 36 grown to test techniques for eliminating troublesome taproots. This example spent its first three months in a Rootmaker propagation container that limits growth at the bottom and sides of the root ball. It was then planted in a fabric container made of polyester knit. The grow-bag kept root growth even and eliminated tap roots without sacrificing general health. At the end of three years the tree’s diameter was about 2.5 inches and it was pulled from the field with a tractor and a nylon strap. The grow bag was removed, and the tree was replanted and allowed to grow three more years with added fertilizer but no irrigation. After six years of growth the tree was inspected to evaluate root development. The largest root segment was about one inch in diameter at the outer face of the root ball, with the roots evenly spaced to facilitate transplanting.

Conditions in a Nursery

Growing conditions in the nursery are very different because moisture and nutrients are provided and weeds are controlled. There is no need for a deep taproot. A deep taproot is obsolete, not an asset. This is because shallow horizontal roots, those in the upper 10 to 12 inches, are the prime providers of nutrients to leaves since they are in the zone of soil where both oxygen and nutrients are most plentiful. When pruning of the taproot occurs at the proper time and position, horizontal secondary roots are produced and it is highly desirable to maintain these roots in the horizontal position. Trees grown with such procedures produce roots radially as well as downward following transplanting, accelerating establishment, top growth and overall plant health (Figure 1). Such desirable root systems can be created consistently by air-root pruning (dehydration pruning) the tip of the taproot approximately four inches below the seed. This stimulates secondary branch root formation along the entire short taproot (Figures 2 and 3). Pruning the taproot later will stimulate formation of secondary roots at the face of the point of pruning only which is much like development of roots from the cut ends on a tree harvested balled-in-burlap, but never along the vertical axis of the taproot just below the soil surface. This critical opportunity to stimulate secondary branch roots will be forfeited if not accomplished before the taproot is four inches long. However, if provisions are made to stimulate secondary roots but all roots are deflected downward prior to field planting, little is gained (Figure 4).

Figure 2.
The shumard oak, Quercus shumardi, seed at left is in the earliest detectable stage of germination. The seed at center is three days older than the seed to the left. The seed at right is four days older than the seed to the far left. The seedling to the right has experienced air-root pruning at a depth of four inches (note the darkened tip of the tap root) and has begun to produce secondary horizontal roots. The primary shoot is beginning to emerge after just four days.

When air-root pruning is accomplished at the proper time and depth, secondary roots originate positioned horizontal or slightly downward. The challenge has been to find a way to keep these roots growing horizontally. In bottomless milk cartons, plastic tubes or sleeves, and open bottom plug trays, there is no opportunity to maintain horizontal root growth. These types of containers deflect all secondary branch roots down, leaving few, if any, roots to grow horizontal following transplanting. Trees grown in plug or milk carton type containers and planted into larger containers promptly develop a complex mat of roots at the bottom and modest roots above (Figure 5). Similarly, when trees grown in plugs or milk carton type containers are planted in the field, most roots extend downward, further reducing the amount of roots in the root ball when harvested balled-in-burlap or with tree spades.

Figure 3.
After seven days, the center shumard oak seed in Figure 2 was allowed to extend and had reached a depth of nine inches. The few secondary roots near the seed were no more developed than on day three. By contrast, the tip of the taproot that had been air-root pruned at a depth of four inches had produced horizontal roots above the point of pruning that were not evident on day four. In addition, four branch roots have been produced just behind the point of pruning of the taproot. These secondary roots are larger in diameter than the others and will reestablish a multiple taproot if left unchecked. It is only after air-root-pruning of these secondary and sometimes even tertiary attempts to reestablish a taproot that the horizontal secondary roots along the vertical axis of the taproot begin more rapid growth.

Figure 4.
The lacebark elm, Ulmus parvifolia, seedling at right was grown under conditions that stimulated optimum horizontal root development. The three seedlings to the left were grown under conditions less and less favorable to the stimulation of horizontal secondary roots. Most striking of all is the fact that all four trees are the same age and, following the early seedling stage, were grown for two years under identical conditions in the field. Results from extensive studies leave no doubt as to the desirability of having many roots form at the base of the stem, as opposed to just a taproot or a taproot plus a few large secondary roots.

Improved Container Technology

To consistently improve root branching and horizontal root development a seedling container would need to air-root pruned at several levels on the sides as well as at the bottom. The original RootMaker design was an injection molded container 2.5 by 2.5 by four inches deep, with a series of saw tooth like ledges and openings in the sides and four bottom openings for air-root-pruning. Seeds planted in the these propagation containers develop roots in all directions following transplanting, not just down (Figure 5). A new container design with a 32 or 18 cell tray accomplishes similar results but costs less and is easier to handle (Figure 6). Trees that develop large numbers of roots at the root-stem juncture and along the vertical axis of a short taproot consistently establish and grow faster than trees with fewer roots arising from this point (Figure 2) and far outperform bare root seedlings.

Figure 5.
Following transplanting, roots extend in the direction they were oriented in the previous container. The shumard oak seedlings shown here were transplanted from the plug and seedling container into three-gallon containers, and then removed after three weeks to observe root development. In the case of the plug-type containers, all of the roots grew downward.

With timely air-root pruning of seedlings that include provisions for continued horizontal root development, trees can be consistently produced with highly efficient fibrous root systems that extend in all directions following transplanting. Not only do trees grown this way require little if any staking, they establish more quickly following transplanting and because of their many small roots, more efficiently utilize the volume of soil available. For anyone still thinking that a taproot is necessary, consider which is stronger, a single steel rod or a multi-strand cable of the same diameter. Remember, the greater the support of roots for the top, the faster both top and root grow and the healthier the plant.