Alteration of Root Architecture in Field-grown Liners

The primary root emerges from the seed and grows straight down in response to gravity. When more compacted, less aerated conditions in deeper soils are encountered, growth of the primary root will slow. This may occur quite close to the surface in dense or poorly drained soils, and in species with weak primary roots, or somewhat deeper in strong taprooted species, and on well drained sites. When growth of the primary root slows, growth of the small lateral roots near the soil surface increases. As growth shifts, these lateral roots start to develop into the large, shallow, more-or-less horizontal roots that form the well-developed root flare that is seen on nearly every tree in the forest.

Not all root systems are perfect in nature. In fact, very few. When 80 Fraxinus (Ash), Ilex (Holly), Larix (Larch), Quercus (Oak), Taxus (Yew) and Aesculus (Buckeye) seedlings were dug from a woodland, 79 had root architecture that would not have been accepted by a nursery. (Single, LBG [pdf])

Seedlings with inferior root systems do not survive for very long in nature. Those that do grow to mature trees (less than 1%) are the ones with strong, well-formed root systems. In the nursery, the majority of seedlings are expected to become mature landscape trees with good root systems.

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The first step in production of tree lining out stock (liners) begins with producing seedlings in dense beds. Seeds are planted very shallow, so the roots develop very close to the soil surface. Most species are grown for one season in these seedling beds.

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At the end of the first season, the seedlings are mechanically harvested and the primary root is pruned to approximately 10 cm. The next spring, seedlings are replanted in rows, maintaining the same depth as in the seedling bed. Late in the summer of this second year, cultivar buds may be grafted on to these rootstock plants, 1-2 inches above ground. (photo by: J. Frank Schmidt & Son, Co.)

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Before growth begins in the third year, the stem of the rootstock plant is cut back to just above the bud graft. Trees sold as seedling stock (not grafted), are also cut back to a low lateral bud. Evidence of cutting the stem back will be visible for several years.

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Since the root system has already been established for a year before cutting back, the new growth is rapid and the stem can be trained into a single straight trunk in one year. If harvested at this stage, these liners are referred to as whips. They may also be grown 1 or 2 years longer to be sold as branched liners.

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The transplanting and root pruning operations at the end of the first year in the nursery are never experienced by trees in nature. How does the root system react to this process? Roots are produced at the cut end of the primary root. Soil conditions around this cut end are ideal for root growth and the regenerated roots grow rapidly. Transplanting results in the loss of many of the small natural lateral roots. Honeylocust, sugar maple, and pear can lose up to 60% of these lateral roots in the year they are transplanted (Hewitt, LBG [pdf]). After transplanting, dry exposed surface soils in the liner fields can also contribute to the loss of natural laterals in the upper 5 cm (2 in; (Warren, LBG [pdf]).

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The roots regenerated from the cut primary root are adventitious roots, induced by pruning. By the time field-grown liners are harvested, there are approximately three times the number of adventitious roots at the pruned end compared to the remaining natural laterals, and they are approximately four times larger. They begin to dominate the architecture of the root system even as young liners. These vigorously growing adventitious roots have the potential to develop into an “adventitious root flare” (ARF) somewhat deeper in the soil than the natural root flare. (Hewitt, LBG [pdf]) We are still learning about species variation.

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The distance between the ground line and adventitious root flare must be as small as possible. If the primary root is pruned at 10 cm (4 in) below the soil line (root-stem transition) as a seedling, thickening of the ARF roots will gradually reduce the root shank length. By the time the tree is ready for the landscape, the structural roots/root flare will be 8 cm (3 in) or less from the soil surface. If the root shank is too long, the uppermost root of the adventitious root flare will be too deep (more than 2 inches).

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