Deep Roots of Landscape Trees
In the Nursery » Liner Production

Alteration of Root Architecture in Field-grown Liners

Alteration of the architecture of structural roots by early stages of nursery production is only beginning to be recognized as a possible contributor to deep structural roots. To understand how this affects root system architecture, an understanding of development of root architecture under natural conditions is needed.

In Nature

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 root flare.

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In Nursery

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 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 regenerated from the cut end of the primary root. Growing conditions in the soil around this cut end are ideal for root growth and the regenerated roots grow rapidly. Most of the shallow laterals do not persist.

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Adventitious Root Flare

The roots regenerated from the cut primary root are adventitious roots, induced by pruning. As the tree increases in size, they form what can be called an adventitious root flare several inches below the natural location for the root flare. Sometimes a few laterals remain on the “root shank” (the remaining portion of the primary root above the adventitious root flare). The pattern seems to be set at time of seedling transplanting, but much more information is needed. 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 distance is too large, the adventitious root flare will develop too deep.

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