Nurseries that sell landscape sized B&B and containerized trees typically buy bare root ‘liners’ from other nurseries specializing in liner production. For most northern states, the most common source of liners is the Pacific Northwest where they can be produced rapidly and inexpensively in the field.

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In warm climates, such as the southern US, tree liners are often produced in containers. Typically, the stems are not cut back in this production method, as they are in field-produced liners.

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Soil cultivation in the nursery fields can affect root depth in two ways. Frequent cultivation will usually keep roots from developing in the first couple of inches of soil. Unlike the forest where the soil is protected from drying by leaf litter ‘mulch’, the frequent drying of this bare soil between rainfall events or irrigation cycles also discourages root development at the surface.

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Cultivation practices can also mound soil around the base of trees. If not removed before harvesting, it will add soil to the root ball above the structural roots.

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Planting liners too deep has been a major reason for deep structural roots, though the situation is improving rapidly. Some growers would purposely plant the liners too deep in order to control sprouts from rootstock, to protect the bud union, to hide the crook in the stem from the grafting and cutback process, and to reduce the need for staking. The need for staking may be overstated. Fraxinus and Malus leaned no more when planted at grade than up to 15 cm (6 in) deep. Betula experienced more instability than all other species, but were more stable when planted as little as 5 cm (2 in) deep. (Giblin, LBG [pdf])

Growers may have assumed that new adventitious roots would form on the root shank above the adventitious root flare. Only a few species will do this. Usually those adapted to floodplains where the soil level is periodically raised by added silt. They may have simply been unaware that this could be a problem since the trees would grow well in their high quality nursery soils.

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Field-grown bare root liners are often planted in containers and sold after a season in the container. If not planted correctly, the structural roots can easily be too deep. (photo by: D. Fare)

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The graft union should be visible, just as when the liner was growing in the field.

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Containerized trees with deep roots can develop large amounts of fine roots above the structural roots. This may lead to some difficult decisions when planting. (photo by: D. Fare)

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