Study suggests pot-grown trees best for city schemes

A US study that has produced a list of nine “Power Trees” capable of surviving extreme urban and environmental stresses used nursery trials to compare the growth rates of container-grown and field-grown trees.

Initial trials have led scientists to suggest that container-grown trees produce better early growth rates than field-grown ones. Scientist John Hammond from the US Department of Agriculture’s chief scientific agency, the Agricultural Research Service, headed a team that aimed to produce attractive trees capable of withstanding urban stresses as well as not being a nuisance to utilities. Optimum attributes were small to medium size, pest and disease resistance, at least 45° upright branching structure, tolerance of different soil conditions and low litter production with no large fruits or leaves. Hammond and his team selected nine trees from cultivars bred at the US National Arboretum, Washington DC. They included varieties of crab apple, flowering cherry and red maple. Hammond said: “I would expect all of these varieties to be suitable for anywhere in the UK.” The team established a nursery at the US National Arboretum to grow the nine new tree varieties and used both container and ground planting to compare which method best prepared saplings for a life on the streets. Hammond said: “We saw significantly more growth in the first year for the elms and maples in pots compared to those on the ground.” He added: “Growing trees in pots is increasingly common in the US. There are a lot of advantages. They can be moved and transported all year round — unlike field-grown trees, which are limited to spring and fall. There is also not as much damage to the root system.” In total, 24 samples of each cultivar were planted at sites across Maryland and the District of Columbia. Half of each group were pot grown and the other half field grown. The department will evaluate survival and growth rates to see whether the growth advantage of the container-raised trees persists. * Visit for further details.

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