"And yet the majority of container plants grown today still have what I would call 'bad roots'," John Cooley of Proptek told delegates to the IPPS International Conference in Kilkenny last week.
"Roots tend to circle the bottom of the container, producing a root architecture that is very different to that produced in open ground. The curled root system is retained when the plant is potted on or planted out and the plant is doomed. It will be poorly anchored and very liable to blow over," he said.
Cooley described methods that had been developed to overcome the problem including root trimming, copper treatment of the container, pots that let light through and pots with ridges to make the roots grow downwards.
Yet he has been most impressed by the effects of air-pruning - particularly if the pot is designed to allow air-pruning of the sides as well as the base. "Air-pruning encourages secondary roots, which are in turn air-pruned to give a mass of young roots," he said.
The benefits of this type of air-pruning can be seen clearly in trees grown in Superoot Spring Ring containers with the high-density plastic side walls moulded like perforated egg cartons, Cooley said.
When roots grow out to the sides, they are deflected towards the holes where they are air-pruned and branch out. The end result is a mass of roots throughout the rootball, ideal for establishment in soil.
Cooley has been developing the root-pruning idea for smaller plants based on the use of cells, which have become very popular for young plant production. "Cell trays encourage bad roots," he said.
His company, Proptek, has developed a tray that elevates the cells' bases and keeps the sides open, so there is good air-pruning all round.