However, if the containers are simply stood on the ground, they are easily blown over and root-zone temperatures may become dangerously low in winter.
Alternatives include the "bag-in-pot" - the container still sits on the ground but the rooting medium is in a bag - and "pot-in-pot", in which a "socket pot" is sunk into the ground and both plant and container are then stood inside it.
In the research described here, bare-root whips of the crab apple 'Donald Wyman' and plants of lilac (Syringa vulgaris 'Monge') were planted either in the field (Minnesota, USA) or in various container systems containing a pine-bark and peat mixture. After two years, there was little difference between field-grown and container-grown plants of crab apple.
Even so, the "pot-in-pot" plants had the heaviest root dry weight and their roots were never as cold as those in other container systems. With lilac, the biggest plants had been field-grown or were from in the "pot-in-pot" system.
"Root circling" was not a problem in the field-grown plants or the "bag-in-pot" ones and their roots grew out well when planted. Nevertheless, the "pot-in-pot" system was preferred.
Crab Apple and Lilac Growth and Root-Zone Temperatures in Northern Nursery Production Systems by Neal (2010). HortScience 45 (1): 30-35. ISHS members can view HortScience from the website www.ishs.org
Dr Ken Cockshull is emeritus fellow at Warwick HRI