A root pruning container is an aid to the cultivation of young plants and trees in nurseries. Many pot designs train the roots. One example is a truncated plastic cone in which a seedling is planted. There is a drainage hole at the bottom and the main tap root tends to grow towards this.
What this achieves is to encourage the roots the grow a denser system of root hairs. How it does this is to have the pots designed so as to air prune the roots.The advantage is when the plant is planted into its home environment it has a stronger root base to start with.
When polythene bags are used instead, this root tends to go through the bag into the ground and is then broken off when the tree is moved for planting. The other roots are insufficiently developed to cope with the shock caused by this and so the tree's chances of survival are reduced. The root trainer is mounted in a stand above ground so that, when the tap root emerges, it is dried by the air. This air pruning causes the root inside the pot to thicken with stored carbohydrates that support vigorous root growth when the plant is put in the ground. The other lateral roots of the plant grow to compensate for this—so a stronger root ball forms, which improves the sapling's chances.
When raising multiple seedlings, the root trainers are commonly placed in trays or racks. The size of each trainer depends upon the species but, for broad-leaved trees, the capacity is about a cup. Vertical ribs inside the trainer are positioned to train the roots to grow downwards and so prevent root spiralling.