Significant gains can be made in terms of quality and environmental impact through the study of plant stress dynamics. In the root environment, for example, containers with punctured side-walls promote ‘air-pruning’ of roots as a result of stress; a technique that is increasingly being accepted for woody, nursery-stock plants.

by Edward Bent

The first, registered Air-Pot introduced into the European and US markets is based on an Australian patent and is called Superoots from the Caledonian Tree Company Ltd. in Scotland. The profile of the plastic sidewalls consist of cones, which point both inwards and outwards; the cones on the inside are used to guide the roots towards the outwardly protruding cones that have their apex removed to create an air hole. Once the roots reach to within 2-3 mm of the air holes, the higher air density of the substrate (in the absence of high humidity) causes the root tips to dehydrate and the root-cap meristem dies. This process is referred to as air-pruning.
A secondary effect is the growth of new, lateral fibrous roots, all of which are potential white-tip feeder roots. The response can be so strong that from any one root that is air-pruned, up to 10 new fibrous roots can appear. The response does vary by plant species, but the general overall result is very positive, even in genera such as Eucalyptus that ordinarily produce very dominant tap roots. A better transplant response is noted by Jamie Single, CEO of the Caledonian Tree Company, to be one of the advantages from the faster development of a fibrous root system. At the same time, since 30% more fine compost by volume can be packed into Air-Pots, plants can actually be left to grow in the same container for much longer periods, saving time and expense. More trials are required to evaluate the effect on other tropical plants such as Ficus, Palms, Dracaena, Croton etc.


Initial scepticism changed
The manufacturer further comments that there was a considerable degree of grower scepticism regarding the cost effectiveness of Air-Pots initially; the price is up to twice that of conventional plastic pots. Today, however, after several years of working with Air-Pots, growers have changed their opinions in favour of their use. In fact growers such as Cherry Lake Tree Farm in Florida with 600 ha of land and A. Hill & Sons in the UK, also well known for their quality products, practically only use Air-Pots and are competitive in the market.
Trial data comparing the growth of conventional and Air-Pot plants is not available, but the growers do report a potential 30-50% reduction in production time through to ready-for-sale plants. It is added that quicker growth is not necessarily always implied with the use of Air-Pots; a reference is made however to the constant, superior quality of the plants. Air-Pots are available in different sizes from the smaller Air-Cells with a volume of 0.3 to 1 litre for young stock production (from seed or cuttings) through to large containers for medium and mature trees with large root-balls.
The high density, recycled polyethylene is actually supplied as either a roll or in pre-determined lengths of various breadths. This also makes it ideal for larger trees with irregular shaped root-balls. For example, thousands of square metres of Air-Pot material has been sent to Spain to containerize the root-balls of ancient olive trees.


An Italian competitor
The Uniko container, manufactured by the Italian company Barghini Plast, is following the same principle. While considerably cheaper it is not yet clear whether it will provide the same advantages seen with Air-Pots. The Uniko container has only recently been introduced and has not yet been adequately tried and tested. In the Uniko container, the inward and outward pointing cones are less pronounced than in Superoots, and the plastic is a little more rigid. A plus point is a bottom ring of rectangular holes for lifting purposes. The Uniko design uses a mixture of high and low density, recycled polyethylene and comes in panels that clip together; an exception is a 2 litre container that consists of a single panel plus base. The panels can be produced in a variety of colours.