The Circle of Death
The standard nursery pot has been a, well, standard for years, and will, no doubt, continue as such for many years to come. While those ubiquitous containers are excellent for annuals, herbaceous perennials and bulb or tuber grown plants, they can be a death sentence for woody shrubs and trees grown in them. This is because of the circling roots that form as a result of the standard pot design. Not only that, but the death will be a slow, protracted one, resulting from a gradual decline in overall health. A recent Dave's Garden article by Lois Tilton, Planting Rootbound Shrubs, highlighted this problem and offered some solutions.
The reason for this doomsaying is simple; standard pots do not allow the development of a healthy radial root system. Instead, they encourage the development of roots that inevitably circle around the sides and base of the pot. That's not all because in addition, most of the roots end up near the pot wall, so most of the potting media is unused by roots. The proximity to the nursery pot wall (often black) makes the roots susceptible to burning from an overheated pot wall also. In herbaceous or annual plants, this is not a serious problem, but in woody plants, all these factors can add up to a slow death sentence.
You can do better than that!
A few nurseries have begun using a type of growing container known as a root-pruning or air-pruning pot. These innovative products work by stopping the root tip at the pot wall, forcing root branching and eliminating circling of the roots. A number of designs are available to the professional nursery plant grower. Among them are, essentially, plastic pots complete with various configurations of holes and root-directing channels or protuberances, and grow bags made from a special fabric that strangles the root tips as they try to penetrate it. I have tested several of the "plastic pot with holes" type myself and have come to favor the Superoots Air-Pot (picture at left). Some very informative videos about the Air-Pots are available at Superoots.com.
With the root constriction fabric bags, aggressive plant roots may get hung up in the bag fabric, requiring the cutting away of the bag in order to plant the tree or shrub. The Air-Pots have the advantage of easy reuse, even when a tree or shrub has been growing in them for years.
If you can find a nursery or garden center that stocks plants grown in root pruning containers, you are in luck. The root system will be immensely superior to that found on a plant grown in the standard nursery pot. However, I urge you to be of an inquiring nature because the final pot the plant was grown in may have pruned the roots, but the liner or smaller pot that it grew in previously may have been a standard container. For example, if your plant started as a standard liner, then was potted up to a one gallon nursery pot, and finally to a 3 gallon root pruning container, the plant may have two concentric areas of circling roots deep within the radial portion of the root ball. Those placed into a standard 3 gallon container will have as many as three concentric areas of circling roots! The very best you can get would be a tree or shrub that has been grown in root pruning containers since seedling or cutting days. Sadly, very few nurseries grow their plants this way.
Turning over a new leaf
Knowing this, and especially if you have a propensity for growing your own nursery stock, you can start yours in a root pruning container. The quality of your trees or shrubs may surpass anything you can buy at most nurseries, and your plants will have the best start and chance for abundant growth. At this time, retail availability of the root pruning containers is limited; however, the Superoots Air-Pot is available at retail in the western U.S. at Tree Amigos and in the eastern U.S. at Aroidia Research.
Photo credits: Linda Dodge, Department of Plant Sciences, UC Davis and LariAnn Garner, Aroidia Research