Allen D. Owings
Traditional pot-in-pot production in a nursery attempts to combine field and container-growing techniques and offers advantages over both production systems. A "socket" container is placed in the ground, and a second pot containing the growing plant is placed into the socket.
Advantages of pot-in-pot production, when compared to traditional container production, include reduced irrigation requirements, reduced heat stress to the root system during the summer, elimination of container blow-over and minimization of root-zone temperature fluctuations during the winter.
Growers across the Southeast have been implementing pot-in-pot nursery production systems to a varying degree for 10 to 15 years. The practice has gained wide use in some areas more than others. Pot-in-pot is normally used in shade tree production, but shrubs can also be grown successfully using this system.
In-ground pot-in-pot production produces a larger plant root mass when compared to conventional container production. A larger root mass translates into improved transplant success and faster establishment in the landscape.
A primary requirement for an in-ground pot-in-pot system is a well-drained soil base or an installed drainage system to remove excess rainfall or irrigation water on poorly drained soils. Normally, a sandy soil, or most certainly a soil no finer textured than a sandy loam or silty loam, is needed if drainage improvements are not made. Additionally, installing an in-ground pot-in-pot system is labor intensive and requires considerable equipment. The system also is permanent once installed, so future production plans and nursery layouts need to be carefully considered before installation.
The mid 1990s saw the introduction of above-ground pot-in-pot production systems – also referred to as "nested containers" – with 7- and 15-gallon containers most common. This method is intended to overcome some of the disadvantages associated with in-ground pot-in-pot while still taking advantage of the insulation value of a socket container.
The above-ground system places a potted container in a socket pot on the surface of a container yard, ground cover-clothed area or field. The socket pot has flared sides to prevent blow-over and only needs a little soil or mulch at the base to be held in place.
Compared to in-ground pot-in-pot systems, above-ground pot-in-pot systems provide significant labor and cost savings at installation. They also allow for adjustments in container spacings and eliminate the need for ideally drained soils or installed drainage systems.
Escape roots can sometimes be a problem with in-ground pot-in-pot plants, whereas escape roots are normally less of a problem in the above-ground system.
Blow-over may occur once in a while with above-ground pot-in-pot systems; it never occurs with in-ground pot-in-pot, if the socket pots are installed properly. Both of these methods will save significant labor picking up blown-over plants. Retail garden centers and wholesale yards should consider an above-ground pot-in-pot system for short-term holding of large trees and shrubs that are prone to blow over.
Studies comparing root-zone temperatures and size of the root mass usually show that a plant properly maintained under an in-ground pot-in-pot production method will have minimal root-zone temperature fluctuations and increased root growth compared to an above-ground pot-in-pot system. The insulation capability of the above-ground pot-in-pot system is considerably less than the in-ground pot-in-pot system, but the above-ground method has been shown in some instances to reduce the amount of root stress on the southwest side of containers effectively when compared to traditional container growing. More research is needed.
Allen D. Owings, Professor, Department of Horticulture, LSU AgCenter, Baton Rouge, La.
(This article appeared in the spring 2005 issue of Louisiana Agriculture.)