Evaluation of Pot-in-Pot in Nursery Crop Production
Billy Carriker

Pot-in-Pot production is a fairly new concept in our area. It was first seen in North Carolina in the late1980’s. It is beginning to gain popularity among growers, but is still not widely used due mainly to the lack of information available and the tremendous amount of capital to install and start up an operation employing this technique. The reason for doing this project was to compile as much information as was available and arrange it in an understandable manner.
What exactly is Pot-in-Pot production? Pot-in-Pot, commonly referred to as PiP, was developed to combine the benefits of container production; growing in a highly managed substrate and the ability to harvest year round with ease, with the benefits of field production; insulated root zone and less water usage. This was accomplished by burying a pot, the socket pot, in the ground with about three inches protruding and placing the pot the plant will be grown in, the insert pot, inside it, hence the name pot-in-pot. All of this was developed for one main reason. When the building boom of the 1990’s hit, the size of the average newly constructed home grew by almost 1,000sqft. This caused a dramatic increase in the demand for larger plant material. Growers needed a way to grow plant material to a larger size efficiently and still be able to harvest it year round. To do this plants needed to be grown in a container, but use less water than tree boxes and large above ground containers, as well as grow quicker than conventional container produced plants. Though the building boom has begun to slow, the demand for large plant material is higher than ever.
The advantages of pot-in-pot are numerous. The most noticeable advantage is an increased growth rate. Shrubs will not only grow to a larger overall size in less time, but they will also be more filled out and have more dense foliage. In trees a similar result is noticeable. The tree will not only grow in caliper more quickly, but will grow a larger head and denser canopy in less time. Besides increased growth rate, pot-in-pot produced plants require little if any winter protection while in the socket pot. There is also no need for any type of blow over prevention or spacing of plants every year because the spacing is preset. Pot-in-pot production systems do not require any specialized harvesting equipment because harvesting is reduced to simply lifting the insert pot out of the socket pot and placing it on any type of vehicle. Aside from being able to harvest year round, pot-in-pot actually extends the growing season because it insulates the root zone. This insulated root zone causes a dramatic decrease in the amount of water that the plant needs during high temperatures compared to above ground containers. These are the main advantages of pot-in-pot that were shared by all growers in this study, but each grower also experienced other advantages unique to their system.
As there is with any type of production system there are some draw backs to pot-in-pot production. These consist of issues such as what is called blow-out, where the roots of the plant break through the insert pot and grow into the socket pot making it difficult to harvest the plant. Another disadvantage to a pot-in-pot system is that the spacing of the field is permanent. This makes crop scheduling more difficult because the grower must consider the fact that the plants can’t be moved around on the production pad. The biggest deterrent of pot-in-pot production is the cost. It costs approximately ten times as much to install a pot-in-pot field as it does a conventional above ground system. To install fifteen to twenty-five gallon pots, it cost approximately $20,000 to $25,000 per acre. This is what causes most growers to steer away from pot-in-pot, but if all other factors are considered, this high initial cost is quickly offset by all of the benefits it brings.
There is no set method for constructing a pot-in-pot field. Each grower has his own way of doing it and usually changes methods several times before completing the whole operation. Each method contains different ways of completing the same basic principles required to successfully install a pot-in-pot system. These basic functions are as follows. The growing area is laid out in the same method as it would be for an above ground area. After the perimeter is established and the orientation of the pots is decided on, the spacing of the pots must be determined. After determining the spacing of the pots each row must be marked. A trench approximately two feet deep is then dug at the location of each row and a corrugated drain pipe is placed in the trench, covered in gravel, and then filled in with soil. After the drain lines have been installed the rows need to be re-marked and the spacing within each row established. The location of each pot should be marked and then a hole the size of the socket pot is dug at each location with an auger, stopping approximately four to six inches above the drain pipe. A pair of hand held post hole diggers are then used to dig a small hole from the bottom of the large hole to the drain pipe. This small hole is then filled with gravel and forms the drain for each pot. A socket pot is then placed in each hole and backfilled. After all of the socket pots have been installed, a small trench is dug beside each row for the irrigation line. This trench only needs to be about two inches deep so many growers use a single disc to cut it. After the irrigation lines are installed a ground cloth is laid over the entire field, an "X" is cut at the location of each socket pot, and the points are folded downward into the socket pot. This is the basic method for installing a pot-in-pot field, but there are many variations that work well.
Irrigation of a pot-in-pot operation must be handled by drip irrigation. Spray stakes are employed in most situations because traditional drip emitters simply will not supply enough water to large containers. The spray stakes are placed in each pot and connected to the buried irrigation line with a spaghetti tube. Most spray stakes have an adjustment on them to control the amount of water being applied to an extent, but in large pots several stakes can be placed in the same pot to supply ample water. The advantage of using drip irrigation rather than overhead irrigation in a pot-in-pot system is that it requires significantly less water to be applied to reach the same moisture level in the growing media. There is no deflection caused by the plan’s canopy and less evaporation due to the fact that the water droplets travel a shorter distance through the air. More water can be reclaimed because any unused water travels through the pot and into the drain system. It is then returned directly to a retention basin. With overhead irrigation much of the water never enters the pot and a lot of the runoff is never reclaimed. The major disadvantage to drip irrigation is that it requires a clean water source. Silt particles can clog the narrow spaghetti tubes and spray nozzles. This can be dealt with one of two ways. For a smaller operation a well can be used to supply water to the drip irrigation system. Well water is normally clean enough not to need any filtering and works well in these systems. For larger operation where surface water must be used a filter system can be installed to clean the water. There are several types of systems on the market now but the most popular are sand filters and disc bank filters. Drip irrigation is the superior method of irrigating a pot-in-pot operation due to the more accurate placement of water.
Growing in a pot-in-pot system is very similar to growing in conventional above ground containers. Propagation and liner production are handled the same as for conventional production. After the liners have been grown to the desired size, they are transplanted into the insert pot that they will be sold in. The insert pot containing the plant is then taken to the production area and placed into a socket pot of the same size. The plant is grown in that location until it is ready to sell. It is then lifted out of the socket pot and sold. There are no special steps that must be taken to grow in a pot-in-pot system, it actually reduces the number of processes required to produce a finished plant.
Pot-in-pot production has many advantages over above ground container production. It produces a higher quality plant in less time and requires less manpower than conventional production. Pot-in-pot requires less water for irrigation and no winter protection for most plants. It does have a high initial start-up cost and requires much more labor to install than an above ground container production facility, but it quickly overcomes these disadvantages with the ability to cut production time by one-third.

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