Pot-in-pot operations expensive to establish

Pot-in-pot production is growing rapidly in the United States, most commonly among growers of shade, fruit and flowering trees, though the system is suitable for shrub production as well. Probably the biggest deterrent to establishing a pot-in-pot (PnP) operation involves the expenses of establishment. Each plant requires 2 pots, the in-ground socket or moat pot, and the removable liner pot. In Tennessee it has been estimated that it costs roughly $20,000 per acre to set up PnP operation involving 10 gallon (#10s) containers (1). While nursery supply houses (2) can furnish many different pot sizes, #15s (15 gallons) are quite common, as well as #25s. One of the largest size PnP pots is a #45.

Cost to establish
It's hard to generalize on the establishment costs to establish a PnP operation. There are many variables in setting up this type of nursery, as seen in the Table 1.

With the number of different variable involved in setting up a PnP system, growers will have to design their own budgets. Nursery supply houses can easily provide quotes on the cost of supplies for materials. A drainage contractor can provide an estimate of the costs involved for installing a buried drain tile system, if one is needed.

Plastic pots
The number of pots required for PnP operation will depend upon the expected size of plant at harvest, and the in-row and between row spacing. Common spacing in both directions has been 4'-6' for shade trees. Remember to include alleyways between blocks in calculating the number of pots needed per acres. Blocks refer to groups of rows. For larger socket pots a block could be 4 rows wide. Alleyways can be kept in sod if the site has good soil drainage. A firm base during the wet months of the winter will be important as a surface on which to operate machinery. Alleyways can also be surfaced with pea gravel.

The steep initial expense for all the pots can be partially off-set by the life of the socket pots. Industry proponents of PnP report that the socket pots can last as long as 15 years (3). Sockets pots have typically been injection molded for durability. The liner pots used in a PnP operation are usually blow molded, and are not as stiff as the socket pots (1). Nonetheless they are built well and can withstand long distance shipment. They can also be used in retail settings.

Drainage
Unless the PnP operation is established on sandy soils, supplemental in-ground drainage will need to be considered. If there is a slight slope to the field rain and irrigation water will collect in the base of the socket pots. It's not uncommon to find growers having to install PVC drain tile under every row of socket pots. Depending upon the spacing of the rows this practice can result in having to lay considerably more tile than would be the case for a typical orchard or pasture. Tile lines have to be installed before any pots are placed in the field (4).

A very nicely designed PnP nursery raising 25 gallon basswood trees for the landscaping industry

Tile lines are typically set in with laser leveling and mechanical trenchers. Tile lines are generally installed during the dry months of the year. After the tile lines have been installed it's important not to avoid compacting the soil above the tile lines as this can retard water infiltration rates (5). An agricultural engineer with a major tile company (6) can assist with the design of a tile system. Tile lines are typically set 2-3' deep. A properly installed drainage system will keep the water soil water level at the depth of the tile line, thus protecting the socket pots from flooding. Taller socket pots will offer better total container mix aeration and thus moisture retention. Taller pots will add to the cost of establishment (3).

Irrigation
PnP operations will require supplemental water during the growing season, though much less than traditional above ground container stock. The plants will be growing in hard sided impervious container. Even though some of the plants will send roots out through both the liner and socket pot which will take up soil moisture, growers try to reduce the so called "rooting-out" as much as they can. It can be extremely difficult to remove these liner pots from their sockets when the tree or shrub is ready for sale. Growers have tried painting the inside of the liner pot with copper (7) to reduce root escapes and circling, as well as placing fabric liners (8) inside of the socket pots to cover the drain holes.

There is generally no irrigation runoff from a PnP farm growers will not have to build ditches and confinement basins as would the above ground container grower.

PnP operations typically rely on either trickle emitters or spray stakes to water the pots. This type of irrigation requires careful attention to water quality than would be the case with an overhead system. Sand filtration units (9) will need to be installed in order to keep the emitters from plugging. Workers will need to be assigned to walking each row every day during the irrigation season to check for clogged emitters.

On the fringe of urban areas a PnP operation can be irrigated with the public water supply. Farm properties with valid Water Rights are often not available. If the PNP nursery is less than 2-3 acres city water will be affordable.

Fabric weed barriers
Given the tight spacing between the socket pots some growers revert to using heavy duty woven polypropylene ground cloth to cover the blocks before the socket pots are installed. In a year 2000 (1) survey it was estimated that ground cloth could cost as much as $1,000/acre. This is a considerably higher expense than traditional use of residual herbicides that would be used in a field grown operation. Given the longevity of the PnP operation growers will need to be constantly aware of the potential for perennial weeds to establish themselves between the pots. If the blanket of woven ground cloth remains intact, weed growth should be greatly diminished.

Comparing different propagation methods
In a 2002 study at Auburn University (10) a comparison was made between establishing 3 different woody ornamentals (crape myrtle, dogwood, and Leyland cypress) using either PnP, aboveground containers, or conventional field production. This study was based on a number of assumptions related to starting a commercial nursery. These included purchasing farm ground, building an office for the nursery headquarters and a machine storage building for the implements, building greenhouses for propagation, purchasing tractors.

Under these southern U.S. conditions, it was estimated that it would take approximately $27,000/acre to start either a PnP or an above ground container operation. Establishing the nursery stock in the field directly however, only required approximately $6,500/acre. At the end of the 3 year growing period both the PnP and the aboveground containers operations had the highest fixed and variable costs per acre, while the field operation was the lowest. The study then shows that establishing a container operation, whether it involves above or within ground production, is an expensive proposition.

The above ground container operation was considered the easiest to start, and had the least impact on the site chosen. The PnP operation involved the most work as the site had to be leveled and the socket pots augured in. The PnP system was also considered the most inflexible as there was no way to alter the spacing and layout of the socket pots without digging them up.

If prospective growers have determined the type of woody ornamental species they would like to propagate the PNP technique, they are advised to set up a small test block to determine the feasibility objectively before they commit to a larger operation.

This study shows that the producing woody ornamentals in the field is probably the best design for producers hoping to minimize initial capital outlay. While the field operation had the lowest establishment cost, producers will have to own, or purchase a tree spade to dig up stock at maturity.

References
1. The pot-in-pot production system. 2003. Mark Holcomb, University of Tennessee.

2. McConkey Company, Sumner, WA.

3. Nursery Supplies, Incorporated. McMinnville, OR.

4. Planning an agricultural subsurface drainage system. 2001. Jerry Wright and Gary Sands, University of Minnesota Cooperative Service.

5. Management of drained fields. 1990. Ontario Ministry of Agriculture and Food, Adex #: 555.

6. Advanced Drainage Systems, Inc., Hilliard, Ohio.

7. Physical and economic requirements for pot-in-pot nursery production. 2002. Robert McNiel, Department of Horticulture and Landscape Architecture, University of Kentucky.

8. Production of 'Rotundiloba' sweetgum using Tex-R-Agroliners. 1998. John Ruter, University of Georgia, Department of Horticulture.

9. Nursery stock production using the pot-in-pot technique. 2002. Hannah Mathers, Department of Horticulture, Ohio State University.

10. The economics of producing nursery crops using the pot-in-pot production system: Two case studies. 2002. Charles Hall, John Haydu, and Ken Tilt, Southern Cooperative Series Bulletin #402.

First posted: December, 2004