Physical and Economic Requirements for Pot-in-pot Nursery Production

Dr. Robert McNiel, Department of Horticulture and Landscape Architecture
Dr. Bridget Behe, Department of Horticulture, Michigan State University
Chris C. Montgomery, Buffalo Co. Inc., Charlotte, NC

Nature of Work

The pot-in-pot production system has been increasing in acres steadily during the l990s in the eastern two-thirds of the United States. The system is centered around the production of caliper sized shade and flowering trees and large shrubs in containers which are placed in the ground. A permanent container (socket pot) is placed in the ground, where it may last for a decade or longer. A production container (insert pot), with plant, is inserted into the socket pot. The time a plant is in a production container can be from six to nine months but not more than two years. Unit sizes have varied from 3 to 30 gallons. On an acre basis, more units in the 7, to, and 15 gallon sizes appear to be in production today. The trend is away from smaller sizes and into larger sizes. The socket pot and production pot need to be two distinctly manufactured units. Compatible units during the past few years have been manufactured only in the mid-range to large sizes. Container media consists of the common bark-based media existing in the industry.

Holes are angered using a size related to the size of the container when installed on well-drained soils, such as sandy soils. When well-drained soils (most others) are not available, a trench is formed. Tile is usually laid in the bottom of the trench just below the bottom of the socket container. Insufficient drainage is the reason for plant loss in most early established pot-in-pot sites.

Spacing is dependent upon the size of the canopy at harvest time. Common spacing in both directions has been 4, 5, or 6 feet. The socket pot is inserted into the angered hole and leveled. It should rest on firm soil. The socket pot determines the orientation of the plant in production, and new plant growth needs to continue the axis formed by the trunk. A spacer is set in the bottom of the socket pot. The spacer should be % inch taller than the height of the air gap between the bottoms of the 11 two containers. This prevents the weight of the production container and plant from becoming wedged into the socket container. Once the plant is placed in the production container, it can be transported to the production site and inserted into the socket container. Weed control and fertilization practices are similar to those existing in container production.

Micro irrigation nozzles are used, and the nozzle should be one which sprays water across the entire media surface. Plant growth can be maximized and plant stress can be reduced by making multiple water applications per day.

Escaped roots destroy the socket pot and thus disrupt the value of the system. Copper-treated paper or fabric is placed on the bottom of the socket pot to cover the drain holes in order to keep roots from escaping. The production pot is treated on its inner surface with copper in order to reduce circling roots and root escapes.

The industry may or may not place fabric over the entire production area. Fabric (landscape fabric) appears to be more common in the southeastern United States. An X is cut in the fabric above each socket container it covers. The fabric reduces erosion, eliminates weed maintenance, allows movement around the area at all times with a dry surface, and keeps UV light from reaching the socket pot.

Most acreage in the northeast and central parts of the United States has been left uncovered. Weeds or turf are allowed to grow between containers and then maintained on an as-needed basis.

Results and Discussion

Production costs were synthesized for a hypothetical 15acre nursery with lo acres in crop production. An economic comparison was made as to whether this nursery was producing the crop in the field, as aboveground container plants, or as pot-n-pol container plants. In-field production based on 19 ft2 per plant yielded 18,990 salable plant. Above ground container production based on 18.6 ft2 per plant yielded 21,430 salable plants. Pot-n-pot production based on 16.8 ft2 yielded 23,338 salable plants. Capital requirements and annualized fixed and variable costs were determined for each system. Table 1 summarizes these costs and illustrates the total cost per salable plant for each production system.

Significance to the Industry

There are more plants per acre in the pot-n-pot system than in the field production. Plants can be harvested any day of the year. The system produces a container-grown plant that should not be root-bound. Plants have the advantage that they are anchored in the ground and do not blow over in the wind, as compared to above ground container production. Pot-n-pot uses the natural ground temperature, like in-field, without the need to heal-in or move plants to a structure for overwintering or to shade the container wall from summer sun. Moisture and nutrients can be monitored better than in a filed situation. Total cost of production per salable plant was similar between in-field ($23.73), aboveground container ($23.71), and pot-in-pot ($21.52).

Table 1. Capital requirements and cost comparisons for crapemyrtle production using alternative production systems, 15 acre nursery with 10-acre production area, 3-year production cycle with 1 year in liner development, USDA hardiness zone 8, 1996.

Production System ($)
Item IFz AGCy PNPx
Capital Requirement $210,840.00 $223,170.00 $224,260.00
Machinery/equipment operation $26,370.00 $15,650.00 $18,700.00
Fixed Cost $352,880.00 $350,450.00 $374,525.00
per plant $18.58 $16.35 $16.05
Variable Cost $97,790.00 $157,650.00 $127,680.00
per plant $5.15 $7.36 $5.47
Total Cost $450,670.00 $508,100.00 $502,205.00
per plant $23.73 $23.71 $21.52

z in-field.
y above ground container
x pot-in-pot