Oregon farmer Rob Miller, owner of Mt. Jefferson Farms, is always searching for new and innovative ways of integrating forestry with agriculture, and vice versa (see the July 1997 issue of Temperate Agroforester to learn about his "working" riparian forest buffers).

For the last two years, Rob has experimented with the production of containerized nursery stock under the natural shade of a poplar plantation at his nursery near Salem, Oregon. Using the "Pot-in-Pot" (PNP) system, a relatively new practice for the nursery industry, he is testing the feasibility of growing ornamental plants as an alley crop in an agroforest.

Pros and Cons of PNP

The PNP system combines elements of both container and field-grown methods. A plastic container, the "socket pot," is placed permanently in the ground and a second container of the same size, the "insert pot," which contains the plant in a soilless medium, is placed into the socket pot. Standard sizes are used for both. A drip irrigation system is set in place to water each container, thereby reducing water use and weed growth compared to sprinkler irrigation.

One of the main advantages of PNP versus bareroot production in the field is lower cost o­nce the system is established, both in labor costs in harvesting - the insert pot is merely lifted out of the socket pot - and in shipping costs associated with the lightweight artificial medium. The root systems of plants in PNP are less subject to the extremes of temperature than are container plants grown above ground, and PNP eliminates problems with blow-over. Plants are able to progress more quickly from liner to saleable size.

However, cost of establishment is the biggest drawback of PNP since holes must be augured in the soil, containers must be inserted and a drip system installed. Nevertheless, this o­ne-time initial investment can be recouped by lower costs during production and faster plant growth. The other potential problem with PNP is root elongation out of the insert and socket pots into the surrounding soil. Several methods are used to control root escape, such as copper paint inside the socket pot or fabric barriers.

Trial at Mt. Jefferson Farms

The PNP system established two years ago o­n a trial basis has worked well so far, according to Shirley Dague, nursery manager for Mt. Jefferson Farms. It has eliminated problems with blow-over and, combined with drip rather than sprinkler irrigation, has resulted in better plant growth compared to growing nursery stock in a conventional "can yard." Container plants sitting in the outer rows of a can yard are exposed to more sun and therefore require more irrigation, she explained, a fault alleviated by PNP.

Three rows of socket pots are spaced at 3 X 3 ft. within a poplar clone bank adjacent to the nursery. The poplars, now 4-6 years old, have been thinned to 16 X 16 ft. spacing and their lower branches pruned. A variety of native trees and shrubs are being tested in the PNP system to determine which species do best under the natural shade of the poplars.

"We’ve had no problems so far with root escape," Shirley said, "expect, not surprisingly, for some red alder that have been in place for two years." However, she added that deer browse has been a problem since the poplar plantation is a more protected area for wildlife compared to the open nursery growing grounds. Seed shed from some of the poplar clones has been prolific, she said, and this has required some extra time to pull small seedlings from the pots.

Irrigation can also be made more efficient, Shirley suggested, by placing species with similar water needs o­n separate drip lines operated o­n different schedules, or by using emitters with different flow rates. Using emitters o­n "spaghetti" tubing rather than plugged directly into the above-ground plastic hoses is the better option, she said.

Although weed growth in the soil between the socket pots is much reduced with drip compared to sprinkler irrigation, Shirley said that residual weeds are controlled by a pre-emergent herbicide application plus o­ne glyphosate spray per year. Herbicide spraying around the woodlot trees is also facilitated by the ease with which the insert pots can be removed and replaced.

PNP as Alley Crop

Beyond its use by commercial nurseries, the PNP system could be used in an agroforestry context by forest or plantation owners to produce a saleable understory crop during the years before tree harvest. Although it has a higher initial investment than annual row crops, it may have a higher profit potential due to the greater value of the plants produced.

As with forest farming, alley cropping and silvopasture, the level of shade cast o­n the ground below depends o­n tree species, canopy age and size, spacing and management activities such as pruning and thinning. The challenge is to match the light requirements of the crops grown in the understory to the shade levels cast by the trees as they grow.

With PNP, different species of understory plants could be grown to take advantage of different shade levels as the trees grow. Plants needing full sun could be easily replaced in later years by those benefitting from more shade after tree canopy closure. Shirley suggested that the first few rows around the perimeter of the plantation or agroforest could be used throughout the rotation since there would always be more light available compared to interior rows.

The socket pots and drip irrigation components are relatively durable, Shirley said, and could be reused for another rotation after tree harvest. Following harvest, holes for the socket pots would need to be redug, and drip tubing and emitters replaced as necessary.

Growth of tree roots from the soil into the socket pots is a potential problem with PNP in an agroforest setting. This has not been observed so far in the trial at Mt. Jefferson Farms, Shirley said, despite the pots being bordered by some fast-growing poplar clones. Adequate distance between the tree row and outer socket pots is needed both to lessen root competition and to allow equipment access.

Of course, before starting PNP landowners should undertake a careful investigation of potential markets for containerized ornamental plants. There would likely be opportunities for growing o­n contract to retail nurseries or direct marketing to customers through o­n-farm or subscription sales. PNP has been used in the South to grow living Christmas trees which can later be planted outdoors as landscape trees, an added appeal to customers.

Based o­n positive results to date, Mt. Jefferson Farms plans to expand its trial of PNP under poplar trees next year. More field testing is needed to determine the criteria necessary for the PNP system to be a biologically and economically viable production method in an agroforest setting.

Reference: Pot-in-Pot Production of Nursery Crops and Christmas Trees, Alabama Cooperative Extension System (www.aces.edu/department/extcomm/publications/anr/ANR-893/anr893.html).

(This article originally appeared in the April, 2001 issue of the Temperate Agroforester.)