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.
Tree crop 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 an 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.
