COLUMBUS, Ohio - A relatively new landscape and nursery stock production technique that combines conventional field and container practices is finding a successful niche in the industry.
Pot-in-pot (PIP), the technique of growing container trees and shrubs in a holding pot permanently placed in the ground, is an alternative system to field culture, or growing trees and shrubs directly in the soil. Pot-in-pot is becoming very popular but it has some critical issues that need to be considered.
"Pot-in-pot is an increasing trend in the United States," said Hannah Mathers, an Ohio State University nursery and landscape specialist. "Container production is getting to be a bigger deal. On the West Coast, container production represents about 45 percent of nursery production. It's anticipated that container production will continue to grow and eventually pass field production, and pot-in-pot is part of that growth." Mathers speculates the growing popularity of pot-in-pot production is due in part to the many advantages the technique offers landscape and nursery professionals over field production.
"For one thing a tree can be sold year-round in container production," said Mathers. "If you put a tree in the ground, you basically only have two time periods to lift that tree for sale, spring or fall. But with pot-in-pot, you've got a lot more flexibility with the material." Trees and shrubs raised in pot-in-pot production also tend to grow faster because the soil-less medium used to grow the plants enables professionals to add additional fertilizer and water, something that can't be done with soils. "What used to take two years in the field could take only one year in a container," said Mathers. "So you can turn more units off that acreage of land, about three times the production versus field culture." Pot-in-pot production is also more environmentally friendly than field production because it reduces soil mining, a condition where the topsoil is removed with the root balls in conventional field culture.
Research has shown that soil removal due to 'mining' has enormous implications to the economic viability of a field nursery. It is estimated that the harvesting of 44-inch diameter balled-and-burlapped stock can result in the loss of 470 tons of soil per acre. This is an average of 94 tons of soil lost per acre over a 5-year rotation or 2.8 inches of topsoil lost in 5 years. "Of course pot-in-pot requires relatively permanent modifications to a nursery field that result in soil profile changes," said Mathers. "But if for some reason you reverted a field from pot-in-pot back to conventional culture, the soil levels would be virtually unchanged." Despite such advantages, however, the biggest drawback in developing a pot-in-pot system is the cost, which can run $30,000-$32,000 an acre. Mathers said drainage and irrigation are the two biggest driving forces behind the high cost. "A drainage line needs to be placed under every row of pots, because whenever you grow a plant in a container, you get what's called perched water, where a certain amount of water in the container never evaporates," she said. "So you need the drainage system to eliminate as much of that water as possible." Trees and shrubs grown using the pot-in-pot technique may be of higher quality, but Mathers said the market for such quality has to exist for a business to be successful in selling its products.
"One of the disadvantages of pot-in-pot is that trees need to be grown for a certain market size because once they reach that size, they either need to be transplanted to a bigger pot or moved out. So the market has to be there to absorb that constant turnaround," she said. "You also have to have an established clientele that finds quality important and is willing to pay a higher price." Such a market, however, is one reason Mathers believes pot-in-pot will catch on in Ohio. "Ohio already has a good deal of nursery history and many businesses already deal with established clientele," she said. "Ohio is a shipping hub for nursery stock. One of the main market niches for nurseries is taking stock from the west, growing it, and shipping it to major population centers on the East Coast. Pot-in-pot fits that market really well." Pot-in-pot production came on the U.S. market in the 1980s and has existed in Ohio for approximately five years.