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College of Food, Agricultural, and Environmental Sciences


Tree Liners Give Purpose to Retractable Roof Greenhouses

January 11, 2005

COLUMBUS, Ohio -- Nursery professionals looking to economically produce tree liners, boost profits and get their product out to customers more efficiently may have found the answer in retractable roof greenhouses.

The relatively new technology, popular in areas like the South, Southeast and Pacific Northwest, is still finding its place in the Midwest. But nursery professionals may now have found a purpose behind the high-end technology -- to use it to produce their high-end tree liners.

"The structures fit in well with the expanding tree liner market of the Midwest," said Hannah Mathers, an Ohio State University nursery and landscape specialist. "Growers are looking for ways to produce plant material suitable for their region profitably and more efficiently. Retractable roof greenhouses may help them do that."

The structures, which come in either a flat-roof or a peak-roof variety, can open or close those roofs and roll up their sides to help control ambient temperatures. Such capabilities allow for better temperature and humidity control and improved wind and light conditions. Current Ohio State University research has shown that tree liners grown in retractable roof greenhouses produce greater height, greater diameter and greater root mass than those grown outside of such a structure.

"The moderating affect of the climate in the retractable roof greenhouses is the key to the success of the plants," said Mathers. "Plants grown outside during a normal day are subjected to various stresses because of various changes in temperature and humidity. In a retractable roof greenhouse, the plants are not going through those extremes."

The research, funded by an Ohio Department of Agriculture and U.S. Department of Agriculture Specialty Crop Block grant, was conducted to determine the feasibility of using the structures in liner production, and comparing the liners produced in those structures to those produced apart from the structures, as well as those received from Western outlets.

A tree liner, or whip, is referred to as a small plant that is transplanted or grown on to become a larger plant, and marketed as caliper nursery stock. The West has cornered the market in liner production, producing field-grown whips then shipping them to the Midwest and East for spring planting.

Research has shown that the retractable roof greenhouses offer benefits in tree liner production that markets in the West cannot provide.

One benefit is the availability of the liners for fall planting. "West coast production is designed to supply Midwest growers with whips in March. In Ohio, with heavy clay soils, the ground will still be frozen or the fields will be too wet to plough and plant," said Mathers. "Also, in some nurseries, 50 percent or more of the yearly sales occur during the spring. If labor can be diverted from planting to conducting shipping operations, this would be a major resource savings."

Liners grown in retractable roof greenhouses are ready for fall planting and, in many cases, are at least one year ahead in growth to their Western counterparts.

"This spells profits for nursery professionals," said Mathers. "It's all about growing the biggest tree possible in the shortest period of time. If it normally takes three years to grow a tree in the field, with a $75 average price on that tree, and you can reduce that growth time to a year or two years, that's a $25 profit in the grower's pocket each time."

Another benefit is the availability of tree species specific to that growing region. "Species grown in Western markets are ones that respond well to that region and may not do so well when planted in the Midwest or elsewhere," said Mathers. "Plus, some species that are popular here are not produced and shipped from the West because they are just too difficult to grow in that environment." Additional Ohio State studies involve producing Ohio species that are difficult to grow and measuring their success in retractable roof greenhouses. Such species include Ivory silk, red oak, yellow wood, lilac and green spire.

A third benefit is the incorporation of pot-in-pot production into the system. Pot-in-pot production is a technique of growing container trees and shrubs in a holding pot permanently placed in the ground. Such a technique eliminates bare-root production, which is popular out West.

"Bare root plants suffer more from transplant shock than containerized material," said Mathers. "Also, sometimes whips are dug up before they reach true dormancy and can be subject to late winter freezing temperatures when they are shipped to the Midwest for planting."

In the past, the Ohio nursery industry was not growing its own whips because the growing season is too short. But retractable roof greenhouses are beginning to change that. Nurseries across Ohio, like Willoway Nurseries, Inc., in Avon, are seeing the benefits of using the technology for tree liner production.

The company, which was using standard poly greenhouses, was able to double its production of tree liners in one season using one retractable roof greenhouse. They quickly added another structure to their production.

Mathers said with retractable roof greenhouses, seedlings can be grown into whips under full cover through fall, winter and early spring and, when conditions become more favorable, retract the roofs and/or walls and turn the nursery stock into a commercial production site without ever having to move the trees.

Such convenience, Mathers said, decreases labor and equipment costs. The greenhouses also are affordable to build, costing only $1 a foot for flat-roof houses and $3 a foot for peaked-roof houses.

"When the average price of a liner is going for $15, it makes economical sense to build a retractable roof greenhouse. If that tree is established in one year, then that greenhouse is already paid for," said Mathers. "The nursery professionals can focus on buying other products. So what's good for the industry is good for the individual grower.

Ohio growers import approximately $14 million worth of tree liners from the West annually. It is estimated that Oregon liner sales, alone, into Ohio and surrounding states total $50 million a year. The wholesale nursery industry in Ohio is worth about $580 million. The landscape, garden center and nursery sectors combined are worth approximately $2.5 billion annually. Nursery production is estimated to be the fourth largest agricultural industry in Ohio, behind corn, soybeans and dairy.

Candace Pollock
Hannah Mathers