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UMaine Today Magazine

Extension Connection

Marjorie Peronto
Marjorie Peronto

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Winter planting

Think a Maine winter is tough? Pity the poor shrub: freezes, thaws, frost heaves. It's enough to make a woody perennial curl up and die.

Which is a bit of a problem for growers.

Enter Marjorie Peronto of University of Maine Cooperative Extension, who has teamed up with organic perennial growers Peter and Julie Beckford of Rebel Hill Farm in Clifton, Maine, to research the effectiveness of pot-in-pot overwintering. The technique is widely used in the South to reduce heat-induced stress on roots. In Maine, cold is the stressor.

"When you're growing perennials, you're relying on them to be able to make it through the winter, and our winters fluctuate in terms of temperature and precipitation," Peronto says. "Some years, we'll have a nice blanket of snow. Some winters we don't get any snow and temperatures drop to the point where severe cold penetrates the ground and causes root damage."

There are several ways to prevent root damage, but each has its drawbacks. If growers raise woody perennials in pots, they can insulate the pots with thermal blankets above ground, but mice often nest under the blankets and girdle the stems. They can place potted shrubs in a greenhouse over the winter, but the cost of maintaining a greenhouse is prohibitive for some growers. Shrubs can thrive when raised in a field, but when growers dig them up, most of the roots are left in the ground.

"With a field-grown woody plant, the root system extends up to three times as wide as the drip line of the plant," says Peronto. "There's no way you can dig up all the roots and get them in a pot. You often leave between 75 percent and 90 percent of the root system behind. That is monstrously traumatic."

In Maine, many growers don't like to sell plants that have undergone such trauma, so they often purchase dormant bare root plants and pot them to sell in the spring. As a result, the plants haven't had enough time to develop much of a root system.

The Beckfords, who specialize in native herbaceous perennials, wanted to expand into shrubs. Because Peronto has an affinity for both native plants and woody perennials, she jumped at the chance to research pot-in-pot, which combines the insulating benefits of field growing with the plant health benefits of starting and leaving plants in a pot.

In June 2008, she began planting native rose and chokeberry seedlings in 3-gallon pots, which were then placed in 3-gallon "socket pots" that are permanently sunk in the ground. She also planted seedlings in the field to see how the plants will fare in two different conditions.

The plants will stay where they are over the winter. This spring, if they have reached a marketable size, Peronto will unearth them and compare the results.

UMaine Today Magazine
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