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
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
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
Department of University Relations
5761 Howard A. Keyo Public Affairs Building
Phone: (207) 581-3744 | Fax: (207) 581-3776