Otho Wells harvests strawberries from May 1 to June 15, at least six weeks before and for two weeks longer than is the norm in New Hampshire. Wells, professor of plant biology at the University of New Hampshire, developed this technique of overwintering strawberry plants under 6- to 7-foot-high plastic-covered tunnels, or hoop houses. While his method promises to support an important niche for market gardeners, ambitious home gardeners can benefit, too. It's not magic. Wells has simply adapted planting techniques long used in the South and West and added a hoop house.
The first week in October, Wells builds raised beds (10 inches high and 14 inches wide) within a series of tunnels at the university's horticulture farm in Durham, New Hampshire. He adds a 1-inch layer of compost to the soil, as well as any nutrients needed as indicated deficient by soil tests. Once the soil is ready, he tamps the beds with a sheet of plywood so the soil is firm and flat. He then lays drip irrigation hoses on the beds and covers them with black plastic, which he later cuts to insert plugs.
The black plastic and the hoop house work together to keep both soil and plants warm. "When it's -15? F outside, the soil inside is only frozen 1/4 inch deep," says Wells. That means there's no need to mulch the plants to protect the strawberry crowns from winter cold.
As soon as the beds are prepared, Wells plants young transplants rooted from runners, or "plugs," of 'Chandler' strawberries. This, Wells notes, is the key to his success. "'Chandler' has superb quality," he says, "with good size and flavor and excellent productivity." Originally developed to overwinter in California, 'Chandler' has been shown to have a 99 percent survival rate in New Hampshire hoop houses, and it produces nearly a pound of berries-that's a level quart-per plant. Wells has tried several varieties and plant types, including northern-bred June-bearers and bare-root plants. Productivity of other tested varieties was significantly less, and bare-root plants proved much less winter hardy. Although 'Chandler' strawberries are early producers, gardeners seeking the earliest harvest can plant the variety 'Sweet Charlie' as well. Though not as productive, 'Sweet Charlie' ripens one week earlier than 'Chandler', around the last week in April. Wells emphasizes that strawberry plugs are essential to getting the strawberries off to a good start. The strawberries grow in the plug trays - filled with a peat moss and vermiculite soil mix - for four to five weeks. Unlike bare-root plants, the plugs are easy to slip through 2-inch by 2-inch slits in the plastic and their strong roots won't be damaged in the process.
Wells recommends planting in double staggered-rows with 12 inches between the rows and 10 inches between the plants in each row. Planted this way, you'll need 72 plants for a 30-foot bed.
Though he's only tested his system in southern New Hampshire, he believes the technique is viable throughout the North. "It'll work up to the Yukon Territory," he says, then pauses and backtracks a bit. "But if it gets to -30° to -40°, you might have to mulch the plants."
After planting and one or two waterings, nothing is done until spring, other than rolling down the sides of the tunnel in late October, and opening them again in early April. In February, as the days get longer, the strawberries start growing again. By late March, flowers appear. "You have to protect those flowers from frost, so cover the plants with a row cover," says Wells.
"Strawberries are wind pollinated, but there's little or no wind in the hoop houses," says Wells. Pollinating insects can't be relied on either, so he uses a leaf blower to circulate the pollen. In the hoop houses, pests are not a problem, but incomplete pollination is. On warm and w days, Wells rolls up the sides of the hoop house, allowing wind pollination.
To keep the strawbery hoop house plants healthy and growing fast, Wells applies fertilizer in late April or early May. He recommends a water-soluble fertilizer with 15 percent nitrogen, diluted at the rate of 2 tablespoons per gallon. Soak the roots of each plant with this solution to ensure good growth throughout the harvest season.
After harvest, Wells pulls most of the plants out so that he can start fresh in the fall. "We've tried to grow plants for two years. The yield falls to less than half of what it was the first year," notes Wells. He leaves just enough plants to produce runners in summer. He roots those in mid- to late August then plants them in October.
Once you have a supply of 'Chandler' plants, it's easy to grow plugs. Cut off "daughter" plants, the tiny plants that form on runners. Set them into a 50/50 peat moss-vermiculite soil mix and place them in a bright, humid location until roots form. A misting chamber in full sunlight is ideal.
"The cost of growing strawberries this way is about $1.25 per square foot," says Wells. "And this includes time and every other detail, including the cost of the hoop house." Each square foot yields 2 pounds of strawberries, valued at approximately $4 to $5. That's a $2.75 to $3.75 profit per square foot!
Hoop houses are available in a range of lengths and for different prices. "A serious home gardener should look at buying a ready-made system. It's so much simpler. If you make something out of PVC and get a foot of heavy, wet snow, chances are it will collapse."
Besides cost savings, Wells sees the benefits of the system another way. "Last year I bought a pint of out-of-region strawberries and compared them with ours. They tasted like rubber, and ours were fresh and tasty."
Article published on June 23, 2008.