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Grubinger: Greenhouse Tomatoes

07/09/12 7:55AM By Vern Grubinger
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(Host) Visit just about any roadside stand or farmers' market in Vermont and you'll find fresh local tomatoes available early in the summer. Commentator Vern Grubinger, UVM Extension vegetable and berry specialist, explains how Vermont growers do it.

(Grubinger) There's nothing quite like a ripe tomato, picked fresh from the garden or farm field. Except maybe a ripe greenhouse tomato, locally grown.

In Vermont, the harvest period for outdoor tomatoes is pretty short, typically August and September. But vine-ripened tomatoes from inside a local high tunnel or greenhouse can be harvested from late May through November. A high tunnel is just a simple greenhouse, without a permanent foundation. These protected environments not only extend the harvest season, they also keep out the rain, avoiding diseases that attack the plants and fruits. Yes, a tomato has seeds so it's technically a fruit.
To have ripe fruit by early summer, Vermont growers start their seedlings in February and then transplant them into the ground, inside the greenhouse, in March.

Several hundred high tunnels and greenhouses across Vermont produce about three million dollars worth of tomatoes each year. Worldwide, most greenhouse tomatoes are grown using hydroponics, where plants sit in an inert material, like rock wool or peat moss, and are fed a solution of synthetic fertilizers. In Vermont, most greenhouse tomatoes grow in real live soil, amended with compost. Most of our growers also use beneficial insects and organic sprays to manage bugs and diseases if necessary.

Because the plants grow in soil instead of a sterile medium, root diseases have been common in the past. Now, most growers use grafted plants to prevent this problem. They take the top of a plant, with the desirable fruit characteristics, and splice it onto the stem of a variety with strong, disease-resistant roots. This is done at the seedling stage, and once healed, the plants are set out in the greenhouse.

Outdoors, wind takes care of tomato pollination, but inside the greenhouse growers have to vibrate the flowers manually to get pollen from the male anthers to the female stigmas. It turns out that bumblebees do a better job than human pollinators, so growers put hives of commercially-reared bumblebees their tomato houses during flowering.
Side shoots, or suckers, are removed from greenhouse tomato plants so they'll grow upright with just one or two stems. The plants get trellised using twine and special clips, sometimes climbing ten or 12 feet high. This makes good use of the greenhouse growing space compared to sprawling on the ground. As the plants grow, each flower cluster is thinned to 4 or 5 fruits when they're still small, so that production is balanced over the season, and a heavy fruit load early won't reduce yield later.
It does require some fuel to heat Vermont's greenhouses in March and April, but this seems a reasonable trade off in order to have fresh local tomatoes in June and July. Growing the crop thousands of miles away also uses energy for shipping and storing. Besides, some of our growers use wood, corn or waste vegetable oil as greenhouse fuel. That means their fruits are renewable as well as delicious.
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