GALENA, Kan. — Temperatures haven’t risen much above freezing in the past week on a patch of land between Galena and Riverton, but that didn’t stop Tim and Violet Green from harvesting a few hundred pounds of tomatoes.

The bounty was picked from plants that are not being grown hydroponically or in pots. They are growing at ground level in Kansas soil.

Bright red, shiny and the size of grapefruit, the tomatoes were the product of seed the couple started in July.

“We’re the only ones I know of who grow all winter within about 200 miles,” Tim Green said.

Their secret?

“They’re called high tunnels,” he said of the long, rounded structures in which the retired couple also grow several varieties of lettuce, radishes, onions, cucumbers and green peppers.

“We have all the fixin’s for a salad, any time you want it,” Violet Green said.

A high tunnel, or hoop house, is a low-cost version of a greenhouse that can help market gardeners extend their growing season in order to improve the profitability of their farms.

According to the Missouri Vegetable Growers Association, high tunnels can be as simple as pipes or other framework covered by a single layer of greenhouse-grade 4-millimeter to 6-millimeter plastic sheathing. Typically, they aren’t outfitted with electricity for heating or cooling.

In 2001, research by the Bradford Research and Extension Center at the University of Missouri-Columbia evaluated the yield performance of several tomato cultivars within a high tunnel and in the field. The study found that high tunnels significantly enhanced the yield of the tomatoes.

“Based on the result in this research,” the study concluded, “it is possible for a grower to have vine-ripe tomatoes from mid-June until October in the central Midwest by using high tunnels as a complement to field production.”

But the Greens took it one step further. “We installed wood-fired furnaces for times when the temperatures really dip,” Tim Green said. “Our only cost is labor.”

Hence their crop of cucumbers, romaine lettuce, green peppers, and row after row of Red Deuce, Red Bounty and Carolina Gold tomatoes that are producing in late December.

During the recent onset of colder weather, Tim Green has bunked in a sleeping bag on a cot next to one of the furnaces, and he wakes several times a night to feed them more wood.

“When you wake up cold, that’s when you know you need to feed it,” he said.

The Greens use drip irrigation via a network of tiny spouts to each and every plant, and they practice heavy pruning to keep the plants from getting too wet and overgrown.

“They must have good air circulation,” Violet Green said.

The couple have relied on a fair amount of trial and error.

“We learned that in here, the time the plants take to mature is about 20 days longer,” Tim Green said. “The seed packet says 73 days, but in here we plan on 93. There is less light — it’s diffused — so it slows down photosynthesis.”

He also puts into practice years of knowledge learned while shadowing his grandfather, Vert Charles Dudley, starting when he was 6 years old.

“I followed him around his garden north of Stone’s Corner in Joplin,” he said. “We were both having a good time. He was teaching, and I was learning, and neither one of us realized it.”

Tim Green, who worked in the grocery business and for the highway department, described the couple’s level of involvement in gardening as “a hobby that got out of control. It was a job I always would have liked to have.”

There have been setbacks. One year, a new heating system went out the first night it was installed, and they lost everything. This year, the plastic that shielded the cabbage was ripped off in strong winds, which killed the delicate vegetable. But the couple’s persistence has paid off. The Greens are regulars at the Webb City (Mo.) Farmers Market throughout the winter, and they will be there today.

“One year we had more than 2,000 pounds of tomatoes,” Tim Green said. “We sell all we can grow.”

They also will play host to a Feb. 5 winter gardening conference under the direction of the University of Missouri and the Webb City Farmers Market. The conference will be open to the public, and attendees will include market gardeners from as far away as New York.

Come spring, the Greens will begin tending a well-established garden plot north of the tunnels, where 35 rows of blackberry bushes and 12 rows of asparagus plants will demand their attention.

They also will set out dozens of onion plants growing in trays in the smallest of their high tunnels, where neighboring Greek and red lettuce, and radishes now are making a showy appearance.

“Our son, Tim Green Jr., and our daughters and grandkids sometimes help,” Tim Green said. “I’m glad for the help, and glad to pass on what I know.”