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Passive heat greenhouse grows veggies in winter
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Passive heat greenhouse grows veggies in winter

NEW ULM, Minn. – For many northerners, the dearth of fresh leafy greens in winter is a trial to endure.

With spring comes the thrill of placing tiny seeds in still cool soils.

Several weeks will pass before the first bite of leaf lettuce, spinach, and kale will be ready to eat. The taste when it’s finally ready is like a drink of cool water to a thirsty person.

Tim Gieseke has found one way to overcome the long wait for leafy greens.

He grows delicious leafy greens in the winter using a passive heat greenhouse system.

No added heat is needed beyond the southern sun shining through the long side of his greenhouse.

During the mild winter of 2020-21, he grew something every month in his outdoor structure. The fresh leafy greens were especially appreciated by his mother and mother-in-law.

“If it is 20 degrees outside, it will be 85 in here, while the sun is out,” Gieseke said during an interview at the greenhouse.

The sunlight might only pour into the greenhouse for a few hours in winter, but he’s added enough passive heat design concepts to keep the south side of the greenhouse above freezing.

Cold-tolerant vegetables grow year-round.

Through the rest of the year, he has tomatoes, cucumbers, herbs, eggplant, Brussels sprouts and many other vegetables in abundance.

“It’s like going out to the grocery store,” he said. “I tell my wife every six weeks what we have available in the greenhouse for cooking.”

His passively-heated greenhouse relies on several concepts for success:

• A 20- by 36-foot greenhouse constructed from 1.25-inch chain link fence piping and covered with 6-milliliter plastic.

• Strong pipe footings sunk into the ground.

• A greenhouse within the greenhouse made of the same materials but 18 inches shorter than the taller frame. The smaller greenhouse is used only in winter.

• High quality composted soil in raised beds.

• Black barrels filled with water that absorb heat from the strong southern sun.

• Cold-tolerant vegetables.

• A brick walkway that holds heat.

• A white blanket that covers the cold-tolerant vegetables. The cloth lets about 60-80 percent of the sunlight in and keeps the vegetables about 10 degrees warmer than the air temperature.

His own greenhouse experiences

Tim Gieseke and his wife, Jenny, wanted a greenhouse on their Nicollet County farm.

Someone suggested they look for a used one, and about 10 years ago, they found a 20- by 36-foot hoop house.

The couple enjoyed the plastic-covered greenhouse that extended the growing season. It was a sunny place where their young sons could play while the adults planted despite snow and cold.

Sitting like a bowl on the ground, the plastic-covered greenhouse wasn’t anchored down securely.

A few years later, a severe thunderstorm hammered the farm. An old barn and the greenhouse were knocked down.

Sometime after the storm, Gieseke decided to use the hard wood from the old barn to build an improved greenhouse.

“When you do it a second time, you know what you want, so that is the benefit of it going down in the wind,” he said.

Work started by pounding chain-link fence piping into the ground as footings.

Using boards from the old barn, he designed a strong snow-bearing central pillar system. A central walkway runs between the supports.

Heavy-duty plastic sheeting was permanently installed over the 20- by 36-foot greenhouse. People ask why the plastic doesn’t come off in the warm months, but removing the plastic makes it almost impossible to get a tight fit that keeps wind out.

For water, Gieseke ran a misting system throughout the structure. The system draws water from the nearby cattle water hydrant. Overhead sprinklers apply about 20 gallons of water mist per day to the full garden.

There is iron in the water, so every six weeks, the nozzles are changed. He soaks the iron-clogged nozzles in vinegar so they’re ready to use next.

Then, on just the south side of the greenhouse, chain-link fence piping sits about 18 inches below the outer shell to form a greenhouse in a greenhouse. In the winter, plastic is placed over the pipes. He adds a curtain that faces north.

The smaller winter greenhouse is just tall enough for Gieseke to stand in. He’s continually surprised by the warmth in the southern portion of the greenhouse vs. the more open north side.

“With each sheet of plastic you add, you lose sunlight, but these spring greens don’t care,” he said.

To the north of the greenhouse, a pile of compost and manure sits and decomposes.

“We have livestock and compost, and I scoop up a pretty good dose of compost, take a tiny tiller, and work it into the ground,” he said. “The manure, straw, is quite old, nice, and decomposed, and I think that is a huge part of the growth in here. It is just having nice soil, and when you pull any of the weeds, there is no resistance.”

In the coldest months, he adds a frost blanket over the greens. Lightweight and breathable, the cover protects the hardy annuals.

Water in black barrels, as well as a brick pathway, hold heat that is slowly released through cold nights.

Knowing the temperature of the greenhouse is helpful. Gieseke installed a digital thermometer so he can check the temperature of the greenhouse on his phone. Even in February, the temperature will frequently get up to 85-90 degrees on the south side as the sun becomes stronger.

He goes out to the greenhouse in February to plant the first seeds for the growing season.

As the weeks and months go by, the plants grow strong and healthy. The young seedlings don’t require a lot of water, but Gieseke will dip some water out of the black barrels when needed. The soil in the greenhouse doesn’t freeze to any extent.

“When people are planting during the second week of May, my planting is already done, and I’ve moved on to other things I want to do,” he said.

There is always something to learn with gardening, and that is true of the Gieseke greenhouse, too. He opened the inner plastic curtain during the warm March of 2021. Then temps moved back to 19 degrees in early April, but the only thing killed was the cucumber plants. Everything else looked black and brittle, but soon came back to life.

For very hot days, Gieseke needed to get the air moving. He permanently installed a fan system with louvers that draw heat out from the top of the greenhouse. It’s set to go on at 105 degrees.

The plastic on the north side of the greenhouse can also be rolled up 2-3 feet for more airflow.

“On most summer days, the two open doors on each end and the rolled-up side provides enough ventilation to keep the temps down,” he said.

For fertilizer, he uses a little 20-20-20 starter fertilizer, but the compost provides the bulk of the fertility. He can watch the plants to see how they are doing.

The plants grew like they were in the jungle this summer. Leaves were 1.5 times bigger than average. Zucchini leaves looked like elephant ears. Brussels sprouts were as tall as a man. Pea plants grew to 6-8 feet.

In early October 2021, the greenhouse looked like a conservatory – lush and overflowing with greenery.

“I think the misting helped a lot in this hot summer,” Gieseke said. He was surprised how well the plants handled the heat, and the gentle mist kept the plants a little cooler.

Final notes

Gieseke mentioned that the plastic for the greenhouse cost $400-$500, but it should last 10 years. The fan system and louvers were $300-$400. He didn’t say the cost of other supplies, but he added it would be easy to spend a few thousand dollars on the system.

He suggests not looking at the cost of raising each bell pepper, or tomato, or carrot, unless the mental health benefits of spending time in the beautiful greenhouse are also included.

The greenhouse holds potential for communities, as well, he suggested. In a 60- by 20-foot greenhouse, 12 families could each grow 100 square feet of vegetables. The greenhouse could serve as a place to socialize, too.

“Even if you didn’t grow anything, if you get the winter blues, just come out here in January and it’s going to be 75 degrees,” he said. “It’s a nice break.”

He asks that anyone thinking about building a greenhouse to consider its design and take strong winds into account. If the plastic starts to whip in the wind, and the design isn’t sturdy enough, it will become disappointing.

A passive heat greenhouse takes a commitment, observational skills, and knowledge for success.

“If you have the opportunity, you should do this – who doesn’t like a greenhouse? Just make sure you put it together properly,” he said.

For anyone with questions, Gieseke will take e-mails at tim@AgRSLLC.com.

Minnesota Farm Guide Weekly Update

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