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How To Grow Crops Year Round In a Greenhouse or High Tunnel

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Winter poses a lot of challenges to those who love fresh foods! While the summer months are filled with loads of crops from a vegetable garden, there’s something very special about locally grown winter vegetables on the kitchen table! 

Back when we only had a home vegetable garden, eating with the seasons was something we looked forward to every year. BLT’s on repeat during July with fresh leafy greens and tomatoes, sun warm melons in August… we even appreciated the winter squash and potatoes from November-February while the garden rested under a blanket of snow. We anxiously waited for the garden beds to warm up in late spring to get our outdoor garden planted again. 

Once we began selling produce in 2018, and then saw the many supply chain gaps in grocery stores in 2020, we knew that we needed to extend the times of the year we were able to grow plants and harvest produce for sale. Que in the winter greenhouse! 

While our farm offers loads of root vegetables and storage crops like onions, winter squash, potatoes, and garlic, we are also able to grow some cold hardy crops year-round through the use of a high tunnel– an unheated structure with a thermal mass that allows growers to extend their growing season over 6 weeks on either side of their traditional outside growing season. Read on to hear about how we have successfully extended our growing season, the challenges we have faced, and our best suggestions and recommendations. Plus, learn how you can do all this on a smaller scale in your own backyard too! 

What is the difference between a greenhouse and a high tunnel?

The terms greenhouse and high tunnel are used interchangeably sometimes, but a high tunnel typically means the crops are grown in the ground, and greenhouse means the crops are grown in pots. 

How we grow year-round- using a high tunnel space

We have four high tunnels that are 30 feet by 96 feet wide. Each structure has 8 beds inside that are 30 inches wide with a 15-inch walking path. Since these spaces give us the most protected planting space, we try to rotate through crops quickly using intercropping methods. We plant everything 6 weeks ahead of the outdoor schedule– so cold-tolerant plants like spinach can go into the ground in an unheated greenhouse in February in Iowa. In March we are planting heat-loving plants like peppers, that can normally not go into the ground until June. Many times people assume the produce we are selling at market ahead of the season is purchased from other places- but really we are able to sell crops 4-6 weeks earlier because of our extended growing season in the high tunnels! 

Ashley in a high tunnel picking tomatoes

Extending the season in both directions 

Using a high tunnel or unheated greenhouse allows to extend the harvest of crops by about one month on both the spring and the fall harvest. For example in Iowa, it’s common to not have tomatoes ready to pick until mid-July and they are done producing by September. We can pick tomatoes in June and the last few years, thanks to mild winters, all the way into December! 

What we grow in winter

In the winter, the daylight actually limits the crops ability to grow more than the temperature. We fill our high tunnels with kale, spinach, hardy herbs, green onions, swiss chard and lettuce for winter to be able to harvest from September-April. These are all crops that tolerate cold weather well as an unheated high tunnel is typically about 10 degrees warmer than outside. If there is a night that will be extremely cold, we will use a row cover over the plants to protect them from frost. 

The plant growth is very slow in the winter, so it’s imperative to time out the plantings so the crop is well established after November when the day length is short. In February the days start getting longer, meaning the growth will increase. Basically between November and Feb, the plants are growing extremely slow, but stay alive and fresh in the soil.

How we interplant to always be harvesting 

The space in a high tunnel is very valuable and we are still experimenting with interplanting to make the most of our space in the tunnels. For example, if we want the kale to be established by September, it is transplanted at the base of the tomato plants. Once the production of the tomato plants slows down, we remove the tomato plant and are left with the kale. When we prep for the next spring, we will transplant the next crop in the center of the kale to continue to process. We have one tunnel with low sides so we have large pots of perennial herbs that we can keep alive in Iowa’s cold winter months in the tunnel. 

Ashley in the high tunnel with lettuce

Benefits of Growing in a greenhouse

Not only are fresh vegetables available for a long season, but there are also numerous other benefits to greenhouse owners. 

  • Crops are protected from severe weather, so you are less likely to experience damage to your crops. 
  • Kids have a place to burn off energy in winter months. We love sending our kids into the high tunnels to run around and play in an area that is nice and warm and out of the winter elements. 
  • Increased yield. We are able to grow and harvest an abundance of fruits and vegetables because of the longer seasons. 
  • Growers can control the water- for example, in 2024 all outdoor gardens had terrible tomatoes due to the rainy early spring, followed by unpredictable large rain showers that made tomatoes crack. Our tomatoes that were under cover did not have this problem. 

Challenges of growing in a greenhouse

Strong winds- This is a big consideration when putting up a structure made of plastic! 

Pests- Since the tunnel never freezes, the pest pressure is very high. We’ve not yet developed a good system to avoid chemicals and grow in these structures. We typically allow a tunnel to sit empty for a period of time in winter to be able to kill off pests, but the growing space is so valuable that it needs to be filled year-round. 

Choosing a type of greenhouse

Your land and budget will determine most structure choices. We wanted the 30*96 structures because we can use the tractor inside if necessary and it was the best option for our farm’s overall topography since they need to be on a level space. However, structures this size are generally $15,000. We actually were able to find all of our structures used and reasonable prices if we did the labor of removing them from the property. Three of our tunnels are the Quonset half circle shape and one is a gothic style. If we had the budget to buy whatever we wanted, we would definitely NOT have a gothic style due to the wind. 

How a year-round greenhouse is heated

On our farm, we provide supplemental heat via a wood boiler to one structure so we can start our own vegetables from seed without using an electric heat mat. The other structures do not have a supplemental source of heat, other than the sunny days transferring heat through the plastic. However, we do find that two layers of plastic with a greenhouse fan is the easiest way to avoid very low nighttime temperatures. One could also consider using large black barrels as passive solar collectors, but we do not see the benefit as it uses valuable floor space.

How to have a year-round harvest for a home gardener

A cold frame is a great way to extend the garden season on a small scale. A cold frame is similar to how a high tunnel works but on a much smaller scale. A cold frame houses crops that are planted in the ground, where a greenhouse houses crops that are planted in pots. The structure of a cold frame is a box that is built around the garden that is planted in the ground and then either has a glass or plastic covering that will help warm up and trap heat for the plants to grow. The cold frame can be a very inexpensive option and it is easy to DIY. 

You could have fresh greens all winter, even in Iowa, with the use of a simple cold frame! We built our own greenhouse in 2017 with the direction of Four Seasons Harvest by Eliot Coleman and it held very well in our high-wind area. We opted to craft ours with plastic and found it was super simple to do. Our only feedback is to use plastic from a greenhouse supply company. The plastic from hardware stores does not seem to be as strong. It’s worth the investment! A family could easily get fresh produce during winter months in a small space like a cold frame or small greenhouse. 

If you are considering building your own cold frame, we highly recommend using the methods outlined in the book Four Seasons Harvest by Eliot Coleman. It walks you through how to build it and what to grow so you can enjoy a homegrown harvest all year! 

Crops to grow over winter in a greenhouse

It is important to consider that not all crops will survive in the winter, even in a greenhouse or high tunnel. While it can depend on where you live, generally, hardy frost-tolerant crops do the best. Crops such as root vegetables, herbs, and hearty greens are all great for winter planting in a greenhouse. Here is a list of crops to consider growing:

  • Root vegetables such as carrots, potatoes, beets or parsnips 
  • Onions and garlic
  • Hearty greens such as kale, Swiss chard, or collard greens
  • Leafy greens such as spinach and arugula 
  • Brussels sprouts, celery, peas and broccoli 
  • Herbs such as parsley, chives, or mint 

More gardening resources

We love sharing the knowledge that we have learned through the years planting and harvesting fruits and vegetables here at Pleasant Grove Homestead! You can find all of our blog posts about gardening here

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