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When you are in love with growing plants, it’s hard to see the end of the growing season. With no more seeds to start or plants to tend, cleaning up the garden does not have the same excitement that watching plants grow does. Inevitably, the thoughts start—what if I had a greenhouse? Okay, that’s a big what if. But what if I had a hoophouse? Something simple, something that let me keep on growing.

So what happens if you have an unheated tunnel, often called a hoop house because of its shape? And what changes if you add heat? 

What is a hoop house?

Sometimes called a low tunnel, a hoop house is built using arch shaped ribs to form a tunnel over the plants. This can be as low as 2-3 feet off the ground, just barely taller than the plants, or as high as 12 feet tall. It can then be covered with row cover, which is a permeable fabric which allows light, air, and moisture in to the plants, or a greenhouse plastic, which allows light in to the plants. With a plastic cover, you have more protection from  the wind. The plastic will also be significantly warmer during the day with passive solar heat that is magnified by the plastic. However, you need to provide water. The supports can be thin or several inches in diameter, depending on how long you want the structure to be in place.

In our growing zone 7, the plastic is sufficient to keep the ground from freezing. We find it gives about 4 degrees of protection at night—16 nighttime temperature equals 20 in the hoop house. The daytime temperature will depend on how much sunshine there is. On a sunny day, it will be about 20 degrees warmer in the tunnel than outside. On a cloudy day, it will be about 4 degrees warmer. 

Typically a hoop house has no electricity, so no fans for additional air circulation. If you do not have a hoop house, you can build a cold frame with a glass or plastic top—basically a raised bed with a clear cover to let light in. This will act in much the same way as a hoop house, just on a smaller scale. The plants are in the ground or in containers which are completely covered by the hoop house.Mint’s growing habit can be invasive, so it is critical to keep the plant where you want it.

What is a heated high tunnel?

This structure is generally larger and has fans in addition to heat to keep the plants healthy. The plants are still planted in the ground or in containers within. The heat can be set at whatever temperature you desire and are willing to accommodate. It is always a balance of how much does it cost to heat the structure and how much will you harvest to make that cost worthwhile. This is where economy of scale becomes important. The larger the space, with an efficient furnace, the more you will be able to harvest at a modest addition in cost. A classic size is about 30×100 feet, rising to 16 feet tall. The volume of air heated holds longer into the evening than a smaller house, so the plants are less stressed and you can wait longer for the heater to kick on. 

Growing in the cool months

Growing in the cool months when frosts and freezes happen is a completely different thing from growing during the main season. 

First, there are fewer daylight hours and the sun is less intense. 

Two, the temperatures are much lower.

Three, everything grows much more slowly due to the first two factors. It can take up to  twice as long for plants to reach maturity as it does during the prime growing season.

What can you grow in an unheated space?

Select plants that can thrive in both cooler temperatures and fewer daylight hours. This means leafy greens, root crops, and bulbs (think tulips, lilies, ranunculus). Look for the items that can handle the nighttime temperatures without going dormant. 

Your goal is to plant early enough in the fall that the plants can reach 90% maturity by the time frost comes to your area. This allows the plants to stay in the cold frame or hoop house, grow very slowly, and you can harvest as you need to.In the unheated tunnels, we typically have perennial herbs such as rosemary, thyme, and oregano, as well as some cold hardy crops such as Brussel sprouts or fava beans. .

In an unheated space, you will see very little growth until your growing zone reaches 10 hours of daylight and temperatures at night are over freezing at least half of the time. For us, that’s January 10 for the daylight hours, and about February 20 for the temperature rise. By March 1, we have frosty nights only once or twice a week, and the plants start growing very quickly. 

This does mean that you can start cool-loving plants like broccoli, cabbage, kale, spinach, or lettuce even before frost is done with your area. 

What can you grow in a heated space?

In addition to providing all  the benefits of an unheated tunnel, adding heat makes it possible for your plants to actually grow during the winter. For example, while lettuce survives 20F weather, it does not actively grow until temperatures hit 45F. By providing a heated area, lettuce can grow during the winter months. Every 5 degrees over 45 will increase the rate of growth. Again, it’s a balancing act between what it costs to heat the tunnel and what the harvest is.

With a heated tunnel, you can add some of the greens that are a bit more sensitive to low temperatures or might show cold damage to the leaves. Our typical crops include lettuce, escarole, fennel, salad turnips, kale, chard, arugula, baby bok choy, scallions, radishes, and beets. We also have tulips, lilies, ranunculus, and gladiolas tucked into the sides and corners.

While plants grow more slowly than during spring or summer, they are actually growing and you can continue to harvest throughout the winter. Kale plants, for example, will continue to grow new leaves as you harvest the outer leaves. This is true for all of the crops where you can cut and the plant keeps growing, such as spinach, parsley, chard, arugula, or collards. 

Conclusion

Winter growing is a joy, and being able to harvest while temperatures are frozen outside is a lot of fun. Remember to get your plants started early. In our area, that means starting them between August 15 and September 15, about 45-60 days before frost is expected. Have your area winterized ahead of time, 

Make sure you have a way to vent the area on warm days to reduce condensation build up and provide some air circulation. That can be as simple as opening a door or lid, but remember to close up for the night. Select the crops that thrive in cooler temperatures and lower light levels. You will learn what is the best timing for  your growing area and the crops that do well  for you.

Have fun with winter experiments!

~ Ruth 

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