Updated: Mar 25, 2021
Our very first year growing, we planted hundreds of ranunculus in mounded field rows in early October and covered them with agribon (frost-cloth). We knew that they should be planted in autumn in our mild climate and we also knew they could survive some cold but nothing extreme. Then we watched as a freak cold system dropped the temperatures to 17 with single digit wind chills. We watched as the capillary action from a wet winter and high water table turned our clay soil into a bog. Out of a hundred ranunculus, only a handful survived. It was heartbreaking. I'm not one to wring my hands for long, however, and sometimes I throw caution to the wind and run full into a solution that may or may not work. But I had to try something. So, we bought a high tunnel.
Now, a high tunnel is a crazy big investment for a first year farmer and if I had been sensible at all, I would not have done this. We only have 1/2 acre in a very urban environment. The amount of profit we're able to make is directly tied to keeping operating expenses very low because our production volume is so limited. So to drop $3,000 on a high tunnel was not a sound business choice. In hindsight, we should've mounded much higher and doubled the agribon. In a normal winter, this would've been enough. But we bought a high tunnel and this is what happened next:
We planted twice the number of Ranunculus corms the following year and in the high tunnel they thrived. Each plant gave us multiple blooms for months in a row. We made back half the cost of the high tunnel just from the robust Ranunculus crop we grew in it's first year. We're now going into our third year of production using the high tunnel and it's already paid for itself.
A high tunnel provides a warmer environment than the outside and some protection against the elements. When deciding to build one, you must consider your environment. We are in North Texas, where summers routinely give us temperatures above 105+ F (41 C). When you think about a space covered in white plastic magnifying the suns rays, it's not hard to imagine the kind of extreme heat you'll end up with. That's why using a high tunnel in North Texas must be for the primary advantage of winter growing. If you'd like to use it in the summer, shade cloth is a must, as well as the ability to fully open the end walls (in other words, have openings much larger than just a door at the ends.) You will also be very limited in what you can grow in the summer in your high tunnel. However, the shade cloth, even in Spring will benefit your crops by elongating the stems and we highly recommend adding it in any growing circumstance.
Certain plants will bloom earlier when grown in the high tunnel in the Spring. Most farmer's use high tunnels for season extension. It allows them to have blooms earlier in the Spring before the last frost and later in the Autumn, even after a hard freeze. Plants like Colibri poppies, Snapdragons, Anemones, Tulips (pre-chilled), and Ranunculus will bloom earlier because their environment will warm up earlier in the tunnel than outside in the fields. However, many of the cold hardy annuals that you could overwinter in your high tunnel are day-length sensitive and wouldn't bloom any earlier if planted in a tunnel. Many cool hardy annuals also need vernalization (a cold period) so protecting them from the cold by growing them in a high tunnel wouldn't make sense. You can use your high tunnel to overwinter sensitive perennial crops like Eucalyptus and Dahlias, where cold protection from year to year will result in more robust harvests.
Even with plastic greenhouse film, the temperature protection in an unheated high tunnel is roughly 4-5 degrees. This means in the event of an unusual cold spell, you should plan on adding a layer of agribon (frost-cloth) protection IN the high tunnel. In the event of a polar vortex, you will probably have to go one step further and bring in special heaters powered by propane to keep temperatures above single-digits. It's also a good reason why we don't use our high-tunnel as a greenhouse for seed starting. If you have a heated tunnel, this would be a great idea, but with an unheated tunnel, it simply gets too cold for seedlings. We would have to frequently ferry seedling trays in and out with our crazy North Texas temperature swings. The space is much better utilized growing profitable Spring crops.
For summer growing, we must grow heat-loving plants in our high tunnel. Gomphrena, Celosia, and basil do great for us. With the shade cloth, our Celosia routinely gets over 4 feet tall and our Gomphrena is so happy, a few plants turn into a veritable jungle. Zinnias do not do well because the area in the tunnel doesn't have as much airflow as the open field and Zinnias are very susceptible to mildew without really good airflow.
The ability to make profit from a high tunnel is correlated to the crops you choose to plant in it. They must be plants that you can charge a premium for and plants which greatly benefit from a high tunnel environment. In other words, growing them in the high tunnel must result in a more robust plant and harvest, otherwise, you would just save the money and grow them in the open field. Ranunculus is definitely one of those plants for us. Sunflowers are not.
The cost will also depend on the make/model, size, and whether you install it yourself. We purchased a hoop bender from Johnny's and built our high tunnel. The shape is Caterpillar and not Gothic. It's also not heated, it doesn't have a fan, and is not automatically vented. All these things kept the cost lower. We did go with side walls that you roll up and while it was more expensive, it has been COMPLETELY worth it. Imagine with a quick flick of your wrist covering 4 whole rows from inclement weather in literally 2 minutes. Farmer's Friend has a great selection of affordable high tunnels and really easy-to-use instructions about how to build one yourself. A quick note: we are in an urban area and have lots of natural wind breaks. If you are in an open field, be very sure you get a high tunnel that can withstand fierce winds.
Growing in a high tunnel is not all unicorns and Rose's :) There are particular challenges. As you overwinter plants, you also provide protection for pests, who sneak in and overwinter too. This often means much stronger pest pressure in a high tunnel. Our friendly worm-eating birds can't easily get into the high tunnel and help us out, so we have to be very vigilant with pest control. We buy in beneficial insects, spend more labor on weeding, and always spray compost tea in the high tunnel first before any other plants get it. There's extra cost associated with all these practices.
It can be very tempting in our storm prone area to let the sides down to protect the plants, but doing this reduces airflow which is crucial for disease prevention. It also creates weaker stems because the stem isn't forced to grow strong in response to wind. They also can't build up natural resistance to cold (plant cells actually make their own antifreeze in response to cold) so they are more susceptible to cold damage during freak polar vortexes if not allowed to get cold with mild frosts. Nature is thwarted in good ways but also in bad ways, and we have to be mindful of this. Side note: when we have straight line winds over 40mph forecast or tornadoes, we close the sides to keep the wind from ripping the roof off. You must take your weather situation seriously when figuring out if you can use a high tunnel and what type you should get. Our high tunnel is surrounded by mature trees, even with the natural shade and wind protection, we must account for our area's tornado activity (falling trees!) and hot Texas weather.
High tunnels can also easily collapse under the weight of as little as 3 inches of wet snow. Be sure you're prepared in the event of a major snowstorm to be out in the elements brushing snow off your high tunnel. If your irrigation freezes, you need a plan to get water to your plants since they won't get it from melting snow. Lastly, even on a bitterly cold day, if the sun comes out and reflects off the snow, you could experience searing hot temperatures inside your tunnel. It MUST be vented on a sunny day, even in single digit temperature (even without snow!).
Rain provides lots of micro-nutrients and a high tunnel prevents rain from falling directly on your plants which means your soil can quickly get depleted of these important micro-nutrients. Diseases can take hold and without rain flushing them through the soil they can hang around for years. It's good practice to take the plastic off your high tunnel every 4 or so years and let it sit open for a season. If you can't really do that, you can utilize solarization (great article here) and cover crops to help with soil maintenance. There's also an old (somewhat dangerous) practice of steaming your soil to get rid of disease and the method has experienced a resurgence recently as more farmer's begin to use high tunnels.
Growing in a high tunnel in North Texas can be a great way to increase Spring productivity if you're willing to add some additional maintenance to your plate, both for the tunnel itself and for the crops growing inside it. I must admit, when we reached a record -2 degrees this past February with a record 8 days below freezing, I was never more thankful to have a high tunnel!!
Growing for Market Magazine has an excellent series of articles on high tunnel crop management, pests, and soil nutrients/disease written by Steve & Gretel from Sunny Meadows Flower Farm in Ohio. Other articles on high tunnel growing are sprinkled through-out the issues. I would highly recommend subscribing to this magazine if you're a flower farmer!
Gardener's Workshop offers an online course also taught by Steve & Gretel about high tunnel growing.
ASCFG: honestly, it's the best resource for everything about everything! :D