Extend your growing season with a high tunnel

WORLAND – Laramie County horticulturist Catherine Wissner gave a lecture via video conferencing on gardening in a high tunnel during the Grow Washakie event March 4 at the Washakie Museum & Cultural Center.

Her presentation grabbed viewers' attention with stories and pictures of high tunnel gardeners that were starting their tomatoes in February, had every inch of ground covered in lush vegetation through the summer, and even had summer crops fruiting in the fall.

Wissner began her workshop by defining exactly what a high tunnel is. A high tunnel, also known as a hoop house, is a type of greenhouse classified by being a nonpermanent structure that lacks any electrical, ventilation or heating systems to maintain low costs while still providing a controlled environment for crops.

It is typically comprised of a single layer of plastic – although more can be used – supported by a frame of light-weight arches. Wissner stressed the importance of the plastic sheeting having UV protection in order for it to last longer without breaking down. She also said she does not recommend using glass because of the risk of hail damage.

Wissner said that location is key when planning for a high tunnel. She claimed that a high tunnel works best when oriented along a north-and-south axis, which allows wind to roll over the curved sides of the tunnel. Plants that are more sensitive should be on the side of the tunnel that is least exposed to the wind (this becomes relevant if you plan to lift the sides of the high tunnel for ventilation.)

Wissner's method involves planting crops in your high tunnel directly into native soil, so soil composition and fertilizers are also important factors to consider. Wissner recommended getting your soil tested to determine both the pH level and its contents of nitrogen, phosphorus and potassium. This will give you an idea of how you can improve your soil if need be.

As for additives, Wissner strongly recommended against using manure or wood ash, as they can both introduce a new set of problems from high salts and high pH. Manure can also harbor pathogens. She also said not to use Miracle Gro, claiming that it contains too much nitrogen and too many salts.

What you should use, according to Wissner, is organic matter such as leaf litter or grass clippings that have not been treated with pesticides. Incorporate a one-inch layer of organic material into your soil between growing seasons. She also advocates using waste materials such as coffee grounds and kitchen scraps.

Next on the list is water. Before doing anything, Wissner said to test your water quality at a local lab. Amanda O'keefe, executive director of Washakie County Conservation District, said that the district can help with this. Once you do that, you can start to think about the watering system for your high tunnel. Wissner said that a drip system on a timer works best for her. She described her current setup as a length of polyvinyl chloride, or PVC, pipe at one end of her tunnel, with outputs to connect a drip line at one-foot intervals along it. This way, you can adjust your watering if you make changes to your crop rows later.

Wissner stressed the importance of keeping water on the ground and avoiding sprinkler systems. In a closed environment, introducing excess water onto your plants can harm them; though one way that you can use sprinklers with a high tunnel is to spray the outside to help with cooling. She said that a grower she knows does this in high temperatures and that they can reduce internal temperatures by as much as 10 degrees with a brief spray cycle.

Although one might think that an enclosed gardening space should be humid, Wissner suggested that you should try to keep it on the dry side, and recommends having sides you can roll up to help vent excess water.

For temperatures, Wissner said that the main issues in high tunnels will be getting rid of excess heat. She claimed that 85 degrees is ideal, and that you can roll up the sides to release heat. Although not permanent cooling solutions, Wissner said that shade cloths, fans and swamp coolers work well to help get rid of heat.

When it comes to managing your crops grown inside of a high tunnel, Wissner said that they are capable of growing just about anything that you could normally grow outside and more. She gave as an example beefsteak tomatoes, which typically struggle in Wyoming because to fully mature they require more time than allowed within the natural growing season. A high tunnel can extend the growing season by providing a controlled environment with consistent warm temperatures, and can even accelerate fruit production due to raising night-time temperatures.

Above all else, Wissner said that keeping the space inside your high tunnel clean is crucial. You should regularly remove unwanted material and moisture from your high tunnel to prevent disease or pests. You should also inspect crops you plan on transplanting to make sure you aren't introducing anything harmful to your high tunnel environment.

For fertilizer to use on an as-needed basis, Wissner recommended a mix that you can make in a five-gallon bucket: fill with four gallons of water, one pound of alfalfa pellets, and one cup of sugar and mix. She gives one cup of this fertilizer to each plant in her high tunnel per week.

For pest control, Wissner recommends growing plants that will attract helpful insects. Plant flowers that will attract pollinators, and "trap crops" or plants that you will not harvest for food but that will entice pests away from your food crops. She also recommended using neem oil if you need to use a pesticide. This is a non-selective, non-chemically reactive substance that works by coating insects breathing apparatus and suffocating them.

Lastly, Wissner recommends keeping records of your high tunnel: maps of where you plant crops, how many crops you planted, watering schedule, soil temperature, crop yield; all of it. This will help to pinpoint areas you can adjust to improve future yields, and lets you keep track of your recipe for success.