Retractable-roof greenhouses are a great way to get the benefits of protected climates, as well as more toned plant growth from outdoor environments.
Photo courtesy of Christopher J. Currey

As you look around your facility trying to figure out where you are going to get more space for next year’s spring crops, look beyond the walls — or glazing material. While greenhouses offer the most controllable (and protected) climates, they are also expensive spaces to build and operate. For shoulder-season production, look for different spaces to grow your crops.

Retractable-roof greenhouses

Retractable-roof greenhouses offer opportunities to take advantage of both a controlled and outdoor environment. When outdoor conditions are agreeable with crop requirements, growing plants under outdoor conditions can result in improved plant quality and lower operating costs. However, for those shoulder seasons where weather is neither consistently poor nor great, retractable-roof greenhouses are an excellent choice.

Retractable-roof greenhouses can provide all the benefits of an outdoor environment — air movement, bright light, etc. — which results in toned growth and high-quality finished plants. However, retractable roofs also mitigate the risks associated with outdoor production by excluding unwanted precipitation, so you can control the substrate moisture and fertility. Additionally, unit heaters installed in retractable-roof greenhouses can keep temperatures in ranges from keeping warm to actively growing. An efficient heating method for retractable-roof greenhouses is to use root-zone heating or plants grown on the ground.

High tunnels

High tunnels or hoop houses, for the purpose of this article, are single-wall film-plastic-covered structures, usually with roll-up or -down sidewalls. These have been increasing in popularity for extending the season for field-grown fruits and vegetables, as well as cut flower production. However, they also are useful for finishing seasonal crops.

Recent research shows high-quality annual bedding plants can be grown in high tunnels. Like with a retractable-roof structure, you can get the benefits of an outdoor-grown crop along with some protection.

Even though you have a protective structure, you’ll still need to watch for temperatures in high tunnels. There are several approaches to this. First, watch your planting date. While plants in a high tunnel can experience very warm days when outdoor temperatures are cold, plants can cool down to the ambient outdoor temperature (or lower!) during the night due to radiant heat loss. Make sure early plantings focus on more cold-tolerant crops.

High tunnels are not for every crop — finishing cold-tolerant petunias (Petunia × hybrida) can be great, while cold-sensitive angelonia (Angelonia angustifolia) are less amenable to this strategy. Additionally, those approaches used to protect plants from frost outdoors, such as pulling spun-fabric netting over crops, can be used in high tunnels on cold nights.

Outdoor finishing

Finishing flowering crops outside is not unusual. In fact, it can be quite common in several locations where the climate is favorable to outdoor production. But even in less-than-ideal climates, there are opportunities to use outdoor space to finish your crops. Maybe you have some room on your fall mum finishing pads that could be used for some of your production? You’ll need to look at the outdoor temperatures and the species you are growing to determine when you can move production outdoors. While it likely will not be used for your earliest crops, outdoor space becomes more useful with each successive turn, as outdoor temperatures improve.

For crops grown outdoors, you will need to think about fertilization in addition to temperatures. Unprotected crops outdoors can leach nutrients when rain falls. As a result, using controlled-release fertilizers (CRFs) can help in providing some nutrition after precipitation, when fertigating may be undesirable. However, for early-season spring crops and late-season fall crops, when outdoor temperatures are cooler, CRF release will have slower nutrient release due to the cooler temperatures.

While retractable-roof greenhouses, high tunnels and outdoors are less-predictable and controllable than greenhouses, they provide opportunities to produce high quality plants while reducing costs. Careful crop selection, planning and scheduling, and a watchful eye can enable growers to take advantage of these affordable production spaces.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.