Plant growers have always striven to control the environment in which plants grow. The hoop house is one of several technologies growers have adopted in an effort to master mother nature.
Hoop houses (referred to as over-wintering structures by the nursery trade) are generally quonset-shaped structures constructed of metal or plastic hoops (bows). They are covered with one layer of 6-mil greenhouse grade polyethylene film and are ventilated by rolling up the sides. There is no permanent heating system and no electrical connections. The only external connection is the irrigation supply line. The ends of the houses are framed and covered with poly-film or a rigid material such as fiberglass. Most end walls are fitted with doors, however a few can be removed entirely permitting greater access. Hoop houses range in width from 14-20 feet and up to 96 feet in length. Compared to greenhouses, hoop houses are relatively inexpensive ranging in price from $1.25- $2.00 per square foot.
Hoop houses can be grouped into two general categories, permanent and movable. The majority of houses in use today are permanent. The permanent structures are usually stronger and more airtight. They are ideally adapted to small enclosed spaces i.e. your back yard. Moveable hoop houses are designed to be moved by hand or tractor forward or backward along the long axis of the house. Because they are mobile, these structures provide for more efficient use of land. Movable houses require a minimum of double the space utilized by similar sized permanent houses.
Our experience to date has been on permanent hoop houses only.
Traditionally, high value crops such as tomatoes are grown in hoop houses to justify the additional expense. Other crops being grown include strawberries, raspberries, cut flowers, melons, eggplant, pepper, and summer squash. These crops are typically marketed direct to the public.
Because they are not heated, hoop houses should not be regarded as freeze protection devices although they can provide limited protection. On the morning of April 12, 1997, our hoop house tomato crop survived despite the ambient (outside) air temperature dropping to a low of 27 degrees F.
One benefit of the hoop house often overlooked is storm protection. A crop is only worth as much as the protection provided by a 6-mil sheet of plastic. During the fall of 1996 our hoop house tomato crop escaped the ravages of a hailstorm accompanied by 70-mph wind. Seeing is believing.
There is less incidence of foliar disease in hoop houses primarily because crops are sheltered from rainfall. Our 1997 hoop house tomato crop never required a foliar fungicide application. Consequently we saved on chemical and labor expense.
Crow and mockingbird damage to tomato fruit is an annual occurrence in our outside plots. Vertebrate pest damage to crops grown in our hoop houses hasn't been a problem to date, even when the houses have been fully vented. Evidently crows are suspicious enough of the covering they don't venture inside. If animals become a problem, covering the side vents with a fabric mesh should remedy the situation. It's a good idea to have some plastic mesh on hand even if you aren't experiencing animal problems. During the spring it makes an excellent windbreak for young transplants.
The plastic film is the component of the hoop house that is primarily responsible for crop protection. Consequently we have chosen not to remove film from our houses during the summer as some growers do.
Hoop houses have proven effective in extending the growing season in both spring and fall. The primary benefit is earliness. Based on our research and the experiences of other growers about one month of earliness is normal for tomato and pepper. Earliness is a combination of being able to plant two weeks earlier than field planting and faster maturation of the crop inside the house.
Temperature management within the hoop house is the single most difficult task. First time growers often underestimate the capacity of the house to gain heat especially on cloudy days. Because many factors affect heat gain in a hoop house, there are no hard and fast rules regulating ventilation and shade fabric application. Some of these factors include:
- Size of House: Generally speaking the larger the house the greater the heat gain. All things being equal larger houses will require ventilating earlier in the morning to prevent crop damage. Most experts suggest limiting the width of the house to 20 feet to ensure adequate cross ventilation. To date we haven't experienced any problems ventilating our 20-foot-wide houses.
- Film transparency: Greater heat gain can be expected in houses covered with clear film as opposed to opaque or white film. High quality opaque film provides all the benefits of clear film in addition to reducing solar load during the summer. Shade fabric was not required on our 1997 tomato crop grown in a house covered with opaque film.
- Condition of film: Dirty film delays heat gain while perforated film isn't as efficient in preventing heat loss at night.
- Cloud cover: Don't be fooled! Although heat gain is slower on cloudy days house temperatures may reach detrimental levels before you realize it.
- Wind: When the ambient air temperature is lower than the air temperature within the hoop house heat is conducted through the plastic to the outside. Wind makes this process more efficient. On cold, windy days your hoop house will be slower warming up and on windy nights quicker to cool down.
- Calendar date: As spring progresses, the sun climbs higher in the sky. The resulting heat gain mandates earlier ventilation with each passing week.
As a rule of thumb, hoop houses should be vented before internal air temperature reaches 90 degrees F. The amount of vent opening will depend on the desired growing temperature, the ambient temperature and wind speed. Don't panic if you miss the mark by a few degrees. We noticed no long-term damage to a tomato crop when the air temperature exceeded 100 degrees F for a few hours.
Every crop has its ideal growing temperature. We try and adjust side vents to maintain a growing temperature of between 80 and 85 degrees F for bell pepper and tomato and 90 degrees F for cucurbits, eggplant, and hot pepper.
During cool weather, roll down the sides in the early evening to trap as much heat as possible. As the season progresses, sides can remain open when night temperatures don't fall below 65 degrees F. A minimum-maximum thermometer is excellent for keeping tabs on day and night temperatures. If the forecast calls for a chance of rain during the night go ahead and close up the house before retiring for the evening.
When air temperature exceeds 90 degrees F at plant level (in fully vented house) we start thinking seriously about applying shade fabric. In Oklahoma this can be as early as mid May or as late as mid June. We currently use a 55% shade fabric on any house equipped with clear film.
There is no substitute for experience when it comes to hoop house temperature management. A good way to develop management guidelines for your hoop house operation is to keep good records. To do otherwise is to court failure.
We currently employ the use of raised beds in all of our hoop houses. We find they offer the following advantages:
- Soil warming: Raised beds provide the same soil warming benefits in a hoop house as in the field.
- Best utilization of plastic mulch: Because beds are raised, mulch can be 'wrapped' around soil profile ensuring a snug fit thus promoting efficient heat transfer.
- Ease of harvest: Beds elevate low growing crops reducing back stress during harvest.
- Effective soil solarization: Temperature in beds covered with 1-mil clear plastic mulch can reach 140 degrees F at one inch depth. Six weeks of this treatment during the summer does an excellent job controlling soil borne pathogens. Closing the house increases the effectiveness of solarization.
- Limited freeze protection: Heat radiating from beds moderates temperature around young transplants.
- Site flexibility: Raised beds permit placement of hoop houses on sites with poor soil.
All of our beds are bordered by 1-inch by 6-inch treated lumber or equivalent sized rubber lumber. This eliminates the need to reform the beds. Beds are 40 inches wide and constructed on 5-foot centers. Each bed runs the length of the house. The growing medium consists of a mixture of loam soil and compost.
Our 17-foot and 20-foot wide hoop houses contain three and four beds respectively. The outer beds are purposely located close to the house sides to facilitate spraying and harvesting from outside when the house is fully vented.
Prior to planting, any nutrient deficiency is corrected based on a soil test. Following fertilizer incorporation each bed is fitted with two drip lines buried 1 inch deep. Next the surface of each bed is firmed using a turf roller after which the plastic mulch is applied. A soil thermometer is inserted through the plastic to record bed temperature. To encourage earliness, transplants grown in 4-inch pots are used. Planting holes are prepared using a post hole digger. A starter fertilizer solution is applied at planting.
When the crop has reached the appropriate stage of growth, support structures (cages, trellises, etc.) are installed. As the crop matures, supplemental nitrogen is applied via the drip system based on a fertilizer schedule developed by the University of Florida. Irrigation scheduling is determined by tensiometer readings.
Beds inside a hoop house are not subject to leaching by rainfall. Consequently salts should be monitored on a yearly basis. Consider soaking beds using a sprinkler or water wand prior to soil solarization.
In an effort to offset greater expenses associated with hoop house production growers often increase plant population. However, too much of a good thing isn't always beneficial. Plant spacing recommendations based on yield trials and harvest efficiency are given in table 1.
To justify using a hoop house, yields must be greater than field-grown crops. Growers should focus on growing only the highest quality produce. Also, growers must obtain higher returns for their early market produce. Table 2 summarizes yield results of tomato and pepper crops grown during 1997 in our hoop houses.
Compared with other countries and the northeastern U.S., hoop house culture in Oklahoma has found limited use. However, because these structures reduce the production risk by providing greater control over the environment, the potential for expanded use in our geographic area is tremendous.