A question I often ask tour groups at the Noble Foundation Horticulture Center is, "Why doesn't Oklahoma have a horticultural industry rivaling that of California's Central Valley?" Some common responses include lack of irrigation, markets, labor, poor soil and a short growing season. Although all of these have some legitimacy, they are not paramount. The single greatest deterrent to sustainable horticulture in Oklahoma is extreme weather.
The Horticulture Center was developed in part to address the problems gardeners face in extreme weather regions. During the mid-1990s, our emphasis shifted from researching and demonstrating the benefits of permanent raised beds to addressing weather-based problems, primarily temperature-related ones.
In response to these problems, horticultural researchers have joined forces with plastic film manufacturers to develop protective covers for both commercial and hobby gardens.
There are two types of plastic crop covers. The first consists of polyethylene film supported on frames or hoops to form a tunnel over a row or bed. The second is a fabric-like polypropylene material that floats or can rest directly on the crop. The edges of both kinds of cover are anchored in the soil to combat wind.
The polyethylene row covers are generally cheaper, but more labor-intensive because of the support structure. They create higher daytime temperatures, which is desirable for early- and late-season production, but require venting to control heat buildup on warm, sunny days. Some manufacturers offer row covers with preinstalled slits or circular perforations for automatic venting. Some even offer pigmented (opaque) covers for added cooling. The standard width for row covers is 6 feet.
Because of their porous, lightweight design, floating covers are self-venting and usually don't require a support structure, making them more versatile than row covers. Their width varies from about 6 feet up to about 60 feet, large enough to cover an entire garden. However, if whipped by high winds, floating covers can injure young seedlings. If your garden is exposed, consider using a support structure or erecting a windbreak.
Floating covers have a variety of functions based on the weight of the fabric. Lightweight covers are used primarily for insect exclusion; heavier covers are used for elevating day and night temperature; and the heaviest material is used for frost control.
Don't expect crop covers to save your plants from heavy freezes. Row covers and medium-weight floating covers will keep the temperature only 2° to 3°F warmer than the ambient (outside) air. Wind speed and soil temperature influence the degree of protection achieved.
The major benefit of crop covers is their overall growth-enhancing characteristic when used in cool weather. Crops planted one to two weeks ahead of the normal time can mature three weeks early.
Particular attention must be given to cover removal. Depending on the crop and the environmental conditions, row covers can be left in place for about a month. Floating covers can remain in place longer because of their ventilation. Most of the heavier floating covers used for frost control are removed during the day.
Yields of wind-pollinated crops such as tomato, pepper and eggplant can be reduced if covers are left on too long. Temperature under the covers should not exceed 90°F for more than a few hours during the flowering stage. For crops requiring bee pollination such as squash, cucumbers and melon, the covers should be removed when female flowers appear.
Row covers not only enhance crop development but also hasten weed growth. Because of the impracticality of weeding under crop covers, you should use plastic mulch as well. You can't beat this combination for maximum crop growth and weed control.
If you're interested in taking crop protection to the next level, consider constructing a hoop house, whose primary advantage is convenience. You don't have to remove a cover to gain access to your plants; you simply walk in!
Hoop houses are generally Quonset-shaped structures constructed of metal or plastic hoops. They are covered with a single layer of 6-mil greenhouse-grade polyethylene film and are vented by rolling up the sides. There is no permanent heating system, and there are no electrical connections. The ends of the houses are framed and covered with poly film or other transparent materials. You can place a door in the end walls or make them completely detachable, permitting better access. Compared to greenhouses, hoop houses are relatively inexpensive at $1.50 to $3.00 per square foot.
Hoop house temperature management is difficult because the size of the house, film transparency, cloud cover and wind speed affect heat gain or loss. As a rule, houses should be vented before the internal air temperature reaches 90°F. The amount of venting depends on the wind speed, the ambient temperature and the desired growing temperature (80° to 85°F for bell pepper, snap bean and tomato; 90°F for cucurbits, eggplant, hot pepper, southern pea and okra), which can be maintained by side vent adjustments.
Remove the poly film or apply shade fabric when temperature at plant level within a fully vented house reaches 90°F. We have reduced air temperature successfully by using a 50 percent shade fabric over the poly film.
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°F. A minimum/maximum thermometer is excellent for keeping tabs on day and night temperatures. If the forecast calls for rain during the night, close the hoop house before retiring for the evening.
If you're serious about gardening, hoop house and crop cover technology can help you beat those weather woes. We use this technology religiously at the Noble Foundation. It works for us, and it will work for you too.
See the appendix at the end of this book for a list of literature on the use of crop covers and hoop houses (high tunnels). Also included is a list of hoop house and crop cover merchandisers.