Season Extension Considerations

Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu

Season extension techniques and protected culture have been a hot topic recently, spurred on by the availability of cost share money for high tunnel and hoop house structures. With that in mind, a review of techniques that are available to growers to extend production is in order.

As a starting point, it is important to have clear goals for season extension and crop protection. Goals may include one or more of the following:

● Improving overwintering success
● Providing winter freeze protection and increasing fall through early-spring growth
● Providing frost and freeze protection in the spring
● Obtaining earlier production from overwintering
● Obtaining earlier production from spring plantings
● Extending fall harvest seasons
● Producing in the winter
● Producing year-round
● Producing heat susceptible crops in the summer
● Providing protection from heavy rain
● Providing protection from wind
● Providing protection from wildlife damage such as birds

Soil Modifications
Growers should consider ways to modify soils as the first step in season extension. This includes raising beds or ridges – high beds or ridges have better drainage and heat up more quickly in the spring. With ridges, orienting them so that you can plant on the south side will also provide more spring heating.

Plant Selection and Planting Modifications
A second set of options for season extension involves plant selection and planting modifications. Examples include using transplants in the spring, planting windbreaks between beds, selecting earlier maturing varieties for first spring plantings, using more cold tolerant varieties (or heat tolerant for summer) of current vegetables and fruits that you grow, or changing to more cold tolerant species for early or late production (or more heat tolerant species for summer production). You should also consider shallower planting depth for seeds and planting into in your best drained (or “lightest”) soils for early direct seeding.

Mulches
Mulch can be used to increase soil temperature, decrease soil temperature, or provide winter insulation. Plastic mulches are used to increase soil temperature and hold heat during night periods. Heating increases of 5-20 F° can be obtained depending on the color of the mulch. Soil heating obtained with different colors from lowest to highest are Black < Red < Blue < Olive or Brown < Clear. White, white on black, and reflective plastic mulch types are used to during summer periods to reduce heat loads. Organic mulch sources such as straw are used for winter protection and insulation of crowns and roots of overwintering plants.

Covers
Covers are used to protect against wind, accumulate heat and/or insulate and frost or freeze protect plants. Thin clear plastic row covers with slits, holes, or perforations with wire hoops (or soil ridges) as supports have been used successfully for many years to allow for earlier planting. Thicker plastic (greenhouse film) has also been used as covers but must be pulled on and off to avoid over-heating. Floating row covers and supported row covers including spun bond polypropylene, woven poly, and foam plastic thermal blankets are used extensively for winter protection, for frost and freeze prevention, to increase temperatures, and to increase earliness. The heavier the weight of these materials, the more the protection but light penetration is reduced.

Cold Frames
In years past, considerable production was grown in cold frames, generally large wood sided frames 1-2’ tall, 4’ wide partly buried in the soil with clear covering (glass sash has largely been replaced by rigid plastic or fiberglass for cold frames). This still is a viable system for smaller growers to start transplants, get earlier production and extend fall production, especially of cool season salad greens.

Low Tunnels
As mentioned in the cover section, wire supports with clear perforated plastic can be used as a type of low tunnel to protect plants and increase heating. Larger low tunnels using bows made of metal piping, also known as the “quick hoops” system are also possible. These are essentially miniature greenhouses with reduced air volume. They are covered with one or two layers of polypropylene row covers, clear plastic, or a combination. Low tunnels are used extensively for extended cool season production of leafy crops such as lettuce, spinach, endive, and mustards.

Heat Sinks
Heat sinks are materials that can collect heat during the day and then release that heat over night. Collection devices are usually filled with water and may consist of clear plastic containers or tubes or black plastic containers or tubes. These are placed near plants and when used in conjunction with covers can provide considerable heat buffering.

High Tunnels
High tunnels can be considered an unheated greenhouse with extended upright sidewalls and roll up sides where production is done in-ground (plastic mulched beds are often used for crops like tomatoes). Single high tunnels are usually 48’ or 96’ in length and 14’ to 30’ in width. The extended sides allow for small scale equipment to be used for land preparation and allow for better space utilization. Side curtains are rolled up during warm periods for ventilation and closed to conserve heat during cool periods. When combined row covers over crops being grown inside, additional freeze protection and heat conservation is achieved allowing for extension of production into the early winter and with some crops through the winter.

Larger multi-bay “European” style high tunnels are becoming more popular. These structures may be several hundred feet in length and each bay is 18’-30’ wide (many bays can be added for any width desired). In these multi-bay tunnels is possible to vent bays by sliding the roof cover (so it is more open or closed) as well as by opening sides and ends. They are large enough to allow for machinery to operate for land preparation, mulch laying, and spraying. Crops that can be damaged by heavy rains are often grown in these multi-bay tunnels eliminate injury or defects (such as fruit cracking).

Shade Houses
Extending production of heat sensitive crops in the summertime is done often with the used of shade cloth extended above the crop on a pipe framework. Shade cloth with different shading percentage is available from 30-70%. Black or aluminized reflective shade cloth is available.

Combinations
For maximum protection, heating, and heat conservation combinations are used. Examples from lowest to highest protection and heating are:

● Plastic mulch + row cover
● Plastic mulch + clear row cover + floating row cover
● Plastic mulch + row cover + heat sink
● Plastic mulch + clear row cover + floating row cover + heat sink
● High tunnel + plastic mulch + row cover
● High tunnel + plastic mulch + clear row cover + floating row cover
● High tunnel + plastic mulch + row cover + heat sink
● High tunnel + plastic mulch + clear row cover + floating row cover + heat sink

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