Season Extension Techniques for Vegetable Crops

Many vegetable crops, especially heat-loving crops such as cucurbits (vine crops), peppers, tomatoes, eggplant and sweet corn, benefit from the use of plastic mulch and row covers. Some benefits associated with season extension include:

• earlier harvest
• increased yield
• higher-quality produce
• soil moisture retention
• reduced disease pressure
• reduced fertilizer leaching
• increased season-long nutrient availability
• weed control

Plastic Mulch

Most mulches are made of polyethylene. Table 1, Plastic Mulch Types, describes the different types. They are available in widths of 1.2-1.5 m (4-5 ft) with a thickness of 1-1.5 mil (thousandths of an inch).

Several commercial plastic mulch layers are available. Proper set-up of the mulch layer is essential. The edges of the mulch should be well covered, and the plastic should be tight on the soil surface to permit heat transfer to the soil. Angle the discs and press wheels of the mulch layer to ensure a tight and uniform fit.

Do not lay mulch on dry soil. If the soil is very dry, irrigate or wait for rainfall before laying. Soil moisture is important for heat retention. Lay mulch 2-3 weeks prior to planting for maximum soil heating.

Table 1. Plastic Mulch Types

Type	Average Soil Warming at 2 in.depth	Advantages	Disadvantages
Black	2°C-4°C	Prevents weed seed germination	Good soil-to-plastic contact is necessary to ensure maximum heat transfer.
Clear	4°C-8°C	Can be used with direct-seeded crops such as sweet corn.	Weed germination under the mulch requires good preplant (residual) weed control.
White	-1°C (cools the soil)	. Keeps the soil temperature lower to minimize bolting in cool season crops.	Weed germination may occur under the mulch. Requires good preplant weed control.
Infra-red transmitting (IRT)	6°C	Absorbs certain wavelengths of light, preventing weed seed germination and growth.	Expensive. Tomato and pepper yields may be lower on IRT mulches than on black.
Photo-degradable	Same as undegradable mulch	Breaks down with exposure to sunlight, eliminating the need for retrieval and disposal at the end of the season.	Rate of breakdown can be inconsistent. Buried edges frequently do not break down, leaving plastic residue in the field. These plastic residues are often difficult to retrieve.
Biodegradable	Same as undegradable mulch	Usually a starch-based plastic. Broken down by soil micro-organisims.	Most of these mulches are relatively new in the marketplace, and only limited quantities are available. There is a big range in the quality and degradability of products. Always field-test new products on a limited scale first.

Row Covers

Row covers can be used to promote earlier production by increasing the canopy air temperature and protecting young transplants from wind damage. There are two basic types: floating row covers and low tunnels.

Regardless of the type of row cover used, heat can build up inside the covers and damage crops. Temperatures should be monitored. Remove or ventilate the covers when the temperature exceeds 32°C-35°C (90°F-95°F). For crops that require bee pollination (vine crops), row covers must be removed or opened up at flowering time.

High temperatures under the row cover during flowering may cause fruit deformities or a decrease in pollen viability.

Floating Covers

These are made of polypropylene or various polyester-type fabrics and are laid directly over the crop. They are available in sheets up to 15 m (50 ft) wide to cover multiple rows. Floating covers are well suited for large acreages and low-growing plants. They are not recommended for use on upright-growing crops such as tomatoes and peppers. They may cause abrasions on the plant's shoot tips.

Low Tunnels

These are made of white or clear polyethylene and are supported over the crop on wire hoops. The plastic usually has slits or perforations for ventilation. Commercially available layers install hoops, lay the plastic covers over the hoops and bury the edges of the row cover in one operation.

Research in Ontario has demonstrated that use of low tunnels can result in earlier production of lettuce, peppers, cucumbers and melons. Tunnels used on fresh-market tomatoes should be removed well before flowering. Extended use of row covers on tomatoes may result in reduced fruit set.

High Tunnels

A high tunnel is a semi-permanent, simplified greenhouse. It is typically made of a metal tubing framework and a single sheet of polyethylene plastic. Each end of the tunnel is open or contains a large wooden door frame, allowing for the use of small field equipment inside the tunnel.

A high tunnel may expand the growing season by 3-4 weeks, but will not function well for growing during the winter. It is possible to expand the growing season even more by adding temporary heaters to the structure.

In addition to expanding the growing season, high tunnels may improve the harvest quality of certain vegetable crops. Shoulder-checking (russetting) and cracking of tomatoes may be reduced when the fruit is protected from rain. A reduction in leaf wetness may also help reduce disease problems.

During the hot summer months, the tunnels may be ventilated simply by rolling up the plastic sidewalls, allowing air to move through the structure. As the inside of the tunnels does not receive any natural rainwater, drip irrigation is highly recommended.

Remove the plastic from the high tunnel during the winter months. This will improve the lifespan of the plastic. It will also prevent structural damage to the tunnel due to snow load or inclement weather.

Plastic Disposal

Used plastic mulches are difficult to recycle due to the amount of dirt, plant material and water present. Limited recycling opportunities do exist in some areas. In most areas, disposal at a licensed landfill site is currently the most practical option. Burning or on-farm burial is not recommended.