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:
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
Table 1. Plastic Mulch Types
||Average Soil Warming
at 2 in.depth
||Prevents weed seed germination
||Good soil-to-plastic contact is necessary to ensure maximum
||Can be used with direct-seeded crops such as sweet corn.
||Weed germination under the mulch requires good preplant (residual)
||-1°C (cools the soil)
Keeps the soil temperature lower to minimize bolting in cool
|Weed germination may occur under the mulch. Requires good
preplant weed control.
||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.
|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.
|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 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
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.
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.
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.
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
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.