May 22, 2018
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Extend the garden season with row covers and cold frames

Photo courtesy of Marjorie Peronto
Photo courtesy of Marjorie Peronto
This photo, courtesy of Marjorie Peronto, show Elisabeth Curran, University of Maine Cooperative Extension Master Gardener, inspecting plants in one of the Hancock County Extension greenhouse cold frames.
By Reeser Manley

Last week’s column discussed the use of floating row covers, made of Spun Polypropylene, to exclude insect herbivores from vegetable crops. These row covers are light in weight (less than 0.5 oz. per square yard), admit up to 95 percent of available light, and allow rain and irrigation water to pass through to the soil.

Heavier row cover fabric (exceeding 1 oz. per square yard) can be used to extend the growing season for a variety of crops, including cucurbits, lettuce, edible-pod peas, carrots, radishes and others. Also made of Polypropylene, season-extending row covers can give as much as 8 degrees of frost protection at both ends of the gardening season.

For most crops, the heavyweight row covers can be used as floating row covers, supported by the plants themselves. Exceptions are tomatoes, peppers and other plants with fragile growing tips; for these crops, the fabric should be supported with hoops.

Season-long row covers should never be used for tomatoes and peppers, as temperatures exceeding 86 degrees for more than a few hours may cause blossom drop in these crops. In

my opinion — and here I am parting company with many users of row covers — row covers used for season extension always should be removed when not needed for frost protection.

In the home garden, removal of the row cover during flowering is essential for crops that

require pollination by insects. But to avoid removing the row cover, some large-scale growers

will introduce pollinators under the cover.

Consider the chain of events when crops are left covered for the entire production period. Warmer temperatures under the cover enhances weed growth, leading to installation of a plastic

mulch. The mulch excludes water, requiring a plastic drip irrigation system beneath the mulch.

And to top it off, add a layer of clear plastic over the row cover material for extra protection in the coldest months.

There is a term for this type of vegetable production: plasticulture. Covered in plastic, the garden no longer looks or acts like a garden. It has little to offer pollinators and other beneficial insects. The birds go elsewhere. There may be a place for such an approach to vegetable crop production, but I cannot call that place a garden.

Row covers last for two, possibly three years in the home garden. What happens to used row cover fabric? At present, most ends up in landfills. Recycling options are limited by the contamination of the fabric with soil and burning results in contamination of air, water and soil.

One recycling idea is to tear used row covers into plant ties, using them to stake tomatoes.

Cold Frames

A simple wooden frame set onto the ground, or on top of a raised bed, with a transparent top called the “light” hinged on top, a cold frame creates a microclimate using solar energy and insulation. The top of the frame is often slanted at a 45 degree angle to maximize exposure to the sun.

Gardeners often use old storm windows or discarded shower doors for lights. Corrugated polycarbonate sheets mounted on a lightweight wooded frame also make a durable light, lasting for at least 15 years.

The lights are hinged onto the top of the frame so they can be opened for ventilation on warm, sunny days. Opening can be done manually or you can install a temperature-sensitive automatic vent opener.

In early spring, cold frames can be used to harden off seedlings produced indoors. A week in the cold frame will help acclimate seedlings, bridging the gap between indoor warmth under fluorescent bulbs and the garden’s chilly nights and full-sun days.

Some cold frames are constructed to double as raised beds, once the lights are removed.

Cool-season crops, such as lettuce and cole crops, can be started in late winter from seeds sown in the frames, then later either transplanted to the garden or grown to maturity in the frame, after the lights are removed.

Mobile cold frames can be used to cover garden beds in advance of the first frost, extending the growing season into late fall and early winter. Depending on the severity of the winter, you may be able to continue harvesting cole crops and lettuce well beyond the first hard freeze in the open garden.

Greenhouses covered with polycarbonate panels extend the vegetable production season when the warm air at the peak of the greenhouse is circulated to a heat sink beneath soil in raised beds. Cover these beds with cold frames and you can harvest fresh lettuce in February.


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