Extending The Gardening Season
Want to extend your growing season? Think row covers! The use of protective coverings to produce a miniature greenhouse effect is certainly not a new idea. A quick trip to one of our local antique shops will often lead to the discovery of glass bell jars, which were commonly placed over individual plants in the early 1900s by European gardeners. And more than a few of us recall using newspapers to make paper “hats” to place over tender tomato and pepper seedlings to protect them from a late frost.
The main purpose of row covers in the late winter or early spring is to provide protection from adverse weather conditions and to increase air and soil temperatures during the initial stages of growth. The improved growth of plants under row covers can be attributed to higher air and soil temperatures. Row covers, when utilized in conjunction with raised beds, often allow crops to be planted 3-4 weeks before traditional planting. Now that’s a head start to make my grandmother envious!
Two basic types of row cover materials are available: plastic (which is supported by plastic or metal hoops) and fabric (which also may be supported by plastic or metal hoops but are often let free to float, resting directly on plants). If you decide to use clear plastic, use a length of 5-8′ wide. Plastic comes in a range of thicknesses, measured in millimeters, ranging from 1 ½- 8 mm. The greater the thickness, the higher the insulation value, and thus, the greater heat retention. The downside of a thicker material is that less light passes through to the plants. The sides and ends of the row covers are secured in place by anchoring the edges with one of the following methods:
- Ground stakes
- Row cover hand pegs
- Anchoring pins
- Plastic bottles filled with water or sand
The temperature under plastic row covers needs monitoring because heat will build up and can be 20-25 degrees hotter than the outside air temperature. As a general rule, when the air temperature outside of the row cover reaches 60º-65º F, the ends of the row cover should be opened to provide ventilation and cooling to prevent plant damage.
One advantage of utilizing a supported row cover is that as temperatures rise, you can prevent bolting of lettuce and other plants that are not tolerant of warm temperatures. You simply remove the row cover and drape a shade cloth over the frame, and your lettuce will continue growing — and not going to seed — in that nice, cool shade.
Most floating row covers are thin, lightweight, porous or spun-bonded materials. They are placed directly over the plants, leaving some slack for movement and room for plant growth. As with plastic row covers, the edges of floating row covers are secured by anchoring the edges with dirt or stones or metal staples you can find at gardening stores. Floating row covers provide only a few degrees of protection, but they are an excellent barrier for a wide range of pests. However, if the crop requires pollination, such as squash, the row cover needs to be removed when plants start to flower or you’ll need to pollinate by hand.
There are numerous resources, including various web sites and seed catalogs, that provide “How To” instructions as well as materials for constructing row covers. According to an article by Washington State University, “Row Covers for Vegetable Gardens,” (Horticultural Fact Sheet #19), row cover benefits include:
- Early yields
- Increased yields
- Frost protection
- Pest protection
- Water conservation
This article describes an interesting trial conducted in New Hampshire, that involved both covered and non-covered seedbeds of nine varieties of lettuce and three types of spinach, planted in October. By spring, the beds with floating row covers had perfect stands of lettuce and spinach. In the beds without row covers, not a single plant survived the winter. The researchers were bewildered by the dramatic results because the temperatures recorded under the row covers were the same as without row covers (-2° F). The researchers theorized that the frost heaving and drying was less severe under the row cover material. They are continuing the trials, so perhaps we’ll soon know even more about row covers. In the meantime, I encourage you to try some row covers this spring.
Virginia Tech Publication 426-381 “Season Extenders” http://pubs.ext.vt.edu/426/426-381/426-381.html
“Vegetable Production Under Row Covers,” The Virginia Gardener, Volume 6 Number 2, February 1987 (S.B. Sterrer)
“Extending the Season,” https://blogs.cornell.edu/gblblog/files/2016/04/Extending-the-Season-1zc6nw0.pdf
“Row Covers for Vegetable Gardens,” (Washington State University Community Horticulture Fact Sheet #19) http://ext100.wsu.edu/king/wp-content/uploads/sites/17/2014/02/Row-Covers-for-Vegetable-Gardens1.pdf