The best times to know when to use a row cover are three key windows in your garden year. The first is spring frost when night temps dip below freezing on tender plants. The second is fall freeze when cold weather threatens late-season crops. The third is pest emergence when bugs hatch out and start to feed. A spring row cover is the most common use for new gardeners just starting out.
I have tracked my last frost dates and pest scout calendar for seven years in a notebook by my back door. My last spring frost falls around April 25 in my zone. My first fall frost hits near October 15 most years. I put cabbage moth covers on around May 10 when the moths show up to lay eggs on my brassica beds. This log saves me from guesswork every season.
Covers raise the air temp under the fabric well above the temp in the open air. Per Maryland Extension, temps under cover run 5 to 15°F warmer than outside. A night that hits 28°F in the open feels like 36°F under a medium cover. The boost is enough to save most frost-tender crops from cold damage.
Use a cover in spring once you plant any frost-tender crop in the ground. Tomato, pepper, and squash starts all need cover for the first two weeks. I cover mine the same day I transplant and leave the cover on through the last expected frost date. Cover stays in place day and night if temps stay cool but lifts on warm afternoons to let bees in.
Fall frost cover goes on when the first hard freeze is in the forecast on your weather app. Cover greens, root crops, and any plants you want to harvest later in the year. I get an extra four to six weeks of harvest from kale, spinach, and carrots by using fall covers. Add a heavy weight or layer two light covers when temps drop below 25 degrees Fahrenheit at night.
Watch out for heat under cover on warm days. Per Wisconsin Hort, take covers off flowering tomato and pepper plants once day temps hit 86°F. Higher temps cause blossom drop, which means no fruit set. I check temps under my covers with a cheap thermometer on any sunny day above 70°F. I tested this last May and saved a pepper bed from a hot snap.
Pest emergence timing is the third big reason to put a cover on your beds. Watch for bug hatch dates in your area through your local extension office. Cucumber beetles, squash vine borers, cabbage moths, and flea beetles all show up at set times each spring. Cover your crops a week before the bugs show up so no eggs get laid on the leaves.
Build a simple calendar that pairs your local frost dates with each pest hatch window per crop. Write down when you plan to put covers on and when to take them off. I list each crop in my notebook with its frost dates and pest dates side by side. A good calendar turns guesswork into a clear plan that boosts your yields year after year.
Read the full article: Row Cover Garden Guide: Weights and Timing
