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Using Row Covers Against Insect Pests

Q. Thanks to the clever catalog I received from Gardens Alive that identifies insect pests by photos of the damage they cause, I now know that I need to control:

  • Squash vine borers;
  • Cucumber beetles that carry bacterial wilt; and
  • Mexican bean beetles.
The suggestions for all three pests include covering the crops with thin agricultural covers. At first, I thought, "I can just cover half of my garden!" But then I realized that the vegetables I want to protect require pollination, and the pests are active all season. How can I use these agricultural barriers without blocking pollinators? Thanks!
    ---Diane; Capital District Community Garden at Ridgefield Park in Albany, NY
A. The same way you get to Carnegie Hall, Diane: Practice, practice, practice! Achieving this noble goal requires that the art of gardening come into play.

Commonly known as 'row covers' or 'floating row cover', these protective 'blankets' are composed of a cloth-like material made from spun polyester or similar fabric. You drape them directly over a row of plants (salad greens are a classic), or over hoops or other structures that allow some room at the top for taller plants to grow unimpeded and with better airflow.

Used early and late in the season, row covers trap enough heat to keep the soil and plants underneath them a little warmer at night—protecting the plants from the cold and helping them grow at the pace they would if the weather was actually warmer. This can be a huge help when summer is slow to arrive, or a week of cold weather blows through late in the summer before your plants are ripe and ready.

Like the sheer curtains that thrifty people sometimes use in their place, row covers 'breathe'; they're porous enough for air and water to pass through, so they don't smother the plants underneath. Sunlight also gets through. But insects do not, and that's the benefit you're looking for—prevention of physical contact from the outside.

There's no fancy ciphering involved when you use row covers to protect plants that don't need pollination—like lettuce, spinach, broccoli, potatoes and such. Those plants can stay 'under cover' all season if you like—especially if you switch from a heavyweight cover early and late in the season to a lightweight one for the summer.

But when the crops underneath require pollination, trial and error, experience, luck and personal choice all come into play. One thing is certain; you should put the covers over any young plants you wish to protect as soon as those plants go in the ground outside, and leave the covers on until any flowers start to open. To provide complete protection from pests (and from wind ruining your Big Plan), the edges of the covers should be pegged down or covered with soil.

Using row covers in this manner early in the season will produce bigger, healthier plants. Virtually every published study has shown that plants under row covers grow faster and stronger than similar uncovered ones. Ah, but then the flowers start to open. What do you do then?

Oh wait—that's what you asked. Ahem. Your choices include:

  1. Just take the covers off at this point. The theory here is that the pests will have largely established their turf elsewhere, and your bigger, healthier, stronger plants will better resist and withstand any attack the insects do mount. Just be sure and also practice basic organic pest control techniques: Keep lots of room between your plants and only feed them compost or other gentle organic fertilizers (chemical fertilizers increase pest problems). Use water and shade to attract toads; these beneficial amphibians prey on all three of your pests. (See this previous Question of the Week on toads for more details.) Handpick any adult insects you see. Check the undersides of your bean leaves for egg clusters and destroy any you find. Wipe squash vines with a damp rag once a week to destroy vine borer eggs before they can hatch. Or wrap the vines where they enter the ground with some row cover material to continue your physical protection of the plant at the actual point of attack.

  2. Take the covers off for a short period of time on sunny days (say about an hour at a pop) to allow for some pollination by native bees and other insects. Squish any visible pests that also fly in.

  3. Grow only bush-type beans and consider leaving the covers on at least some of the plants all season. Beans are kind of 'self-pollinating' and you might be surprised at the yield you get.

  4. For the squash and cukes, lift the covers and hand-pollinate the plants, using a little artist's brush to distribute pollen from flower to flower. This is the best plan when pest pressures are strong in the immediate area (as is often the case in community gardens). If you have lots of plants, just do a few each day—and take a little stool or bench out to the garden with you so you can do the job sitting down.
Which tactics you try—and which ones end up working best—will be based on your own personal style and situation. If you visit the garden in question every day, keeping the covers on most of the time will work better for you. If you don't get out to the garden as often as you'd like, you'll probably be better off coddling the plants under row covers early in the season, getting them off to a good strong start and then taking the covers off for the rest of the season. Providing a happy healthy childhood generally pays big dividends.

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