Using barriers to prevent pests from reaching plants is one of the simplest, oldest, and most effective ways to prevent damage. Most of the methods for protecting plants with barriers are familiar to organic gardeners. However, there are recent technical improvements in materials that make barriers more effective than ever before.
Floating Row Covers
Floating row covers don’t really float. They are lengths of synthetic fabric that are so light-weight they appear to float when draped over your plants. The covers look like the white interfacing used in clothing and are made of spun-bonded polypropylene or extruded plastic. They let in more than 80% of the sunlight that shines on them, and rain and irrigation water can pass through them.
Leaving these covers on makes it possible to have season-long pest control and to thwart some species that are particularly trying for organic gardeners. They are most useful for food crops, as you aren’t likely to want to cover your ornamental plants.
Floating row covers were invented to improve plant growth and extend the growing season. The microclimate under a cover is warmer and more humid during the day than the surrounding air. Minimum night temperatures are also higher, and there is less difference between day and night temperatures because heat loss at night is slowed.
In coastal and short-season areas, plants may benefit from the heat-retaining properties of the row cover all season. In regions with hot summers, high temperatures under the cover may be partly offset by reduced evaporation, but you may have to remove the cover when it gets really hot.
The covers do break down in the sunlight, but should last at least 20 weeks. Patch small rips with duct tape or any plastic tape suitable for outdoor use, taping on both sides of the fabric so tape will be sticking to tape.
In southern gardens with long, hot summers, row covers may last only one season. Preserve your covers by removing them from the garden promptly after use, rinsing them with water to remove soil and dust, allowing them to dry thoroughly, and storing them away from sunlight.
You can also use good-quality cheesecloth, fine nylon mesh, or sheer curtain material tacked onto bamboo hoops or wooden lath frames to screen insects away from plants. Because these fabrics block more light than polypropylene does, they are best suited for protecting seedlings. They should be removed once the danger of pest damage has passed or when the plants have become large enough to withstand some minor pest damage.
Floating row covers provide excellent protection for seedlings of all kinds and from pests of all kinds. They will protect cabbage family seedlings from such difficult pests as flea beetles, cabbage root maggots, and the leaf-eating caterpillars. Row covers protect green beans from Mexican bean beetles and potato leafhoppers, shelter early asparagus spears from asparagus beetles, and keep Colorado potato beetles away from tomatoes, eggplant, and potatoes.
Cutworm collars are stiff cylinders made of paper, cardboard, or plastic that encircle transplant stems at soil level. Putting collars around transplants at planting time should be a routine practice in early spring if cutworms are likely to be a problem in your area.
Cutworms rest in the soil during the day and crawl along the soil at night, searching for plant stems to chew. The collars block the cutworms, completely preventing injury. Collars protect against most species of cutworms, but they’re not effective for climbing cutworms.
Filling the collars with wood ashes or diatomaceous earth helps prevent root maggot flies from laying eggs near stem bases. You can cut cutworm collars from a paper towel or toilet paper roll and slip them over the top of small seedlings. Or, curve a strip of lightweight cardboard around a seedling stem and clip it in place with staples or paper clips. Make the collars 2 to 3 inches wide and 1 1/2 to 2 inches in diameter.
As you plant seedlings, encircle the stems with the collars. Push the collars into the soil so that about half the collar is below the soil surface. Paper collars disintegrate eventually, but by the time they do, your plants should be large enough that cutworms will not do significant damage.
Root Fly Barriers
Paper or tar paper squares laid around the stems of cabbage family plants prevent female root flies from laying their eggs near roots (eggs hatch into maggots that burrow into roots). If barriers are placed carefully, these squares protect cabbage family plants from cabbage root flies.