I love cabbage! Not only is it tasty and loaded with nutrients, it’s one of the prettiest vegetables in the garden. Those plump heads in shades of green and reddish-purple nestled in their ruffs of crinkled leaves look so nice; I almost hate to harvest them. Cabbage is also a great crop for young gardeners to grow in school and home gardens. It can be planted both early in the season and grown for fall harvest, so it fits well with school session schedules. And kids are always excited to check on the progress of those rapidly expanding cabbage heads, some as big as basketballs at harvest!
But, alas, we’re not the only creatures that love cabbage. Grow cabbage anywhere in the country and you will most likely have to deal with cabbage worms (they’re actually caterpillars, in spite of their name). You'll know they have paid your cabbage patch a visit if you see large, round or irregular holes chewed in the leaves between the veins and midribs, as well as masses of green or brown frass (the polite word for caterpillar poop) between the leaves. The cabbage heads may also be tunneled into. This damage comes courtesy of green worms that are up to about an inch and a half long and have a light stripe down the center of their backs. They are either cabbage loopers or imported cabbageworms. Most gardeners have seen small, white, day-flying butterflies with a black wing spot flitting through the garden– these butterflies are the adults of the imported cabbageworm. The cabbage looper adult is a brownish moth that flies in the evening.
If your cabbage leaves are riddled with small holes or have areas where only the outer layer of the leaf had been chewed, leaving a translucent spot, the caterpillar of the diamondback moth is at work. Smaller than the other worms, these 1/4-inch long green caterpillars feed on the undersides of the leaves and wriggle rapidly when disturbed.
All of these pests also happily dine on other members of the Cabbage family, so you may find them on broccoli, cauliflower, kale, and collards as well. But fortunately, all three of these pests can be controlled relatively easily and safely on cabbage and related crops.
My preferred method is exclusion. As soon as I put my cabbage seedlings in the ground, I cover the bed with a floating row cover and keep it on until I harvest my crop. These lightweight covers let in light and air, but exclude the adult butterflies and moths. No flying females, no eggs laid, no worms! And since what’s getting harvested are the leaves of the cabbage plant, I don't need to worry about removing the covers to let bees in to pollinate flowers, as I would with fruiting crops like squash.
If you don't want to use row covers (they do keep those pretty cabbage heads from view) and you only have a few plants, you can handpick worms from leaf undersides and drop them into a container of soapy water to kill them. Or you can spray plants with the safe microbial insecticide Bt, which only affects butterfly and moth caterpillars. They need to ingest Bt in order for it to work, so be sure to spray the undersides of leaves where the caterpillars are feeding. Bt works best when the caterpillars are small, so begin spraying as soon as you see the adults flying or notice any leaf chewing. You'll need to make repeat applications throughout the summer, as the adults lay eggs all season long.
Imported cabbageworms and cabbage loopers spend the winter as pupae attached to plant debris, so clean up the garden well in the fall to reduce the number that overwinter. (Loopers may not overwinter in colder areas, but they generally work their way up from south by mid to late summer.) Diamondback moths overwinter as adults under plants debris, so garden clean-up will also help to reduce their numbers.
Cutworms and Root Maggot Control
Cutworms and cabbage root maggots are other cabbage pests that may be encountered in many parts of the country. Damage from these pests can also easily be prevented with the use of barriers. Cutworms are night-feeding caterpillars that chew through the stems of young plants right at soil level. To protect plants, place cutworm collars around the stems of seedlings at transplant time. One easy strategy is to wrap the stems of your cabbage seedlings with 4-5 thicknesses of 3-4 inch wide newspaper strips, placed so that 2 inches of the strip extend below ground when the seedling is planted. You can also encircle stems with collars made from cardboard, yogurt containers, or the like, with two inches pushed into the ground and at least one inch above.
Root-feeding cabbage maggots hatch from eggs laid by small flies in the soil around the base of a cabbage-family plant. Row covers will exclude the female flies and prevent egg-laying, but this method needs to be combined with crop rotation to be successful. The adult flies emerge in the spring after having overwintered as pupae in the soil, so covers only work in areas that were not previously infested; otherwise the adults will emerge under the covering. You can also prevent egg-laying by placing mats made from flat, 6-inch wide squares of weatherproof material around the base of each plant when you set it in the garden. Make a slit in one side leading to a small hole in the center for the stem and tuck the mat around the stem. Make sure the mat is firmly in contact with the soil so flies can’t sneak under to lay eggs.