Row Covers for Cabbage Pests: Keep Moths Off Your Brassicas
By Paul Smith, pemdasgarden.com
The simplest way to keep cabbage worms off broccoli, kale, and cauliflower is to stop the adults from ever reaching the leaves. A cover of insect netting or lightweight floating row cover, put on at planting and sealed at the edges, is a physical barrier: no butterfly or moth lands on the plant, so no eggs are laid and no caterpillars hatch. This guide is about that barrier only — which cover to choose, how to install it so it actually seals, how to check underneath it, and when to lift it. For identifying the caterpillars themselves, see our cabbage worm identification and control guide; pest identification is kept brief here on purpose.
One caution before you cover anything: a covered bed is not a diagnosed bed. If you find ragged holes and a smooth green caterpillar on a brassica leaf, this looks most like imported cabbageworm feeding, but cabbage looper or diamondback moth larva damage is also possible, so treat any single sighting as a working guess, not a settled diagnosis. Covers help in all three cases, because all three start with a flying adult laying eggs on the plant — but if caterpillars are already inside the heads, a cover will not remove them, and you should handpick first (see below).
Why a cover works when sprays disappoint
Cabbage white butterflies and cabbage moths lay eggs on brassica leaves, and the caterpillars that hatch chew holes, tunnel into heads, and leave droppings that stain broccoli and cauliflower curds (Utah State University Extension). A spray aimed at caterpillars has to hit young, actively feeding larvae on leaf undersides, and it does nothing about the next butterfly. A cover works one step earlier, on the adult, and it keeps working as long as it stays sealed. University of New Hampshire Extension lists cabbage worms, cabbage loopers, cabbage maggots, cutworms, and flea beetles among the brassica pests a row cover can exclude, and Utah State University Extension describes row covers as a mechanical control within integrated pest management — a physical barrier that prevents insects from reaching host plants.
That prevention-first position matters for the whole IPM order used in this article: prevention first, then cultural controls, then mechanical and physical controls, then biological controls, and least-toxic chemical options last. A cover is the prevention step. Everything after it — rotation, handpicking, parasitic wasps, and finally selective treatments — is backup for the caterpillars a cover misses, not a replacement for the cover.
Timing decides whether the barrier works at all. Utah State University Extension advises laying the cover over plants before pests are evident in the area, and the University of Maryland Extension advises installing it over plants soon after planting. A cover put on after butterflies have already laid eggs simply protects the caterpillars inside. Inspect every transplant before covering: look under the leaves for eggs and small caterpillars, and pick off anything you find, so you do not seal the pest in with the crop.
Which cover to choose: netting vs floating row cover vs frost fabric
Three materials dominate garden use, and they are not interchangeable. Insect netting is a see-through knitted mesh, usually white, that excludes insects without adding much heat. Lightweight floating row cover is a spun-bonded fabric, also usually white, that is permeable to light, water, and air and adds a little warmth. Heavier frost fabric is a thicker spun-bonded fabric bought mainly for cold protection; it blocks far more light and traps far more heat. The table below compares them on the four factors that decide a brassica planting, using figures stated by the extension sources cited.
| Cover | Pest exclusion | Light and air | Heat | When it wins |
|---|---|---|---|---|
| Fine insect netting | Excludes cabbage white butterflies, moths, and root maggot flies; the finest meshes also exclude aphids and flea beetles (Utah State University Extension) | About 80% light transmission; air moves freely, so little heat builds up (University of Maryland Extension) | Little or no heat gain; the summer choice when row cover would overheat (New England Vegetable Management Guide) | Summer brassicas and hot climates; beds you plan to keep covered all season |
| Lightweight floating row cover (about 0.45 oz per square yard) | Excludes butterflies, moths, and other listed brassica pests when edges are sealed (University of New Hampshire Extension) | 90% to 95% light transmission (University of Maryland Extension) | About 2 degrees Fahrenheit of frost protection; some warming, but modest (University of Maryland Extension) | Spring and fall plantings; young transplants that also benefit from a little warmth and wind shelter |
| Heavier frost fabric (1.5 to 2.2 oz per square yard) | Excludes pests while it is on, but it is bought for cold, not for insects | Only 30% to 50% light transmission (University of Maryland Extension) | Up to 8 degrees Fahrenheit of frost protection; temperature underneath can run 5 to 15 degrees above outside air (University of Maryland Extension) | Overwintering crops and hard frost nights — not a season-long pest cover for brassicas |
| No cover | No barrier; butterflies and moths lay freely | Full light and air | No added heat | Only when butterflies are absent, or when you are actively handpicking and checking daily |
Verdict: choose fine insect netting if you grow brassicas through summer heat or in a hot climate, because it keeps moths and butterflies out without cooking the plants underneath. Choose lightweight floating row cover if you are planting in spring or fall and want pest exclusion plus a little early warmth and wind shelter for young transplants. Choose heavier frost fabric only for cold protection on overwintering crops or frost nights, and lift it when the cold passes — its low light transmission and high heat buildup make it a poor season-long pest cover. Choose no cover only if you accept daily checking and handpicking as the control, because without a barrier every butterfly that visits can lay eggs.
Cover choice by crop stage and timing
The second decision is when each cover goes on, how long it stays, and what mesh or weight the stage calls for. This original compiled table gathers the timing guidance from the extension sources into one place; every figure in it is stated by the source named in the same row.
| Crop stage or situation | Cover and timing guidance | Source figure behind it |
|---|---|---|
| Transplants just set out, spring or fall | Cover at transplanting, before butterflies are flying over the bed; inspect leaves first | Install soon after planting to be effective against pests (University of Maryland Extension) |
| Direct-seeded brassicas | Cover at seeding, so the barrier is in place before emergence and before the first flight | Row covers are installed right after planting or seeding (New England Vegetable Management Guide) |
| Summer brassicas under heat pressure | Switch to insect netting; ventilate or lift spun-bonded cover on hot days | Consider extra ventilation when outside air reaches around 80 degrees Fahrenheit; insect netting is the better choice when heat is a concern (New England Vegetable Management Guide) |
| Root maggot risk in spring | Have the barrier in place by the time adult flies lay eggs, and keep it until egg-laying ends | Set the barrier up by the time adult flies lay eggs, usually early to mid-May, and keep it until the end of the month (University of Minnesota Extension) |
| Mesh choice for moths vs small pests | Any listed insect mesh grade excludes moths; only the finest grade excludes aphids, flea beetles, thrips, and whiteflies | Moths are excluded at every mesh size in the table, including 5 mm by 3 mm; aphids and flea beetles only at 0.35 square mm (Utah State University Extension) |
| Head formation (broccoli, cauliflower) | Keep the cover on and sealed; check underneath for caterpillars that slipped in before covering | Scout weekly from the button stage, checking leaf undersides and plant centers (Utah State University Extension) |
| End of the covered period | Lift the cover when plants outgrow the slack, when heat builds up, or at harvest; brassicas need no insect pollination, so flowering is not a reason to lift it | Brassicas are listed among crops that do not rely on insect pollination, unlike nightshades and cucurbits (University of New Hampshire Extension) |
Two timing traps deserve emphasis. First, rotation: do not cover a bed where brassicas grew last year if root maggot pupae may be waiting in the soil — University of Minnesota Extension warns that a cover over last year's brassica ground traps emerging adults inside with the crop, and the New England Vegetable Management Guide gives the same warning for soil-overwintering insects. Cover a rotated bed instead. Second, heat: University of Maryland Extension reports temperatures under a row cover can run 5 to 15 degrees Fahrenheit above outside air, and University of New Hampshire Extension advises checking that temperatures under covers do not spike past 90 degrees. A sealed cover in a heat wave can hurt plants faster than the caterpillars would have.
Installation walkthrough: measure, support, seal, check, lift
Install on a calm day if you can: wind turns a large sheet of fabric into a sail, and the USDA Natural Resources Conservation Service guide advises delaying installation when winds run over 2 to 3 miles per hour. Work through these steps in order, and do not skip the edge seal — an unsealed edge is the same as no cover.
The photograph above shows the step most often skipped: the edge seal. Fabric lying loose on the soil, or held by a single brick at each corner, will lift in the first gust and admit the very butterflies the cover was bought to stop. Bury it or weight it continuously, and re-check the seal after every storm.
IPM step 1: prevention around the cover
The cover is the centerpiece of prevention, but it works best with two companions. Rotate brassicas to a new bed each year, so soil-overwintering pests do not emerge under the new cover, and clear brassica debris at season's end, since imported cabbageworms overwinter as pupae near host plant debris (Utah State University Extension). When you plan the next covered round, start from seed so the cover goes on from day one: for broccoli, browse our broccoli seeds collection and time sowing so transplants go out with covers already cut and hoops already in place; for rotation beds, our kale seeds and cauliflower seeds collections give you brassicas for ground that did not hold brassicas last year. Planting and harvest times vary by climate region, so time covering by butterfly activity in your own bed and your county extension office, not by a single national date.
IPM step 2: cultural controls
Cultural control here means making the bed less inviting and the cover easier to keep sealed. Keep the bed weeded before covering, since weeds under a cover grow fast and compete with the crop, and lay drip irrigation or soaker hoses under the cover where you can, so watering does not mean unsealing the whole bed every time (University of New Hampshire Extension). Plant a fast succession brassica beside the main covered crop for a short-season slot — our arugula seeds collection suits that gap — and keep flowering borders nearby for beneficial insects, which the next step relies on.
IPM step 3: mechanical controls — checking and handpicking
Mechanical control is the weekly check under the cover. Lift one side, inspect leaf undersides for singly laid pale eggs, and handpick any caterpillars you find into soapy water. If a plant under the cover is heavily infested while its neighbors are clean, remove and discard that plant rather than resealing the pest in with the crop. For hole patterns and caterpillar identification when you are unsure what slipped in, see our holes in cabbage leaves guide and green caterpillars on vegetables guide; for crop-specific behavior, see cabbage worms on kale and cabbage worms on broccoli.
IPM step 4: biological controls — let wasps work outside the cover
Biological control mostly happens outside the covered bed: tiny parasitic wasps that attack cabbageworm eggs and caterpillars need nectar flowers and unsprayed margins to build up, and a covered bed gives them nothing to attack, which is the point. Keep flowering borders unsprayed near the brassica bed, and do not spray the cover or its surroundings broadly. For the wasps themselves — which species attack which stage, and why a caterpillar covered in yellow cocoons should be left alone — see our parasitic wasps for cabbage worms guide. A cover and wasps complement each other: the cover handles the main crop, and wasps reduce the worm population in uncovered succession plantings and neighboring beds.
IPM step 5: least-toxic options last
Only after prevention, cultural, mechanical, and biological steps should other treatments enter the picture, least-toxic first. Bacillus thuringiensis var. kurstaki (Bt) must be eaten by actively feeding larvae to work, and spinosad, the active ingredient, is listed by Utah State University Extension as effective on young caterpillars. Apply only where young larvae are present, cover leaf undersides, apply in the evening, and follow label directions exactly. No treatment works every time, and no result is assured. If a covered plant holds caterpillars, handpick first and treat only the plants that need it; spraying a sealed, clean bed wastes material and risks the beneficial insects outside it.
Common cover failures and how to avoid them
Most cover failures are installation failures, not material failures. Covering after butterflies have laid eggs seals the pest in. Leaving a gap at a corner or an end admits every butterfly that finds it. Draping fabric tight over leaves lets butterflies lay eggs through the mesh wherever a leaf presses against it. Using heavy frost fabric all summer overheats the bed and slows growth under its 30% to 50% light transmission. And covering last year's brassica bed traps emerging root maggot flies inside with the crop. Each of these has the same fix: cover early, on rotated ground, with slack, on hoops where you can, and seal the whole perimeter — then check underneath on a schedule instead of assuming the cover did the job.
Frequently asked questions
Can I leave insect netting on brassicas all season?
Yes, in most gardens. Broccoli, kale, and cauliflower do not need insect pollination, so unlike squash or cucumbers they do not require the cover to come off for flowering. Keep the cover on as long as it stays sealed, plants have room to grow, and temperatures underneath stay safe — and lift it to check underneath weekly.
Do butterflies lay eggs through row cover fabric?
They can, where a leaf presses directly against the fabric. That is why hoops, which hold the fabric off the leaves, and slack, which lets plants grow without pressing hard against the cover, matter. If you drape fabric directly on plants, check leaf undersides near contact points and handpick any eggs you find.
What mesh size do I need to stop cabbage white butterflies?
Any insect mesh grade in the Utah State University Extension table excludes moths, including the coarsest listed grade of 5 mm by 3 mm. Finer mesh is only needed if you also want to exclude smaller pests: aphids and flea beetles require the finest grade, 0.35 square mm. Buy for the smallest pest you actually need to stop, not the finest mesh on the shelf.
Will a row cover overheat my brassicas in summer?
A spun-bonded row cover can: temperatures underneath can run 5 to 15 degrees Fahrenheit above outside air (University of Maryland Extension), and the New England Vegetable Management Guide advises extra ventilation when outside air reaches around 80 degrees Fahrenheit. In summer heat, switch to insect netting, which adds little or no heat, and check that temperatures under any cover do not spike past 90 degrees (University of New Hampshire Extension).
Should I cover a bed where I grew brassicas last year?
No — rotate first. Root maggot pupae overwinter in the soil near last year's brassicas, and University of Minnesota Extension warns that covering that ground traps emerging adults inside with the new crop. Plant brassicas in a bed that held a different crop family last year, then cover that bed.
What do I do if I find caterpillars under the cover?
Handpick them first, then find how they got in. Check the perimeter for gaps, check whether the cover went on after eggs were already laid, and check whether leaves pressed against the fabric allowed eggs through. Reseal the cover only after the bed is clean — otherwise you are protecting the caterpillars.
Do I need to remove the cover for watering?
Not usually. Rain and overhead water pass through spun-bonded fabric and netting, and drip irrigation or soaker hoses under the cover water without unsealing it. If you hand-water, lift one side, water, and reseal the edge the same day so butterflies cannot use the open side.
Sources
- University of New Hampshire Extension — Using row covers in the garden: https://extension.unh.edu/blog/2020/10/using-row-covers-garden
- Utah State University Extension — Row covers: https://extension.usu.edu/planthealth/research/row-covers
- Utah State University Extension — Imported cabbageworm: https://extension.usu.edu/vegetableguide/leafy-greens/imported-cabbageworm
- University of Maryland Extension — Row covers: https://stage.extension.umd.edu/resource/row-covers
- University of Minnesota Extension — Cabbage and onion maggots: http://extension.umn.edu/garden-and-home/yard-and-garden/yard-and-garden-insects/root-maggots
- New England Vegetable Management Guide — Slitted and floating row covers: https://nevegetable.org/book/export/html/119
- USDA Natural Resources Conservation Service — Community garden guide, floating row cover: https://www.nrcs.usda.gov/plantmaterials/mipmcar9780.pdf
Ready to keep the next brassica round moth-free? Cover from day one, seal every edge, and check underneath weekly: start from our broccoli seeds collection, with kale seeds, cauliflower seeds, and arugula seeds for rotation beds.
