Brussels Sprouts

Start seed indoors in early summer for a fall crop

Sow seed a quarter to a half inch deep in a sterile mix, 4 to 6 weeks before you intend to set plants out, and count backward so the transplants go into the ground 90 to 100 days before your average first fall frost. Seedlings are ready to move out at four to five true leaves. Direct sowing works but adds roughly 20 days to the schedule. Sprouts only firm up and sweeten when they mature in cool weather, so the whole crop is timed to land its 80 to 100 day maturity window on the cold end of autumn rather than in summer heat, which produces loose, bitter sprouts.

Set transplants 18 to 24 inches apart in full sun

Move seedlings out once they have four to five true leaves and a well developed root system, setting each at the same depth it grew in its pot. Space plants 18 to 24 inches apart in the row with 24 to 30 inches between rows, in a spot getting at least six hours of direct sun, with soil pH between 6 and 7. A Brussels sprout plant builds a tall stalk carrying dozens of sprouts and needs the room and light to feed all of them; crowded plants shade their own lower stalk, which is exactly where the first and best sprouts form.

Keep moisture even at 1 to 2 inches a week

If the planting does not get an inch of rain in a week, soak the soil deeply at least once; sandy soils need it more often. Water deeply and infrequently rather than in light sprinklings, and use drip where you can. Mulch three to four inches deep with weed-free straw or herbicide-free grass clippings to hold that moisture. Swings between wet and dry drive two specific failures here: hot dry spells turn the sprouts bitter, and interrupted water movement starves the fastest growing leaf margins of calcium, which shows up as tipburn. Drip also keeps the foliage dry, which slows the leaf spot and black rot organisms that spread in splashing water.

Side-dress nitrogen twice, then stop before sprouts form

Work a complete fertilizer into the bed before planting, then side-dress nitrogen at about 4 and 8 weeks after transplanting, roughly half a cup of 21-0-0 per 10 feet of row, or a small band of all-purpose garden fertilizer along the row. Stop nitrogen once sprouts begin to swell on the lower stalk. This is a long-season heavy feeder that must build a big leafy stalk before it fills sprouts, but nitrogen arriving after sprout formation keeps the plant pushing soft new growth and yields loose, splitting sprouts instead of tight ones.

Top the plant when the lowest sprouts reach half an inch

When the sprouts at the bottom of the stalk are a half to one inch across, cut one to two inches off the very top of the plant, taking out the growing point. Do this only inside the last 60 days before your intended harvest. Strip the lowest leaves as you go to speed the sprouts below. Removing the apical growing point stops the plant from spending sugar on new leaves and new sprout buds it will never finish, and redirects it into the sprouts already set, so the upper ones size up and the stalk finishes evenly. Topping too early just costs you the leaf area that fills the crop.

Check leaf undersides weekly for cabbageworm and looper

Starting right after transplanting, check 25 plants chosen at random once a week. A few days after you see the white cabbage butterflies or looper moths flying, look for eggs and small larvae on the undersides of leaves, and check toward the center of the plant near the leaf midribs for larger caterpillars. Bacillus thuringiensis only works on early-instar larvae, so the entire treatment decision depends on catching the generation while the caterpillars are still small. Once they are large and feeding down inside the developing sprouts, no spray reaches them and the damage is already in the harvested part.

Cabbageworm and looper: treat at sprout formation

As sprouts begin to form on the stalk, spray Bacillus thuringiensis subspecies kurstaki if you find one or more caterpillars in 25 plants. Time it to when eggs are present or larvae are still small, cover the leaf undersides, and add a spreader-sticker to help it stay on the waxy leaves. The Bt crystal protein has to be eaten and only binds receptors in the midgut of caterpillars, so it kills the worms while leaving the parasitic wasps and predators that suppress the next generation alive. Sprout formation is the threshold moment because that is when caterpillars stop being a leaf problem and start boring into the part you eat.

⚠ Bt kills only small larvae, so it fails on large caterpillars. If you need a stronger option, spinosad carries a 1-day pre-harvest interval and a 4-hour restricted-entry interval on cole crops. Brussels sprouts are harvested well before flowering, but if any plant has bolted into bloom, do not spray it.

Cover new transplants with floating row cover

Lay a lightweight floating row cover over the bed right after transplanting and seal the edges to the soil. Leave it on through the first weeks of growth, lifting it only to weed or side-dress. Use a lightweight material so temperatures do not get too hot underneath, and vent or remove the cover during summer heat. Flea beetles, cabbage maggot flies, and the moths that lay cabbageworm and looper eggs all find the plants at the seedling stage, when a few days of chewing can set a transplant back for the whole season. A cover is a physical barrier that never needs a pesticide, and since this crop is harvested long before it ever flowers, excluding insects costs no pollination.

⚠ A sealed cover traps heat. Use lightweight material and watch temperatures under it during summer, or the cover will cook the seedlings it is protecting.

Pick bottom-up after frost, and read the low temperature

Twist off sprouts that are firm, compact, and 1 to 1.5 inches across, always working from the bottom of the stalk upward as they size. Leave plants standing through light frosts. They hold in the garden as long as temperatures stay above 20 degrees F; once the mid-twenties arrive, strip the leaves and either harvest everything or pull the whole stalk and stand it upright somewhere cold to pick from over the following weeks. Frost is not damage here, it is the finishing step: cold makes the plant convert starch to sugar in the sprouts, which is why fall-matured sprouts taste sweet and mild and summer ones taste bitter. Below about 20 degrees F, though, the sprouts themselves start taking injury rather than sweetening.