Growing Brassicas in Rotation

Part of Crop Rotation

The brassica family — cabbage, kale, broccoli, turnips, radishes, and mustard — includes some of the most nutritious and versatile crops for a rebuilding community. Understanding their demands and disease vulnerabilities is essential for placing them correctly in a crop rotation.

The brassica family (Brassicaceae, formerly Cruciferae) is enormous. It includes leafy greens, root vegetables, oilseeds, and even condiments. Cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, turnips, rutabagas, radishes, mustard, and canola are all brassicas. They share common nutritional needs, common pests, and common diseases — which is why rotation treats them as a single group.

These are cold-hardy, nutrient-dense crops that can extend your growing season into early spring and late autumn when most other vegetables will not grow. Many can tolerate hard frost. Some, like kale and Brussels sprouts, actually taste better after frost. In a rebuilding scenario, brassicas provide critical vitamins (C, K, A, folate) and dietary fiber that are difficult to get from grains and legumes alone.

The Brassica Family Members

Crop	Edible Part	Season	Days to Harvest	Cold Hardiness
Cabbage	Head (leaves)	Cool	70-120	Hardy to -5°C
Kale	Leaves	Cool-cold	55-75	Hardy to -15°C
Broccoli	Flower buds	Cool	60-90	Moderate (-3°C)
Cauliflower	Flower buds	Cool	70-100	Tender (-2°C)
Brussels sprouts	Axillary buds	Cool-cold	90-130	Hardy to -10°C
Kohlrabi	Swollen stem	Cool	45-60	Moderate (-3°C)
Turnip	Root + greens	Cool	35-60	Hardy to -5°C
Rutabaga	Root	Cool	90-120	Hardy to -5°C
Radish	Root	Any	20-35	Moderate (-2°C)
Mustard (greens)	Leaves	Cool	30-45	Moderate (-3°C)
Mustard (seed)	Seeds	Warm	80-100	N/A (summer crop)

Year-Round Brassicas

In temperate climates, you can have brassicas available every month of the year. Plant radishes and mustard greens for early spring, broccoli and cauliflower for late spring, kohlrabi for summer, cabbage and turnips for autumn, and kale and Brussels sprouts for winter. Succession planting every 2-3 weeks extends the harvest window.

Nutrient Demands

Brassicas are heavy feeders — among the most demanding vegetable crops for soil nutrients. They require abundant nitrogen, potassium, and calcium, plus trace elements including boron and molybdenum.

Nitrogen

Brassicas need 150-250 kg of available nitrogen per hectare (15-25 g per square meter). This is roughly double what carrots or beans need. In rotation, brassicas should follow legumes (beans, peas, clover) that have fixed nitrogen into the soil. Alternatively, apply heavy compost or aged manure before planting.

Signs of nitrogen deficiency: Pale yellow-green leaves, stunted growth, purpling of leaf undersides. Cabbage heads stay small and loose. Broccoli produces tiny, premature heads.

Calcium and pH

Brassicas prefer slightly alkaline soil — pH 6.5-7.5. This is higher than most vegetables prefer (pH 6.0-6.5). Lime the soil if your pH is below 6.5. Calcium also plays a direct role:

• Prevents tip burn in cabbage (brown edges on inner leaves)
• Reduces hollow stem in broccoli
• Raises pH to suppress clubroot (see below)

Boron

Brassicas have the highest boron requirement of any common vegetable. Boron deficiency causes hollow stems in broccoli, brown rot in cauliflower curds, and cracked, corky turnip roots. If boron deficiency appears, dissolve 1 tablespoon of borax in 5 liters of water and apply per 10 square meters. Do not over-apply — boron toxicity is possible at only 2-3x the optimal rate.

Boron is Toxic in Excess

The margin between deficiency and toxicity for boron is narrower than any other plant nutrient. Apply borax solution only once per season and only if deficiency symptoms appear. Never broadcast dry borax — uneven distribution causes toxic spots.

Position in Rotation

Brassicas should follow legumes in the rotation and precede light-feeding crops.

Ideal rotation sequence:

1. Legumes (beans, peas, clover) — fix nitrogen
2. Brassicas (cabbage, broccoli, kale) — use the nitrogen
3. Roots or alliums (carrots, onions) — light feeders, different disease profile
4. Solanaceae or cucurbits (tomatoes, squash) — different family entirely

This sequence gives brassicas access to legume-fixed nitrogen and ensures a minimum 3-year gap before brassicas return to the same ground.

Minimum 4-Year Rotation

Never grow brassicas in the same ground more than once every 4 years. Once every 5-7 years is better, especially where clubroot has been detected. All brassica family crops count — including cover crop mustard, radish, and turnip. A mustard cover crop in the “off” years resets the rotation clock.

Clubroot Prevention

Clubroot (Plasmodiophora brassicae) is the single most devastating disease of brassicas. Roots swell into grotesque clubs, plants wilt and die, and the pathogen’s resting spores persist in soil for 15-20 years. Once established, clubroot makes a field nearly unusable for brassicas.

Prevention is everything:

1. Never transplant from unknown sources. Clubroot is most commonly introduced on infected transplant roots. Grow your own seedlings in clean soil, or buy only from trusted nurseries.
2. Lime to pH 7.2+. Clubroot thrives in acid soil. Raising pH above 7.0 dramatically reduces infection severity. Apply agricultural lime (calcium carbonate) well before planting — it takes 2-3 months to raise pH significantly.
3. Drain wet areas. Clubroot spreads through water movement. Improve drainage in low-lying field areas.
4. Control cruciferous weeds. Shepherd’s purse, wild mustard, charlock, and other wild brassicas host clubroot and maintain the pathogen between crop cycles. Eliminate them.
5. Rotate 7+ years if clubroot has appeared. This is the only rotation interval that meaningfully reduces spore levels.
6. Do not move contaminated soil. Clubroot travels on boots, tools, machinery, and root vegetables. Clean everything when leaving an infected area.

Pest Management

Cabbage Moth and Cabbage White Butterfly

The most common and persistent brassica pests. The adult butterflies and moths lay eggs on brassica leaves. Larvae (green caterpillars) eat holes through leaves and bore into cabbage heads.

Controls:

• Row covers: Lightweight fabric draped over crops and sealed at the edges physically excludes egg-laying adults. The single most effective control. Install at planting and keep covered until harvest.
• Hand-picking: Check leaf undersides twice weekly. Crush egg clusters (yellow dots) and remove caterpillars. Effective for small plantings.
• Bt (Bacillus thuringiensis): A natural bacterial spray that kills caterpillars when they eat treated leaves. Non-toxic to humans, bees, and birds. Apply every 1-2 weeks when caterpillars are active.
• Companion planting: Interplant with strongly scented herbs (thyme, rosemary, sage) or nasturtiums. These do not prevent all damage but may reduce egg-laying.

Flea Beetles

Tiny jumping beetles that chew small round holes in leaves, giving a “shot-hole” appearance. Most damaging to seedlings — established plants outgrow the damage. Spring emergence coincides with brassica transplanting.

Controls:

• Row covers (again, the best solution)
• Delay planting until mid-season when flea beetle populations have peaked
• Trap crops: plant radishes or mustard greens as sacrificial crops around the perimeter — flea beetles prefer these and leave cabbage alone
• Sticky traps: yellow cards coated with petroleum jelly catch adults

Cabbage Root Fly (Root Maggot)

Adults lay eggs at the base of brassica stems. Larvae tunnel into roots, causing wilting and death of young plants. Most damaging in early spring.

Controls:

• Collars: cut 10-15 cm circles from cardboard, carpet, or foam. Slit to the center and fit snugly around each stem at soil level. This prevents egg-laying at the stem base.
• Delayed planting to avoid the spring generation
• Rotation — root fly populations build where brassicas are grown repeatedly

Aphids

Mealy cabbage aphid colonies build rapidly on leaf undersides, especially in warm, dry weather. They stunt growth, spread viruses, and contaminate harvest.

Controls:

• Strong water spray to dislodge colonies
• Encourage ladybugs, lacewings, and hoverflies (natural predators)
• Interplant with flowers to attract beneficial insects
• Inspect weekly and act early — a colony of 50 is manageable, a colony of 5,000 is not

Growing Techniques

Starting Transplants

Most brassicas benefit from transplanting rather than direct seeding. Start seeds indoors or in a seedbed 4-6 weeks before the target planting date. This gives you a head start, avoids flea beetle damage on tiny seedlings, and lets you transplant only the strongest plants.

Seedling care:

• Sow 1 cm deep in fine, weed-free soil
• Keep moist but not waterlogged
• Thin to 5 cm spacing when first true leaves appear
• Harden off for 1 week before transplanting (gradually expose to outdoor conditions)

Direct Seeding

Radishes, turnips, and mustard greens grow fast enough to direct-seed. Sow thinly in rows, 1-2 cm deep. Thin to final spacing when seedlings are 5 cm tall. Eat the thinnings.

Transplanting Distances

Crop	In-Row Spacing	Between Rows
Cabbage	40-50 cm	60-75 cm
Kale	40-50 cm	60 cm
Broccoli	40-50 cm	60-75 cm
Cauliflower	50-60 cm	75 cm
Brussels sprouts	50-60 cm	75-90 cm
Kohlrabi	20-25 cm	30-40 cm
Turnip	10-15 cm	30-40 cm
Rutabaga	20-25 cm	40-50 cm

Watering

Brassicas need consistent moisture — 25-35 mm per week. Drought stress causes premature bolting (going to seed), bitter flavor, tough texture, and tiny heads. Water at the base to keep leaves dry (wet leaves promote fungal diseases).

Mulching

A 5-10 cm layer of straw, hay, or grass clippings around brassica plants retains moisture, suppresses weeds, and keeps roots cool. Pull mulch back slightly from stems to prevent stem rot. Mulch is especially valuable for autumn brassicas that need cool, moist roots during warm late-summer weather.

Frost and Cold

Most brassicas are cold-tolerant, and some are extremely so. Kale survives -15°C. Brussels sprouts improve in flavor after frost converts some of their starches to sugars.

Extending the season:

• Simple row covers or cold frames extend autumn brassicas well into winter
• Mulch heavily around roots before hard freeze — even if tops freeze, protected roots regrow in spring (especially kale)
• Harvest Brussels sprouts from the bottom of the stalk upward over many weeks — they do not need to be harvested all at once

Seed Saving Considerations

Most brassicas are biennial — they grow vegetatively in year one and flower in year two. To save seed, you must overwinter the plant (or the root, in the case of turnips and rutabagas) and let it flower and set seed the following spring.

Cross-Pollination

All brassica species that share the same species name will cross-pollinate freely. Cabbage, broccoli, cauliflower, kale, kohlrabi, and Brussels sprouts are ALL Brassica oleracea — they will cross with each other. To save pure seed, grow only one B. oleracea variety per field, or isolate by at least 500 meters. Turnips (B. rapa) and rutabagas (B. napus) are different species and do not cross with B. oleracea, but turnips will cross with napa cabbage and bok choy (also B. rapa).

Seed saving steps:

1. Select the best 5-10 plants of each variety — healthiest, truest to type, latest to bolt
2. In cold climates, dig roots and store over winter in a root cellar (2-5°C, moist sand). Replant in spring.
3. In mild climates, leave plants in the ground with mulch protection
4. Allow plants to bolt, flower, and form seed pods in spring
5. Harvest pods when they turn brown and dry, before they shatter
6. Thresh by rubbing pods between hands. Winnow chaff by pouring seeds in front of a breeze.
7. Dry seeds thoroughly and store cool and dry — brassica seeds remain viable for 4-5 years

Harvest and Storage

Crop	Harvest Sign	Storage Method	Storage Duration
Cabbage	Head firm and solid	Root cellar, 0-4°C, high humidity	3-6 months
Kale	Leaves full-sized	Harvest fresh as needed	N/A (harvest ongoing)
Broccoli	Buds tight, before flowering	Eat fresh or ferment (sauerkraut relative)	1-2 weeks fresh
Cauliflower	Curd white and tight	Eat fresh or pickle	2-3 weeks fresh
Brussels sprouts	Sprouts firm, 2-3 cm	Harvest as needed from stalk	Weeks on stalk
Turnip	Root 5-8 cm diameter	Root cellar in moist sand	3-5 months
Rutabaga	Root 10-15 cm diameter	Root cellar in moist sand	4-6 months

Fermentation

Cabbage converts to sauerkraut via lacto-fermentation: shred, salt (2% by weight), pack into a crock, and weight down. In 2-4 weeks you have a preserved, probiotic-rich food that stores for months at cool temperatures. This is one of the best ways to preserve the vitamin C content of brassicas through winter.

Summary

Brassicas (cabbage, kale, broccoli, turnips, radishes, mustard) are heavy-feeding, cold-hardy crops that thrive in pH 6.5-7.5 soil rich in nitrogen, calcium, and boron. Place them after legumes in rotation and maintain a minimum 4-year gap between brassica plantings (7+ years if clubroot is present). Prevent clubroot through liming, clean transplants, and weed control. Manage cabbage caterpillars and flea beetles with row covers as the primary defense. Most brassicas are biennial for seed saving, and all B. oleracea species (cabbage, broccoli, kale, cauliflower, kohlrabi, Brussels sprouts) cross-pollinate freely. Store root brassicas in a cool, humid root cellar; preserve cabbage as sauerkraut for winter vitamin C.