Vegetable · Brassicaceae
Mustard
Brassica juncea
Sow it in cool weather and it comes up fast, working as a kitchen green and a soil-cleaning cover crop in one.
Full sun
Every 3–7 days
45–84°F · 7–29°C
40–90 days
Overview
Mustard is a fast, easy brassica grown two very different ways. In the kitchen garden it's a peppery leaf crop and the source of condiment seed, ready to pick within weeks of sowing. On the farm it's one of the best biofumigant cover crops there is, dug in green to clean up the soil before the next planting.
It grows from seed in cool weather and races to flower, with bright yellow four-petaled blooms by spring or fall. Two species do most of the work. Brown mustard, Brassica juncea, carries the highest punch of the compounds behind biofumigation, while white mustard, Sinapis alba, is the one behind much of the condiment seed and a strong weed smotherer in its own right.
The plant's one real fault is its eagerness to spread. Left to flower and pod, it drops seed and comes back as a weedy volunteer, so the trick with mustard is cutting it down at early flowering, before it gets the chance to sow itself.
Care
The short version: full sun and cool weather, direct-sown into well-drained soil rich in organic matter. Keep it moist for fast growth, expect little fuss, and cut the stand down at early flowering before it sets seed.
Light
Mustard wants full sun and grows fastest with it, though it puts up with a little partial shade. The more light it gets, the more leaf and the more biomass you bank for the soil.
Water
Keep the soil evenly moist to push the quick, leafy growth mustard is grown for. Watering matters even more at termination: once you chop a biofumigant stand and dig it in, water it in right away to drive the glucosinolate reaction and trap the gases in the ground.
Soil
Plant in well-drained loam with plenty of organic matter. Mustard is unusually easy about pH, taking anything from 5.5 to 8.5, and it's often grown to mop up leftover nitrogen and sulfur rather than to be fed.
Temperature
This is a cool-season plant, comfortable from about 45°F to 84°F (7°C to 29°C). Sow into soil in that range and it germinates fast, but lasting summer heat pushes it to bolt and flower early.
Humidity
Ordinary outdoor humidity is all it asks, so there's no figure to chase. Airflow matters more, since crowded damp stands invite the cabbage-family diseases mustard already carries.
Feeding
Most of the time mustard needs no feeding at all and is grown precisely to scavenge nutrients other crops left behind. Work in a little nitrogen at planting only if the soil is genuinely poor.
Pruning
There's no pruning to do on an ornamental level. Instead you mow or chop the whole stand at early flowering to terminate it and release the biofumigant compounds before it sets seed.
Repotting
This one doesn't apply. Mustard is a field and garden annual, direct-sown where it grows and never potted up.
Size & spacing
Plants run from 1 to 6 feet (30 to 180 cm) tall depending on type and season, spreading 6 to 18 inches (15 to 45 cm). Grown as a cover crop it's drilled or broadcast in a thick stand rather than spaced out as individuals.
When to plant
Mustard is a cool-weather crop with no taste for summer heat. Direct-sow it in early spring as soon as the soil works, or again in late summer to early fall about four weeks before the first hard freeze. It grows fast and flowers within weeks, so plan to cut it before it pods.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Not typically started indoors; direct-sow in the field/garden.
- Plant: Direct-sow in cool weather - early spring as soil works, or late summer/early fall about 4 weeks before the first hard freeze, into soil 45-85 degrees F.
- Harvest: Bright yellow four-petaled flowers in spring or fall; for biofumigation terminate at early flowering before seed set.
Hardiness
USDA hardiness zones: 3–11
Temperature: down to -40°C
Troubleshooting
Most mustard trouble traces back to two things: letting it go to seed, or rushing the cover-crop steps that make biofumigation work. Match what you're seeing below, then act early while it still helps.
Mustard volunteers popping up across the bed next season
This is self-sowing, and it happens when plants are left to flower and form pods before you cut them down. Feral mustard is a known weed, so the dropped seed can settle in and come up everywhere the following year. Terminate the stand at early flowering by mowing or chopping it in, and pull any stray seedlings while they're still small.
Fix:
- Mow, chop, or till in volunteers before they flower
- Cultivate or hand-pull seedlings while small
- Avoid letting any plants reach mature pods
Cabbage-family pests and diseases carrying over to your cole crops
Mustard is a brassica, so it shares pests and soilborne diseases with cabbage, broccoli, kale, and radish. Grow it right before or after those crops and you build up trouble like flea beetles, aphids, and clubroot, which hides in the soil and infects every brassica it meets. Pull and destroy anything affected, and keep a multi-year gap before another cabbage-family crop goes in the same ground.
Fix:
- Remove and destroy affected plants
- Do not follow mustard immediately with cabbage, broccoli, kale, radish, or other brassicas
- Rotate to a non-brassica crop
Weak biofumigation after a mustard cover crop
When a mustard cover crop fails to clean up the soil, the cause is almost always how it was terminated. The isothiocyanate gases that do the work are volatile, so coarse residue or a slow till lets them escape before they react. Chop the green biomass finely, work it into warm, moist soil within about 20 minutes, water it in to seal the gases, and time the whole thing to early flowering when glucosinolates peak.
Fix:
- Chop the green biomass finely with a flail mower
- Incorporate (till in) immediately, within about 20 minutes
- Irrigate/water in right after to seal in the gases
- Time termination to early flowering
Toxicity
Good news here: mustard is a safe edible crop, eaten as greens and ground into the familiar condiment. Nothing about it poisons people, pets, or animals. The only thing to watch is quantity, since the same heat that makes it pungent can mildly irritate the gut if any animal gorges on the raw leaves or seed.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
Mustard isn't poisonous to cats, and most won't touch the sharp leaves anyway. A cat that chews a lot of the pungent foliage might drool or get a mild upset stomach, but that passes on its own.
Dogs Safe
There's nothing toxic here for a dog. The peppery bite usually puts them off, and the worst a big mouthful of leaves tends to cause is a little gas or a loose stool.
Horses Safe
Mustard is safe for horses to be around, though it's not a feed you'd want them filling up on. A horse that grazes a large quantity of the seed or foliage can get a mildly irritated gut, so keep it as a small part of what they eat.
Rabbits Safe
Mustard greens are fine for rabbits as an occasional leafy treat. Introduce them slowly and in small amounts, the way you would any new green, and don't make them a daily staple.
Livestock Safe
Mustard and other brassica forage are grazed safely as long as animals don't gorge on them. Eaten in quantity the foliage and seed can mildly irritate the gut, so introduce it gradually and keep brassica to no more than about three quarters of the diet.
Birds Safe
Mustard poses no poisoning risk to birds, and chickens will happily pick at the leaves and seed. The greens make a fine bit of variety in the run.
People Safe
Mustard greens and seed are everyday food, eaten raw, cooked, or ground into the condiment. The only catch is the heat: a lot of the raw pungent leaf or seed can sting the mouth and gut, which is just the same chemistry that gives mustard its kick.
Treat normal mustard greens and seed as ordinary food for people. The concern is large-quantity exposure: dogs or cats nibbling a lot of the pungent leaves or seed may get GI/mouth irritation, and livestock grazing large amounts of mustard or other brassica forage can develop bloat, digestive upset, hemolytic anemia, goiter/hypothyroidism, or brassica poisoning. Wear gloves and avoid breathing dust when chopping large amounts of biofumigant mustard, as the isothiocyanate vapors are pungent and irritating.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
United Kingdom
Animal PoisonLine: 01202 509000 (fee applies)
Australia
Animal Poisons Helpline: 1300 869 738
Varieties
Mustard sold for the garden and the field comes down to a few species and named cover-crop blends. These are the ones worth knowing by name.
Brassica juncea (brown / Indian / oriental mustard)
This is the high-glucosinolate biofumigant species, with cultivars like Caliente, Pacific Gold, and Kodiak bred to pack their tissue with the compounds that knock back soil pests and weeds once the plant is chopped and dug in.
Sinapis alba (white / yellow mustard)
This is the white mustard grown for condiment seed and for cover, and cultivars such as IdaGold make the glucosinolate sinalbin that earns it a name for smothering weeds and building biomass.
'Caliente'
A high-glucosinolate Brassica juncea blend sold as a dedicated biofumigant cover crop, throwing up heavy biomass for the chop-and-incorporate green manure that does the soil cleaning.
How to propagate
Mustard is grown one way only, straight from seed, and it could hardly be simpler.
SeedEasy
Direct-sow in cool weather: early spring or late summer/early fall, into soil about 45-85 degrees F. · Germinates in a few days and grows fast, reaching flowering and peak biomass in roughly 6-12 weeks depending on season.
Mustard grows from seed only, and it grows fast. Direct-sow in cool weather, either in early spring or in late summer to early fall, into soil around 45°F to 85°F (7°C to 29°C). Drill it about 5 to 12 pounds an acre at a quarter to three quarters of an inch deep, or broadcast 10 to 15 pounds an acre. Seedlings break ground in a few days and run to flowering and peak biomass in roughly 6 to 12 weeks depending on the season, so cut the stand at early flowering before it sets its own seed and leaves you volunteers.
Myths
Mustard cover crops fight weeds and soil pests just by shading and crowding them out like any thick cover crop.
Mustard is a true biofumigant, and that's a different thing from simple shade. Its tissues are loaded with glucosinolates, and when the chopped plant gets wet the enzyme myrosinase turns them into isothiocyanate gases close to the active ingredient in synthetic soil fumigants like metam sodium. Those gases knock back soilborne pathogens, nematodes, and weed seeds, but only if you chop the residue fine, dig it in at once, and keep the warm soil moist; skip that and the volatile compounds just blow off.