Vegetable · Brassicaceae
Mustard
Brassica juncea
Sow it when it's cool and up it comes fast, pulling double duty as a kitchen green and a soil-cleaning cover crop.
Full sun
Every 3–7 days
45–84°F · 7–29°C
40–90 days
Overview
Mustard is a fast, easy brassica, and it's grown two totally different ways. In the kitchen garden it's a peppery leaf crop plus the source of condiment seed, and you can pick it within weeks of sowing. Out on a plot? It's one of the best biofumigant cover crops around, dug in green to clean up the soil before whatever you plant next.
It comes up from seed in cool weather and rushes to flower, throwing bright yellow four-petaled blooms by spring or fall. Two species do most of the heavy lifting. Brown mustard, Brassica juncea, packs the biggest punch of the compounds behind biofumigation, and white mustard, Sinapis alba, gives you a lot of the condiment seed while smothering weeds nicely on its own.
Its one real weakness? It's a little too keen to spread. Let it flower and pod and it'll drop seed and come back as a weedy volunteer, so the trick with mustard is cutting it down at early flowering, before it gets to sow itself.
Care
The short version: full sun and cool weather, direct-sown into well-drained soil that's rich in organic matter. Keep it moist so it grows fast, expect hardly any fuss, and cut the stand down at early flowering before it sets seed.
Light
Mustard loves full sun and grows fastest in it, though it'll put up with a bit of partial shade. The more light it gets, the more leaf you'll pick and the more biomass you bank for the soil.
Water
Keep the soil evenly moist and you'll get that quick, leafy growth mustard's known for. Watering matters even more at termination, because 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
Give it well-drained loam with plenty of organic matter. Mustard's really easygoing about pH, taking anything from 5.5 to 8.5, and it's often grown to mop up leftover nitrogen and sulfur instead of being fed.
Temperature
It's a cool-season plant, happy from about 45°F to 84°F (7°C to 29°C). Sow into soil in that range and it germinates fast, but a long stretch of summer heat will push it to bolt and flower early.
Humidity
Regular outdoor humidity is all it wants, so there's no number 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 whatsoever, and it's actually grown to scavenge the nutrients other crops left behind. Just work in a little nitrogen at planting if your soil's genuinely poor.
Pruning
There's no ornamental pruning to bother with. What you do instead is mow or chop the whole stand at early flowering to terminate it and release those biofumigant compounds before it sets seed.
Repotting
This one just doesn't apply. Mustard's a field and garden annual, direct-sown right where it grows and never potted up.
Size & spacing
Plants run 1 to 6 feet (30 to 180 cm) tall depending on type and season, and spread 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 one by one.
When to plant
Mustard's a cool-weather crop and it really doesn't like summer heat. Direct-sow it in early spring the moment 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 on cutting 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
Nearly all mustard trouble comes down to two things: letting it go to seed, or rushing the cover-crop steps that make biofumigation actually work. Find what matches below, then jump on it early while it still helps.
Mustard popping back up all over the bed next year
What's going on here is plain old self-sowing. If some plants get to flower and fill their pods before you cut them, they'll drop seed, and since feral mustard is a known weed, that seed happily comes up everywhere the next season. Want to head it off? Cut the stand at early flowering, mow or chop it in, and pull the odd stray seedling while it's still tiny.
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 bugs and diseases hitching a ride to your cole crops
Here's the thing: mustard is a brassica, so it shares its pests and soil diseases with cabbage, broccoli, kale, and radish. Grow it right before or after any of those and you'll stack up trouble like flea beetles, aphids, and clubroot, which sits in the soil and gets into every brassica it can reach. So pull and destroy whatever looks affected, and give it a good multi-year gap before another cabbage-family crop goes back in that same spot.
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
Your mustard cover crop didn't really clean up the soil
If the cover crop didn't do much for the soil, it almost always comes down to how it got terminated. The isothiocyanate gases that do the real work are volatile, so leave the residue coarse or till it in slowly and they'll slip away before they get to react. The fix is easy: chop the green growth fine, work it into warm, moist soil within about 20 minutes, water it in to lock the gases down, and aim for early flowering when the glucosinolates are at their highest.
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 right off the bat: mustard is a safe edible crop, eaten as greens and ground into the condiment everyone knows. It won't poison people, pets, or animals. The only thing to keep an eye on is quantity, because that same heat that makes it pungent can mildly irritate the gut if an animal really goes to town on the raw leaves or seed.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
Good news for cat owners: mustard isn't poisonous, and honestly most cats won't go near those sharp leaves anyway. If one does chow down on a lot of the pungent foliage, you might see a bit of drool or a mild upset tummy, but it sorts itself out.
Dogs Safe
Nothing here is toxic to dogs, so you can relax. That peppery bite usually stops them after a taste, and if a dog does wolf down a big mouthful of leaves, the most you'll get is a little gas or a soft stool.
Horses Safe
Mustard's fine for a horse to be around, but it isn't something you'd want them making a meal of. Graze a big pile of the seed or foliage and a horse can end up with a slightly irritated gut, so just keep it to a small part of what they eat.
Rabbits Safe
Rabbits can have mustard greens as an occasional leafy treat, no problem. Just bring them in slowly and in small amounts, like you would with any new green, and don't let them become an everyday thing.
Livestock Safe
Mustard and other brassica forage are safe to graze, as long as the animals aren't gorging on it. Too much of the foliage and seed can give them a mildly upset gut, so ease it in and keep brassica to no more than about three quarters of the diet.
Birds Safe
There's no poisoning risk to birds at all, and chickens? They'll happily pick away at the leaves and seed. The greens are a nice little bit of variety in the run.
People Safe
For people, mustard greens and seed are everyday food, eaten raw, cooked, or ground into the condiment. The only catch is the heat, since a lot of the raw pungent leaf or seed can sting your 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
The mustard you'll find for the garden and the field really boils down to a couple of species and a few named cover-crop blends. These are the ones worth knowing.
Brassica juncea (brown / Indian / oriental mustard)
This is the high-glucosinolate biofumigant one, and cultivars like Caliente, Pacific Gold, and Kodiak are bred to load their tissue with the very compounds that knock back soil pests and weeds once you chop the plant and dig it 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 gives it its knack for smothering weeds and piling on biomass.
'Caliente'
This is a high-glucosinolate Brassica juncea blend sold as a dedicated biofumigant cover crop, and it throws up loads of biomass for that chop-and-dig-in green manure that does the soil cleaning.
How to propagate
There's just one way to grow mustard, straight from seed, and honestly it could hardly be easier.
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 and only from seed, and it's quick about it. 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 just broadcast 10 to 15 pounds an acre. You'll see seedlings in a few days, and it'll 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 hands you a crop of volunteers.
Myths
Mustard cover crops fight weeds and soil pests just by shading and crowding them out like any thick cover crop.
Not quite, because mustard is a real biofumigant, and that's a whole different trick from just throwing shade. Its tissues are full of glucosinolates, and when the chopped plant gets wet, the enzyme myrosinase turns them into isothiocyanate gases that are close cousins of the active ingredient in synthetic soil fumigants like metam sodium. Those gases go after soilborne pathogens, nematodes, and weed seeds, but only if you chop the residue fine, dig it in right away, and keep the warm soil moist, because skip that and the volatile stuff just drifts off.