Vegetable · Brassicaceae
Mustard
Brassica juncea
Sown in cool weather it shoots up fast, a peppery-leaved green and yellow-flowered cover crop rolled into one.
Full sun
Every 3–7 days
45–84°F · 7–29°C
40–90 days
Overview
Mustard is a fast, easy brassica whose leaves carry a sharp, peppery bite on the tongue. Those leaves run lobed and green, sometimes flushed with a purple tinge along the edges, and they crop within weeks of sowing for the kitchen. The same plant drives one of the best biofumigant cover crops there is, its green mass chopped and dug in to clean the soil before the next planting.
It rises from seed in cool weather and races upward, opening bright yellow four-petaled flowers in loose sprays by spring or fall. Two species carry most of the character. Brown mustard, Brassica juncea, packs the highest charge of the compounds behind biofumigation into its tissue, while white mustard, Sinapis alba, gives much of the pale condiment seed and throws up dense leaf that smothers weeds on its own.
After the yellow bloom fade, mustard forms long slender pods that dry and split, spilling seed across the ground. Left that far, it comes back as a weedy volunteer, so the moment that matters is cutting it at early flowering, before those pods ever set.
Care
The short version: full sun and cool weather, direct-sown into well-drained soil rich in organic matter. Keep it moist so the lobed green leaves come on fast, expect little fuss, and cut the stand down at early flowering before the pods set seed.
Light
Mustard wants full sun and colors up its leaves fastest in it, though it takes a little partial shade. The more light it drinks in, the broader the green foliage and the more biomass it builds for the soil.
Water
Keep the soil evenly moist and the lobed leaves flush out quick and tender, which is what mustard is grown for. Water counts most at termination: once a biofumigant stand is chopped and dug in, water it in right away to drive the glucosinolate reaction and trap the pungent gases in the ground.
Soil
Root it in well-drained loam rich in organic matter. Mustard is easy about pH, taking anything from 5.5 to 8.5, and it often feeds itself on the leftover nitrogen and sulfur in the ground rather than on added fertilizer.
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 the seed splits and greens up fast, while lasting summer heat drives it to bolt and rush into yellow flower.
Humidity
Ordinary outdoor humidity is all its foliage asks, so there is no figure to chase. Airflow matters more, since a crowded, damp mass of leaves invites the cabbage-family diseases mustard already carries.
Feeding
Most of the time mustard needs no feeding at all, and its leaves fill out on nutrients other crops left behind. Work in a little nitrogen at planting only if the soil is genuinely poor.
Pruning
There is no ornamental pruning to shape here. Instead the whole leafy stand is mowed or chopped at early flowering to terminate it and release the biofumigant compounds before the pods form.
Repotting
This does not apply. Mustard is a field and garden annual, direct-sown right where its roots take hold and never potted up.
Size & spacing
Plants stand 1 to 6 feet (30 to 180 cm) tall depending on type and season, their leafy crowns spreading 6 to 18 inches (15 to 45 cm). As a cover crop it is drilled or broadcast into a thick green 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 shoots up fast and lifts its yellow flowers within weeks, so cut it before the pods form.
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 to two things: letting the flowers give way to seed-filled pods, or rushing the cover-crop steps that make biofumigation work. Match the look of what you are seeing below, then act early while it still helps.
Volunteer mustard seedlings scattered across the bed next season
The cause is self-sowing, set off when plants are left to flower and swell their long slender pods before they are cut. Feral mustard is a known weed, so seed spilled from those splitting pods sprouts far and wide the next year. Cut the stand at early flowering, mow or chop it in, and pull the small green seedlings while they are still low to the ground.
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 cole crops
Mustard is a brassica, kin to cabbage, broccoli, kale, and radish, and it shares their pests and soilborne diseases. Grown just before or after those crops, it stacks up flea beetles that punch tiny shot holes through the leaves, aphids, and clubroot, which lingers in the soil and swells the roots of every brassica it reaches. Pull and destroy anything affected, and leave a multi-year gap before another cabbage-family crop takes that 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
Feeble suppression almost always traces to how the green stand was terminated. The isothiocyanate gases that do the work are volatile, so coarse, ragged residue or a slow till lets them slip away before they react. Chop the leafy biomass fine, drive it into warm, moist soil within about 20 minutes, water it in to seal the gases below the surface, and time the cut to early flowering when the glucosinolates run 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 here: mustard is a safe edible crop, its lobed green leaves eaten fresh and its small seed ground into the familiar yellow condiment. Nothing about it poisons people, pets, or animals. The only thing to watch is quantity, since the same sharp heat that fills the raw leaf and seed can mildly irritate the gut if any animal gorges on it.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats Safe
Mustard holds no poison for a cat, and the sharp, peppery bite of the leaves keeps most from taking more than a sniff. A cat that does gnaw through a good deal of the pungent green foliage may drool or turn a little queasy, and that eases off on its own.
Dogs Safe
Nothing in mustard is toxic to a dog. The hot, peppery taste of the raw leaf usually cuts the tasting short, and a big mouthful of the green foliage brings little worse than some gas or a loose stool.
Horses Safe
Mustard is safe around horses, though its pungent leaves and seed make poor bulk feed. A horse that grazes a large helping of either can end up with a mildly irritated gut, so it stays a small part of the ration.
Rabbits Safe
The tender green mustard leaves suit rabbits as an occasional treat. Bring them in slowly and in small amounts, as with any new leaf, rather than piling them on every day.
Livestock Safe
Mustard and other brassica forage graze safely when animals do not gorge on the pungent leaves and seed. In quantity that sharp foliage can mildly irritate the gut, so it is introduced gradually and held to no more than about three quarters of the diet.
Birds Safe
Mustard poses no poisoning risk to birds, and chickens peck cheerfully at both the green leaves and the small round seed. The foliage adds variety in the run.
People Safe
Mustard greens and the small seed are everyday food, eaten raw, cooked, or ground into the yellow condiment. The only catch is the heat, since a lot of the raw pungent leaf or seed can sting the mouth and gut, the same chemistry that gives mustard its sharp bite.
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 grown for the garden and field comes down to a few species and named cover-crop blends, each with its own leaf and seed. These are the ones worth knowing by sight.
Brassica juncea (brown / Indian / oriental mustard)
This is the high-glucosinolate biofumigant species, its lobed green or purple-tinged leaves and cultivars like Caliente, Pacific Gold, and Kodiak bred to pack the 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 pale condiment seed and for cover, and cultivars such as IdaGold make the glucosinolate sinalbin that earns it a name for smothering weeds under dense leafy growth.
'Caliente'
A high-glucosinolate Brassica juncea blend sold as a dedicated biofumigant cover crop, throwing up a heavy mass of leaf and stem for the chop-and-incorporate green manure that cleans the soil.
How to propagate
Mustard is grown one way alone, straight from its small round 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 its small round seed alone, and it comes on fast. Direct-sow in cool weather, 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. Fine green seedlings break the surface in a few days and rush up to flowering and peak biomass in roughly 6 to 12 weeks by season, so cut the stand at early flowering before the long pods form, split, and scatter seed for 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, working through more than the shade of its leafy canopy. Its green tissue is loaded with glucosinolates, and when the chopped plant is wetted 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 the residue is chopped fine, dug in at once, and the warm soil kept moist, since otherwise the volatile compounds lift off and vanish.