Growing Mustard for Seed, and Making Your Own Mustard

A crop that takes a corner of the bed for a season and fills the pantry for a year.

Mustard greens and mustard seed are the same family grown for opposite reasons. Grown for leaf you keep cutting and never let it flower. Grown for seed you do the reverse: you let it bolt, flower and dry, and the flowering you spend all season preventing in a salad crop is the entire point here.

Which mustard to grow

White or yellow mustard. Big pale seed, mild and slightly bitter. This is the bulk of an American ballpark mustard and the base of most English mustard blends.
Brown mustard. Smaller reddish-brown seed, considerably hotter. This is what goes into Dijon-style and most Asian mustards, and it is the same plant sold as mustard greens, so seed from a greens packet will grow it.
Black mustard. Hottest of the three and the traditional choice, but it shatters its pods readily and is awkward to harvest, which is why commercial growing moved to brown.

Start with brown if you want one. It is the most useful, the easiest to find, and you very likely already have a packet labelled mustard greens.

Growing it for seed

Mustard is a fast, unfussy brassica that wants a cool growing season and full sun. It bolts in heat, which for once is what you want, but a plant that bolts too young makes a poor seed head, so sow it early enough to build some size first.

Sow direct. It resents transplanting and grows so quickly that raising seedlings is wasted effort. Scatter or drill about a centimetre deep and thin to a hand's width apart.
Give it room. Seed crops need more space than leaf crops. Crowded plants make small heads.
Do not feed it hard. Rich nitrogen gives leaf at the expense of seed, which is the opposite of the greens crop.
Expect roughly three months from sowing to dry pods, longer in a cool season.
Keep it away from other brassicas going to seed if you want the line to stay true, since they cross readily.

It will flower yellow for weeks, and the bees will find it before you do. That is a decent secondary reason to grow it.

Harvesting before it beats you to it

This is the whole skill. Mustard pods shatter when fully ripe and spill the seed on the ground, and a crop left three days too long is a crop you have sown for next year instead of harvested.

Watch the pods, not the plant. Cut when the lower pods have turned tan and dry but the top of the stem is still slightly green. A few pods splitting is your signal, not a failure.
Cut whole stems in the morning while there is still dew on them, which makes shattering far less likely.
Upend them into a paper sack or a pillowcase, heads down, and hang it somewhere dry and airy for a fortnight. Anything that shatters falls into the bag instead of the garden.
Thresh by rubbing or treading the dry heads inside the bag, then tip everything into a bowl.
Winnow by pouring the mix between two bowls in front of a fan or in a light breeze. The chaff blows off and the heavy seed drops. Two or three passes gets it clean.

Dry the cleaned seed another week before it goes in a jar. Seed that felt dry can still hold enough moisture to spoil a sealed jar. Stored properly in the dark it keeps its heat for a year and stays viable for sowing for several.

Making mustard, and controlling the heat

Mustard is one of the few condiments where the chemistry is worth knowing, because it is the only thing that decides whether you get a mild table mustard or something that clears your sinuses.

The seed keeps two things in separate compartments: a dormant compound, and the enzyme that acts on it. Crushing the seed with liquid brings them together, and what they produce is the heat. Everything else follows from that:

Cold liquid makes it hot. The enzyme stays active and the reaction runs to completion.
Hot liquid makes it mild. Heat destroys the enzyme before it can do much, which is why traditional mild mustards are made with warm liquid.
Acid stops the clock. Vinegar added straight away gives a mild mustard, because it halts the reaction before it has run. Added ten to fifteen minutes in, it locks the heat at whatever level it has reached.

So the heat is a dial, not a variety: same seed, three different mustards depending on the temperature of the liquid and when the vinegar goes in.

A basic mustard

Soak four tablespoons of seed in half a cup of liquid overnight. Water for a plain mustard, or half water and half beer, wine or cider for more depth.
Blend or pound to the texture you want. Leave it coarse for a wholegrain, work it longer for smooth.
Now choose. For a hot mustard, leave it ten to fifteen minutes before adding two tablespoons of vinegar and a teaspoon of salt. For a mild one, add the vinegar immediately.
Jar it and put it in the fridge for at least a day. Fresh mustard tastes harsh and raw, and a day or two turns it into something you would recognise.

From there it is yours: honey, horseradish, garlic, tarragon, whisky, a spoon of your own chilli. Mustard keeps for months in the fridge because vinegar and mustard oil are both hostile to anything that would spoil it, though the heat softens over time.

One thing to know before you scale up: the pungency fades with age. Mustard made from year-old seed is noticeably tamer than mustard made from this season's, which is an argument for growing your own rather than a problem.

Mustard as a green manure

If a seed crop is more than you want, mustard earns its space anyway. Sown thickly and dug in before it flowers it makes a fast green manure that adds bulk to the soil and smothers weeds while it grows. Sow, let it reach knee height, cut it down and turn it in, and leave it a fortnight before planting into that ground.

The one caution is that it is a brassica, so it belongs in the brassica slot in your rotation. Following it with cabbages or broccoli undoes the point of rotating.

See also: Mustard greens for the leaf crop, Seed saving for drying and storing seed properly, and Pickling, where a jar of your own mustard seed earns its keep.