There are three main kinds of mustard. White mustard, Sinapis alba (syn. Brassica alba and Brassica hirta), is also occasionally called yellow mustard (though most people mean the condiment when they say this). White mustard can indeed be used to make the mustard we eat with hot dogs and roast beef, but that is not its only purpose in life... if plants can be said to have a purpose. It's a member of the cabbage family, as is its namesake black mustard (Brassica nigra syn. Sinapis nigra) which is in the same genus white mustard was in before it was moved to Sinapis (black mustard has also moved - from Sinapis to Brassica!) and brown mustard (Brassica juncea).
To distinguish between white mustard, black mustard and brown mustard, you need to look at the leaves. As you can see from the photo, white mustard has deeply cut lobes and are quite spikey - in comparison with black mustard which has a much more rounded shape, but still spikey-looking. Brown mustard (or red mustard or mustard greens) has leaves which are neither lobed nor spikey to look at and often have a reddish cast to them. White and black mustard are generally cultivated for their seeds, whereas brown mustard is more of a leaf vegetable and will not be discussed further here.
Mustards are closely related to salad rocket.
Both black and white mustard are European natives, but naturalized in most parts of the world. Both are hardy perennials; white mustard reaches a height of around 2 feet (60cm) while black mustard reaches around 4 feet (1.2m). They require moist well drained soil but are otherwise unfussy. Black mustard is tolerant of sea air. Neither will grow in full shade.
Mustard is one of the components of mustard and cress - though much of what is sold is, in fact rape seed sprouts. To grow mustard and cress sow the mustard seed thinly on moist blotting paper (or several layers of kitchen towel) and then sow the cress seeds on top about 3-4 days later. Keep the blotting paper/kitchen towel moist and cut with scissors when the sprouts are about 1-2 inches tall. Lovely in sandwiches with egg. Ensure you buy seeds marked as suitable for sprouting, as seeds meant for outdoors may be dressed with stuff you don't want to eat.
Do not use mustard if you have sensitive skin, as it may cause blistering. (Heaven only knows what it does to our insides - when I get a chance at a salt beef sandwich I literally plaster the stuff on.)
Both black and white mustard seeds are generally used externally, usually ground to a powder before use. Mustard powder is often sold in food stores, though it's generally not possible to tell whether the powder is from white, black or a mixture of seeds. However, you can use them interchangeably in most cases.
Mustard oil is said to be a hair restorative - however, it must be diluted with some other oil, as it will burn the scalp otherwise (and may do so in any case). Never allow neat mustard oil to come into contact with the skin. I don't have the dilution details, but for making liniment, the mix is 1 part mustard oil to 40 parts alcohol, so I would go with 1 part in 40 to start with, and amend from there. Use the liniment for rheumatic pain, sciatica, lumbago and gout.
To make a mustard plaster, mix 1 part of mustard powder to 2 parts of wheat or rice flour by volume (to reduce burning), then make a fairly stiff paste with cold water. Take a piece of linen and spread it with a layer of the mixture. Cover the area to be treated with gauze, then apply the plaster and secure. Remove as soon as the burning becomes unbearable and wash the area thoroughly to remove all traces of residue. Powder with rice flour and wrap in a clean dry cotton cloth. This is used to treat congestion of the lungs, pleurisy, arthritic joints, chilblains, skin conditions such as boils, and fungal infections. It must not be used on sensitive areas.
To make a footbath, steep a bag containing 1-2 ounces of crushed seed/powder for 10-15 minutes in hot water then add the liquid to a large bowl of hot water, adjust the temperature, put your feet in it and leave them there until the water cools (you can extend the time by carefully adding boiling water). This is used to help relieve tired feet, and also for colds, feverish conditions and headaches.
To make a stimulating bath, use the same method as for a footbath, but increase the quantities to around 8 ounces of mustard powder.
You can also use mustard to make a steam inhalation for colds and nasal congestion.
A mixture of whole black mustard seed and molasses (called treacle in the UK) is laxative, though I would try other remedies first.
It will probably be no surprise that as with all other plants grown for use in medicine, mustard should be grown organically to retain its efficacy. To find out more about growing organic white mustard or organic black mustard visit the Gardenzone.