Brassica nigra

Brassica nigra
(flowers and seeds)

lots of mustard

Brassica nigra, or Black Mustard, is a common lowland weed, especially around cultivated farmland. It is fairly tall and noticeable, with lots clusters of yellow flowers at the top of it many branches. 

Unsurprisingly, it is a member of the Mustard family, with four petals and narrow seed pods about an inch long. This is typical of the mustard family. The stems are much branched, with clusters of yellow flowers at the top of each branch. The clusters are somewhat small, because it proceeds from bud to flower to seed in a short time, but it keeps growing new buds at the end of the stem, so there can be a lot of seed pods below the flowers. It can be distinguished from other mustards by how the mature seed pods grow pressed up against the stem ("appressed" in botany terms). The leaves are pointed and variously lobed, being quite large and compound at the bottom, and small at the top near the flowers and seed pods. The taste is also very distinctive. The whole plant has the flavor of mustard, and when raw, it is very pungent and spicy-hot.

The mustard flavor is the main reason for eating this plant. The leaves, flowers, buds, and the youngest stems can all be used to add some of that flavor to your food. The pungency is moderated by combining it with other foods or by cooking. It is not a green I would eat as a main course, but it is good when added as a flavoring to other foods.

But this plant is more important as a spice than as a food, and the seeds are the source of spicy-hot mustard. Other species, such as Brassica alba, are also used to make mustard, but they are less spicy-hot than Brassica nigra. 

Collecting the processing the seeds is a longer enterprise. First, you need to collect the branches full of seed pods. It is helpful to let them dry completely before trying to separate the seeds, by leaving them in a dry place for a week or more. Or even much more, if you want to save them for winter when you have less fresh plants to work with. They need to be threshed and winnowed similar to grains, but there are some differences because the seeds are so small. Threshing is fairly easy and similar to wheat. Just put the stems or seed pods in a cloth bag, and beat them with a thick stick for a few minutes. Winnowing is a bit more difficult than wheat because the seeds are small and nearly as light as the chaff. It is helpful to sift them through a screen to separate the seeds from the larger chaff. Then pour the seeds and chaff from one bucket to another in front of a fan to blow away the lighter chaff. Using a fan is better than natural wind, because it is more consistent, and you can move closer or farther from the fan to get the right amount of wind to blow the chaff without blowing away the seeds. Large, deep buckets are helpful, because the seeds have a tendency to bounce. Repeated sifting and winnowing will eventually leave you with clean mustard seeds.

Once you have your seeds, you can make mustard. The general recipe is 1 part mustard seeds to 2 parts water or other liquid. (And add a bit of turmeric if you want the mustard to actually be yellow.) The seeds need to be ground, and this can be done while dry or after soaking the seeds in water. If you are doing a small amount, grinding dry works better, because the wet seeds will stick to the sides of your spice grinder instead of falling into the blades. You could also use a mortal and pestle, but that will be a lot more work either way. Once the seeds are both ground and mixed with water, the chemicals in the mustard seeds start reacting, producing the spicy-hot pungency which is very intense in fresh mustard. It should be left out for a while until it mellows out a bit. The spiciness is also antibacterial, so you don't have to worry about it going bad. It is also common practice to extend the amount of mustard with some filler, such as flour or other meal. I have used wheat flower, almond flour, and other things. It does affect the taste much and gives you more mustard in the end.