Red mustard. Beautiful, tasty and really, really hardy. What's not to love? Just make sure there's no other B. rapa flowering at the same time as they cross like mad.
You will need to leave them in much longer than you'd like, or you won't get viable seed.
I save seed from lots of things and every year I'm taken aback at how huge the plants can get and how painfully long it takes for the seed to ripen.
Growing plants to save seed requires a whole different set of timings, and no, the broad beans won't be out in time to transplant the tomatoes and...

They seem to take forever to dry. They probably won't be dry enough until at least the beginning of June. I'm basing this on Brassica oleracea plants though; mustards may be a little faster. I've let Brussels Sprouts, cabbage, and broccoli go to seed and the pods are full sized early on, but I remember them taking at least a month after this to start drying.

It is sometimes surprising how soon a seed will germinate. I have seen Saguaro germinate in the fruit still on the cactus. I have germinated cosmos seed after frost that still have a few petals with color attached. Some other plants might take two years to ripen. It would be interesting to see what mustard would do. Mustard is so commonplace someone must have published something about it.

Donna, you can plant your summer crops in there. The seeds may be ready in a month or more. Save one or two plants, pull the rest, and harvest the when the pods are dry, yellow and the seeds easily come out when the pod is rubbed.

The seed have to dry on the plant, but the plant does not need to be in the garden. As soon as the pods are filled and maybe a few showing signs of drying, You can pull the plants and place them on a drying rack to finish drying. I have to do this with all brassicas or the house finches will strip them long before the pods dry.

Farmerdilla, if it was anyone but you, I'd discount what you said. My internet search on the subject said over and over that the seed must dry on the plant. Did I misunderstand? Can the plants be pulled out sooner?

glib, that is great advice. I most certainly could just leave one plant. There must be a hundred pods on each plant. (Why didn't I think of that?) This neat freak would hate the untidiness, but it would work....

The seeds have to have their final color before pulling, brown or black. Open a few pods and take a look. Early they will be pale yellow to light green.

I'm unable to tell from a quick search whether this is a hybrid. If so, many seeds won't come true. And if other Brassica in the area, all bets are off.

Dan

Purple Giant as well as Osaka Purple is listed as Brassica juncea. No need to worry about cross pollination unless there are other B. juncea in the area. Open pollinated.
Glib is correct, the seeds have to full grown. But at that point the plant will support final development. I have already pulled dried and threshed turnip. Collards are on line. Radishes are drying. Komatsuna in process.

Thanks to you all. These are Purple Giant, and as best I could find online, they are not hybrids. I left no other brassicas standing, and as far as I know, am the only vegetable gardener for a mile or more. I will pull the extra plants next week when the ground dries more. Thanks, farmerdill for the clarification. I am amazed at how huge they are since they began to bloom!