What is this mystery plant? - mustard

This plant came up in my garden. It seems to be a brassica. At first I though it was an arugula or a wild, sylvetta arugula. But now that it has flowered it is definitely not either. I didn't know that any brassica had purple flowers. Since it is only a single plant, I suppose it can not set viable seeds?

Mustard.

Maybe Wild Radish?

It could very well be a self pollinator. There are many mustards in California that are naturalized invasives from around the world.

That strain is very common in northern California, many consider it a weed. What region are you in?

My second guess is also wild radish.

I'm in Denver, CO.

Radish

The veined flower petals say its wild radish. The real defining characteristics that can be used to tell radish and mustards are the seed pods.If you let it grow, watch for those.

And just for the record, radish is in the mustard family, as are arugula, kale, cabbage, broccoli, cauliflower, raab, horseradish, daikon radish, and so on.  It is a huge family.  Don't know of any poisons produced anywhere in the family.

Here in Western Colorado, we have a little wild mustard with purple flowers, called "little blue mustard"  very delicious greens.

You could have an escaped from cultivation plant, a hybrid of any of the wild and or  cultivated mustards and radishes, several generations out from the original hybridization (if it is an escapee).

Keep an eye on it, save the seeds, see if there is a root, see how it tastes, it might be something you want to keep.

Thanks everyone for the helpful advice! I will see what happens to it.

i found a wild mustard growing in an undsituebd area
i took some home and grew them.
tasted like mustard leaves, and grew taller and faster than normal mustard.

mustard leaves are known to keep nematodes at bay
and be good for the soil.
i love growing them in an area i know i will use later
to condition the soil.
the roots form a mass that keeps soil in place better than grass
and when they rot, they provide excellent nutrition and biomass.
worms appear to like them also.