All the possibilities you mention are Brassicas and the plant in the picture is a Brassica which has bolted i.e. gone to flower before it grew enough leafage to harvest as a vegetable. I can't tell from the photo which of the three is might be but would lean towards the radish from what I can see from the leaves.

On a general note you have really way too many veggies in the planter, unless you are growing for cut and come again saladings.

And you have a heck of a nice view as well.

Thanks guys! Once you said it bolted, I ripped it out. It smelled radishy when I broke it up. Maybe it was a watermelon radish--if so, I guess that's one crop I won't bother with anymore.

Do you think it bolted because it was too crowded? Not enough light? Too hot? I get around 5h direct sunlight from 1pm-7pm. It's started hitting 26 degrees C.

As for the spacing... thank you for the suggestion! I have no idea what I'm doing :) I was trying to follow square foot gardening guidelines; apparently I was too aggressive.

The lettuce tastes fairly bitter, but the little chards are tasty. The peas are taking forever - finally they're showing some pods, though I planted them back in early April. I thought I'd get more food out of two 114L containers... d'oh :/

Thanks for the feedback!

Looks like arugula.

It's a radish. Red base to the stem is where the root has failed to develop. It bolted for all the reasons you thought of.