I have a 3x6ft raised bed in my front yard, and also a community garden plot. I planted some chard (Fordhook Giant and Charbell, which is a magenta variety) and Florida Broadleaf mustard in both places. The ones at the community garden are doing great, but the ones at home are clearly stressed—the leaves are pale, the mustard is bolting without the leaves ever growing bigger than like 4" (they're well over a foot at the community garden), and the Charbell has some minor lesions that might be cercospora or some other disease, not sure. 

I have a couple theories as to the problem:

1. I covered the bed at home with a clear plastic low tunnel over the winter. Around when the mustard was reaching 2-3" tall it was clearly getting too hot to leave that cover on, but I'm still working on fencing to replace it over the summer, and if I leave it uncovered overnight critters will eat everything. So I've been putting the plastic on overnight and uncovering it during the day, but I got to it a bit late some days and things got pretty hot underneath. How likely is it that this is just a case of, once the plants experience heat stress, they're going to bolt no matter what (even if I keep temperatures better controlled after that)? But the chard isn't bolting, so I would expect it to recover better from heat stress. And the Fordhook Giant is doing basically fine—not surprising that it's more robust, that's why I grow it, but still...

2. The soil mix in the bed at home is mostly 50/50 compost/topsoil mix, with about 1/4 peat moss and a bit of coarse vermiculite and perlite. I had just heard a number of things about not wanting to overdo it on organic matter when building raised beds just before I put this one together, thus the relatively compost-"lean" mixture, but is it at all plausible that I've undershot the mark instead and the plants would benefit from a little fertilizer?

Any thoughts which/what combination of these it is, or what else might be going on?

In the chard photo, the dark-green leaves to the right are turnips—those are doing great despite wilting pretty significantly on the hot days. Under the row cover to the left are beets—too soon to tell how they're doing, though I'm concerned they're showing the same pale leaves as the chard. There's also some kale in the bed which is doing okay, and some mache greens which are also bolting, but they're definitely cold-lovers so I'm going to just save the seed and try again in the fall.