I doubt is is the mulch although it does seem quite early to be putting the mulch down, especially if you have been having the cooler and wetter than normal weather most of us have. Like planting out, mulching will be delayed this year by at least a couple of weeks.

The most likely cause of the symptoms you describe is just the weather - overly cool, overly wet, retarded root development, slowed nutrient uptake, etc.

If on the other hand you live in one of the isolated spots in the country having normal weather then I'd pull the mulch back from the plants and let the soil dry out well. Then hope that neither your mulch nor your compost contained any herbicides. Check with the sources for both.

Dave

It has been in the mid-80's here, a couple of 90 days (high desert though, nights are 44-50 - could wide temp swings do that?). I don't think wet weather is an issue, we're at 0.5" for the month, probably all we'll see.

At what point do you recommend putting the mulch down? I didn't realize it shouldn't be put down right away. I was hoping to keep the weeds at bay (which are a HUGE problem for us), but I'll pull it back and see how they respond.