I'm not a perfect companion planter. The guides will tell you what to pair with what, and it will also discuss the plants that shouldn't be together. They don't really mention horseradish, lettuce, marigolds, bee balm and carrots. Still, I had some room, and I knew I wanted to plant more carrots, so in they went.
I try to make use of space in the garden throughout the summer. No beds go unplanted. If I think a bed needs a break, I'll plant it with a cover crop. The lettuce will probably bolt in a few weeks, and I will have to rip it out. By that point, it will allow the carrots to grow bigger. The bed might get overgrown by the end of summer, but I bet everything will live there happily.
The carrots are covered with burlap until they germinate. I learned this fantastic tip from Chiot's Run. I would usually water them afterwards, but I knew we would be getting a big rainstorm in the afternoon (and we did).
(I also planted a few with the asparagus.)