I went back to where I read this and am both clearer on things and more confused at the same time. So - where I read this (my companion planting book
Carrots Love Tomatoes) actually specifies
pot marigold which it says deters tomato hormworm (among others) and it lists
marigold separately which it says deters nematodes (also among others). So, then of course I searched for the difference between these two. The
pot marigold is apparently the same as calendula; yet, although it's recommended in this book as deterring tomato hornwoom one site I read said not to plant it near vegetables because calendula is used by larvae of species such as cabbage moth.
Then there's this old t-ville thread (link below) I found where some folks (looks to me mostly by folks in the warmer states) warn against using
marigolds because they attract spider mites. The last poster in that thread seems to say that it's the
marigold that might do this (i.e. the tagetes) versus the
pot marigold (i.e. the calendula).
http://www.tomatoville.com/showthread.php?t=14002
SO - well - yes, rather confusing. I do have tagetes seeds which I planted up front by my roses. I won't plant any tagetes in with the toms. But at this point I guess I won't bother trying to find calendula either. (By the way, garlic bulbs will deter spider mites).