Question
You may refer me to someone if this doesnt work. I live in New England in zone 5, and none of you seems to have much experience in this climate.

I need to plant cover on a large raised septic field this fall. It is going in as we speak.

I really want a complete field of thyme- but when I say this to local nurseries or the excavator, I am told to simply plant grass.

I would go with ajuga/thyme, or with an annual grass even while the thyme was establishing.

I am really keen on not mowing this huge 100-foot long field. It is to accommodate a housing cluster some day, but I am building alone just now.

I am not planning to own a mower , but would compromise with renting a weed whip until the low-growing stuff took off.

Or, I see ads for dwarf fescue. But I still would prefer flowers.

If you are still reading, can you comment?  

Answer
Ajuga (Bugle Weed), is a shade- & moisture-loving annual, with over 40 varieties, that range in hardiness from a zone 3 or 4 (some sources disagree as to how cold-hardy) to a zone 9.

http://botanicalresources.1afm.com/Ajuga.html

The perennial, Thyme on the other hand, only survives to a zone 5 and loves sunshine.  A severe winter might just knock it down, depending on how protected from winter blasts your septic system area is.

http://www.farm-garden.com/herbs/thyme

You say you live in a USDA zone 5, so sounds like both could work.  I would advise against using an annual grass as a cover crop for these, as you抮e just creating a weed problem later.

Consider also Pachysandra or Plantain Lily (type of Hosta) for a shady location; Wineleaf Potontilla (P. tridentata), Snow on Mountain (Aegopodium) or Fleeceflower (Polygonum reynoutria) for a more sunny exposures - none of which would require mowing or "whipping"

Dwarf Fescue?  Yes, there are lots of varieties, but from you letter, grass would be too boring for your taste.  Hope it helps ~M