A discussion about strewing herbs has been bouncing around the SCA-Herbalism Yahoo!Group.  This nice summary went up today.

Here in Ansteorra, I have found lavender to be particularly
efficacious in repelling mosquitoes and no-see-ums. 

Pennyroyal* is a bit sharp on the nose, but is an excellent repellant
(mostly fleas, but chiggers seem to dislike it as well).  Blended with
lavender, it’s not so bad, and the two work well together,
fragrance-wise. Tansy is as sharp-scented as pennyroyal, and also
helps repel insects like mosquitoes.  I haven’t found an herb yet that
repels flies, though.

Mints will repel most creepy-crawly bugs, but it attracts southern ants.

Southernwood has a lovely fragrance, as does woodruff.  Southernwood
repels stinkbugs – a problem here in some areas of Ansteorra.

Rosemary is pleasant and long lasting – and can be burned in the camp
fire to release the last of the fragrance when you replace the worn
with fresh.

Chamomile gives a light, appley fragrance and lasts well as a strewing
herb.

Thyme and sage also make for lovely strewing herbs.

Rose petals are pretty and briefly pleasant.  I save these two for
special occassions or the visit of favorite people.

Meadowsweet was a common strewing herb, according to John Gerard’s
Herbal. 

We generally lay tarps down, top them with strewing herbs, then cover
the herbs with thin carpets or painted canvas so the fragrance comes
through without splinters embedding themselves in toes and other
tender body parts.

We change the herbs out twice a day:  morning and evening.  Some burn
well, and others not so well.  Sage and rosemary burn the best of all.
The rest are best stuffed into bags and taken home for compost.

Kendweramyr and Itzl of the Ears

References: 
John Gerard’s Herball Republished by Dover Books – ISBN  978-0486231471
History of Perfume by Frances Kennett (George G Harrap & Co, London)
ISBN: 0 245 521356

Well, that tells me how many plants to put into the garden.  LOTS!

*UPDATE – See comments for Theadora’s important safety tip about pennyroyal – thanks, T.  I will think of it together with cat litter changing – NOT FOR PREGNANT WOMEN.