This site lists Thuja plicata as having leaf litter that is slightly allelopathic but no other species are mentioned.
http://plone.urbanforestrysouth.org/Resources/Library/Citation.2004-07-15.3616/file_name/

However, this one states that there is no documented evidence of this occurring:
http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Allelopathic%20wood%20chips.pdf

I canÂt find anything more specific but would suspect that a combination of shade, and water competition from those abundant, strong, fine roots near the surface is more likely the problem.

Sorry, but I donÂt have any plant suggestions for you.

Thanks Rosa-
I think you're right. I find it very frustrating though, this was to be my beautiful border garden with the conifers acting as a landscape foil. As it is, it's my least attractive bed. I think I need to find a plant that also has competitive roots, or else one that can suffer through dry conditions.

Dry shade is sure a tough one. Maybe others in your area have some experience and some good recommendations....
You might try posting for plant ideas on the native plants, perennials and shade gardens forum.
Good luck!!

I do believe that arborvitae litter is not conducive to other plants in some way. I have found that salvia greggi seems to grow happily with it for some reason.

Brenda-
Salvia g. sounds perfect except for it's hardiness. Do you know of a plant with similar tolerances but also cold hardy (I'm in zone3/4)?

Maybe if I do annual salvias?

Maybe an agastache? I love Agastache rupestris, easy from seed, and it comes from the Rocky Mountains, so should take cold.

Great idea!
I don't know if anyone else gets frustrated with the seasons - I wish I could try many plants in one season for this spot, so I'd be sure to have success.

I just checked in my High Country Gardens catalogue. They have A. rupestris - it is very pretty. I love their stuff, but have been hesitant to buy many of their plants because of the difference in rainfall and humidity between Wisconsin and New Mexico.

I do love Agastache. I think I will give it a go. Another problem I have is my garden gets too crowded. I think I need to dig up more lawn this year! (so far I have New Jersey Tea, Baptista australis, and now A. ruprestris slated for that spot)

USDA actaully lists arborvitae as not-allelopathic, but their database is not perfect. Too bad I'm out of the A.rupestris seeds for this year. I have some other Agastaches, but doubt they're hardy for you. Keep me in mind for trading next season!

Thanks Brenda for the info. My soil is the opposite of yours - very sandy which in combination with competition with arborvitae roots might be the problem.

It'd be fun to trade seeds with you. I generally don't collect seeds from my plants - I usually buy all my seeds. But if you're interested, I have alpine plants, native (mostly shade) plants, and a few sun loving natives. (I'm guessing only seeds exchanges are possible) Unfortunately our yard is small - so I'm limited. So next season - I'll look you up!