The native arborvitae, Thuja occidentalis, grows naturally in swampy areas. I don't know if there is a cultivar that will be a size and shape to suit your setting, but moisture shouldn't be an issue for it. The linked page gives some general information on T. occidentalis near the top.

Regardless, most woody plants do best if either the entire area is amended or there are no amendments added at all. Putting sand or rock in the planting holes won't help. Amending just the planting hole is not recommended since the roots may have a difficult time transitioning from the amended soil to the native soil. Also, in clay, there is a bathtub effect as you discovered.

Here is a link that might be useful: arborvitae varieties

Atlantic white cedar (Chamaecyparis thyoides) is a native to eastern VA evergreen wetland conifer you might consider adding to your screen. It would be better to hedge your bets and try a couple of different species to avoid the risks that come with planting just one type. I have a few small ones that came from a county extension tree fair and they seem to be tough as nails and grow fairly quick. I don't think you will be able to run out to a big box and pick one up, but should be able to order small ones to add to the mix. Good luck.

You'll smell root rot and see actual rot if it's present. It's just like rotten food.

When the drainage is poor the hole needs to be dug larger and you should have a "drain" hole at the bottom of the hole. Typically folks fill a drain with rock. It should go below the hardpan of earth so the water has a place to go.

Unless absolutely necessary, your soil should never be amended but there are circumstances where it should be amendment. Never use more than 1/3 amendment to your native soil. A good amendment is sold at home improvement stores and it's called something like "tree and shrub amendment." It has bark fines and peat moss, basically.

You should be using a pick axe to do your plantings. Also, when your hole is opened up, you should drive the pick axe all around the sides of the planting hole creating "pockets" for the roots to go into. To drive thru the hard pan you may need more than a pick axe. I'll provide you with a link that will help you.

Dax

Here is a link that might be useful: Preparing Hardpan For Plantings

Thanks guys for the replies... so far the replanted trees seem to be doing better.

Re the pick axe, on the replanted trees I was using a very heavy digging bar and doing what you said to do with the pick axe (making deep punctures in the bottom and sides of the holes).

I'm not 100% sure that it's a root rot issue now, because I didn't smell anything, however the bottom parts of the roots on trees I replanted were almost as wet as if I had just pulled them out of a bucket of water.

It's definitely interesting to me that parts of the trees I didn't replant seem to be healthy/alive and other parts are just dead. I'll post up some pictures later.

Also thanks for the suggestion of the Atlantic white cedar.