Browning/dead Emerald Thuja trees

I planted several 9' Thuja trees for a friend during the last week of June, 2010. I chose all trees myself from a reputable nursery, and all were in excellent condition when purchased. July and August were very hot and dry in the Alexandria, Va. area. The trees get a southwest sun exposure. I'm told the trees were watered every 10 days or so. Within three months most of the trees have browned and are now dead. What would cause these once vibrant trees to die so quickly? I see no evidence of bugs. Any suggestions?

It could have been too little water. Planting during the summer can be stressful, and if you're not careful to keep things watered enough (and sometimes even if you are careful) it'll just be too much for them and they won't make it. Watering once every 10 days during really hot dry weather when your trees are newly planted doesn't sound like nearly often enough--once they're established that might be fine, but when they're newly planted the rootball can dry out quickly.

There is also a slight downward slope on each side of the trees and hard reflecting surfaces on both sides. Those new plants were hit pretty hard and unless someone stuck their finger down into the soil regularly to make sure the water was penetrating it probably wasn't.

I'm very familiar with Thuja (Arborvitae) and other shrubs in their species. Thuja are usually very hardy shrubs and once established can be watered every 10 days or so. Since these are newly planted shrubs/trees they need more water and should be watered at least twice a week. When planting Thuja it is common practice to make sure you do not damage the feeder roots. Many avid gardeners either roll the black planter pot sideways to gently loosen the root ball. If you check the main trunk of the Thuja you may see new branches growing and they may recover. However the misshaped trees might not be the specimen plants for a long time. If you decide to replant the trees I would suggest you dig the hole about 5 inches or so larger than the pot, add sphagnum peat moss to the hole, and water thoroughly.
Just to be sure, take a piece of white paper and shake the branches on the paper to see if any small insects are present. If this is the case, there are many insecticides that can fix that.

Really sorry about the young trees, I have to agree with all the other comments re watering, I live in a zone equal to about 5 - 6 and when I plant young trees (normally autumn to Spring when cooler weather) I dig the hole 2 times wider and 3 times deeper than the pot size the plans came in, to this I add plenty leaf mould to help retain moisture and a handful of either fish / bone /blood as a slow release feed, mix the bottom soil in the planting hole to break it up as this allows the roots to grow better, (home made or shop bought compost if no leaf mould) before you pop the tree / shrub into the hole, gently tease out some of the roots that are winding their way around the root ball so they can grow outwards instead or turning round the root ball as the grow in the soil, this helps give the plant stability and gives the roots a better chance of reaching moisture when watered.
RE -Watering, for the first few months I always water EVERY evening, especially in your hot temp's, the water evaporates at the rate of naughts and before the roots have had time to drink it up, the best way to help young tree's / shrubs etc to survive the intense heat you have is at the time of planting, get a plastic juice bottle and remove the cap and cut off the bottom, insert the bottle (cap end down) into the planting hole, insert the tree, gently spread the roots you teased out and start to backfill the soil as you do this, firm down the soil so you don't have air pockets, as the hole fills with soil, try to make a shallow indentation around the hole and when it does eventually rain, this shallow indent will trap the water and slowly send it down to the roots, the inserted bottle helps direct the water you add to the bottle every evening way down to where it is required at the roots, , the best way to test IF soil is dry is as mentioned, either stick your finger into the soil or, scrape the top 2 inches away from the root area and I think in your conditions it will be dry as an old bone.

I would suggest you insert the bottle method now as there appears to be some small amount of green shoots still visible and you may be able to save one or two plants, these plants are easy to grow in the right conditions and look magnificent when a few years old upwards.
Hope this is some help and your friend can enjoy her green hedge or screening.
Good luck, WeeNel.