Mark, I'm east of you on LI. I planted a dozen and a half of them last
year around May as well... No problems but I did anticipate a few
losses since these were Home Depot plants.
I used plenty of peat moss, compost and the native soil to make an
ideal planting mixture to compensate for the fact that the potted
Arborvites had what I felt very small rootballs.
I placed a soaker hose (Well, 4 as I had 18 plants) weaved in and out
around each plant on every day for 2 hours. Approx 3" of mulch topped
it all off. Didn't lose a single plant, yet...
I have seen people plant rows and fences and screens with 20, 50 even
100 Arborvite and suddenly lose a couple after a year or 2. If you got
yours at the big orange box, be sure to choose the ones that are just
coming off the delivery truck.