should leylands even be considered zn 6 hardy?
OK, OK, I know some of you will roll your eyes and think "no, not another leyland thread." So, yes, let's not revisit their relative merits even where hardy. My concern is this: they've always been rated zn 6 hardy. Yesterday, I was up in Lancaster and Berks Counties in south central PA. This area is zn 6 rated. I saw many, many damaged leylands. Some severely so...a couple appeared completely killed. Problem was especially evident on trees out in open fields and high, windy spots: a few sheltered ones in towns were less damaged. These were well off the road, so it wasn't salt spray. (and besides, some right by the road were damaged, while other coniferous evergreens like Norway spruce and 'Green Giant' had not a bit of visible damage. Salt spray would have damaged them all. Only saw a couple but Japanese Cedars were also not showing any cold damage.) I wasn't able to get a picture of any both because I didn't have my camera with me.
Granted this was a severe winter, but temperatures in that area did not exceed zn 6 thresholds. In other words, it did not go below -10F...and yes, of course that can happen in zn 6, that's why its an average. It did go below 0F more than once, though. As bboy and others point out...the zone designation of truly permanent plant material (as opposed to things like bulbs and subshrubs) should account for almost any cold weather circumstance in a area: precisely because you don't want to deal with a 100 ft. long row of 20-30 ft. tall sickly looking plants. So IMHO if a once in 20 year winter produces that kind of damage in zn 6, they just aren't zn 6 hardy. Or maybe "Tree hardy" if you will...in other words, you could deal with a 6' shrub having some dieback, not a tree.
Other bit of news is perhaps because they are coning, I spotted several douglas firs. All of them just fine looking, and one right in the middle of Parkesburg, particularly nice looking. However I have never seen these in a non-botanical garden setting anywhere south of the Mason-Dixon, at least in the Piedmont...other than years ago on a steeply sloped bit of construction fill at the 500' elevation Tysons Corner. (highest point in Ffx Co., VA) Proof again of what a climate divide the Mason-Dixon is. Thus perhaps it's not surprising that most of the leylands here along the bay are fine, I've seen a few slightly singed ones. Mine were totally undamaged at 3F, including the related cross C. x ovensii which is generally considered less hardy, but has had much more disease resistant foliage. (mine were planted by the prior owners to help sell the house...I would have chosen something else and in fact removed many of them years ago. I planted the C. ovensii of course!)