I'm on the south shore, zone 6b, and Hydrangea Blue Billow does well here in a reasonably protected shady spot by the house. This photo is from a few years ago; right now it's looking good with small buds.

I have no information on your other questions.

Claire

I've bought lots of stuff at Great Scapes, and have always had good luck with their stuff. No hydrangeas, though...although I've been very tempted.

BBillow has a reputation to be the MOST hardiest of all serratas. It is a reliable bloomer all over z6 and in a warmer part of z5.
Contrary, the Enziandom (IMO, the best deep-blue mophead) as well as Taube, is not reliably bud hardy in z6b and even less so in 6a. In a very sheltered spot (micro z7) it blooms for me in 3 years out of 4.

I had to google H. enziandom, based on Ego rating it as the best deep blue mophead - that's a comment that makes me pay attention. One of the first links to pop up was Great Scapes Nursery, which in turn is making me think that Matapoissett isn't so far away ...

According to http://www.greatscapesnursery.com/hydrangeas.html, H. enziandom is 'A bit sensitive to late frosts' but I have no personal experience with this variety (yet).

Enziandom vs Nikko Blue
Enziandom.

Nikko Blue

Thanks, Geo - Just what I need, more hydrangea temptations!

I DO have one very dark blue mophead, unnamed. In your photos of Enziandom, the white flower centers are quite pronounced; I don't think I've seen that before, so mine must be something else.

My Mathilda Gutges growing in acid pH soil is like that. Intense Cobalt blue with white center. It is also about a foot less tall (4.5' vs 5'). And less leggy.

Mathilda Gutges also has a nice color range, sometimes on the same bush, if you get the lime or ammonium sulphate uneven.

The flowers have white flushed centers before they open, but that intense white dot eye center never seems color.

Strong flower production, cuts well (bred for cut flower industry), nice artistic fall color in past prime flowers that dry well. We have about 80 hydrangeas of select cultivars. And this color chameleon (very easily changed with pH] is among our most favorites of the classic mopheads.

Hydrangea aspera 'Macrophylla' with H. macrophylla 'Mathilda Gutges' outside our former house.

The taller lighter blue-colored hydrangea along the driveway is Nikko Bue.

So many neighbors ask where the heck we even found such dark blue hydrangeas as the Mathilda Gutges.

Enziandom has a looser habit than Mathilda Gutges. Mathilda Gutges can droop a bit before gets fully established. Our plants were grown in bright open shade with ~2 hr direct morning sun a day.

We have recently been seeing another nice mid blue hydrangea being offered locally called 'Blue Danube' which looks more like a compact 4' plant.

Also even more compact (3', 4' in five or ten yr) and always very intensely colored is 'Bottstein', an excellent but relatively unknown older cultivar. It is better known as a UK cut flower. Always intense in color, it blooms strongly and can make a wonderful purple, or dark dusky rose color too. When grown as a blue, it can be purple flushed, unless the pH is acidic enough. I've read that when people see it in bloom, it can outsell about anything else in nurseries. Our little bush has wonderfully dense full growth.

That should be H. 'Mathilda Gutges'. (aka 'Royal Purple') One of the few cultivars that will reliably produce different colored flowers on the same plant.

Thx will correct spelling. Does make a nice purple, but narrow pH range to achieve the color..