'Hamburg' has very deep blue to purple flowers. I also like 'Enziandom' (aka 'Gentian Dome'), both for the deep color and size of the flowerheads. These will need acidic soil to develop that coloring. There are other cultivars that provide very deep, intense color as well, but these two are popular and tend to be more widely available. With the exception of some of the newer varieties that bloom on both old and new wood (and none of these produce very intense color, IMO), macrophyllas in zone 6 should receive winter protection to ensure the flower buds are not damaged. They will be root hardy but the buds - formed during the previous summer and fall - are more susceptible to winter (and spring) cold damage.

thank you all for the suggestions... When the Endless Summer craze hit a few years back, I got rid of quite a few Nikkos, thinking I would have constant bloom from ES's. I cant even get one good flush of blooms on the ES's... I've tried them in sun, part sun, shade, etc. and its basically hit and miss... some years are decent and some years nothing.. I see some beautiful, big old 5ft Nikkos (I'm presuming) around town that bloom year after year on very sturdy, upright, bushy plants. The ES's seem to be somewhat weak, "leggy" growers, often just flopping over, and not getting much larger than 3ft tall. I missed my Nikkos so much that when I stumbled upon a few- by pure luck- I bought 3 nice ones in 7 gallons. I was shocked to find them in Cleveland. I thought I would experiment w/ a few various remontant or very cold hardy cultivars and see if there was much variance between them, but I've heard that there isnt much-- just thought I would check w/ the experts on the forum here. I also saw some ASB's at the same place I found the Nikkos so, I guess I will try them as well.. Hardiness doesnt seem to be an issue, I usually mulch w/a bit of leaf compost, but I cant recall significant dieback this close to Lake Erie. I would love to find a very deep blue like the Enziadom, or some of the other older "standbys" but the problem is locating them, in that every nursery is saturated with Endless Summer. Anyone know a source for Enziandom?

Below are the current hydrangea we are growing at home, many still small. So my direct hydrangea experience comes from this base. I have a few others on my wishlist still. I will pick up unlabeled plants if highly compelling, but try to avoid it. I have seen 'Mathlida Gutges' generically labeled or mislabeled as 'Nikko Blue' at Home Depo. Our List: Hydrangea angustipetala 'MonLongShou' 'Golden Crane' Hydrangea arborescens 'Annabelle' Hydrangea arborescens 'White Dome' Hydrangea aspera 'Macrophylla' Hydrangea aspera 'Plum Passion' Hydrangea aspera 'Sargentianum' Hydrangea aspera 'Villosum' Hydrangea macrophylla ?lavendar pink Hydrangea macrophylla ?peppermint Hydrangea macrophylla ?blue/pink Hydrangea macrophylla 'Amethyst' Hydrangea macrophylla 'Angel Lace' Hydrangea macrophylla 'Ayesha' Hydrangea macrophylla 'Bigleaf White' (Monrovia) Hydrangea macrophylla 'Blaumiese' Hydrangea macrophylla 'Bloomstruck' Hydrangea macrophylla 'Blue Danube' Hydrangea macrophylla 'Bottstein' Hydrangea macrophylla 'Burning Embers' Hydrangea macrophylla 'Let's Dance Diva' Hydrangea macrophylla 'Eisvogel' Hydrangea macrophylla '(Blue) Enchantress' Hydrangea macrophylla 'Endless Summer' Hydrangea macrophylla 'Enziandom' Hydrangea macrophylla 'Fasan' Hydrangea macrophylla 'First White' Hydrangea macrophylla 'Freedom' Hydrangea macrophylla 'Gallilee' Hydrangea macrophylla 'Hamburg' Hydrangea macrophylla 'Hobella' Hydrangea macrophylla 'Homigo' (Hovaria) Hydrangea macrophylla 'Horcos' 'Firefly' Hydrangea macrophylla 'Hot Red' Hydrangea macrophylla 'Jubilee' Hydrangea macrophylla 'Kardinal' Hydrangea macrophylla 'Kluis Superba' Hydrangea macrophylla 'Lady in Red' Hydrangea macrophylla 'Lanarth White' Hydrangea macrophylla 'Leuchtfurer' Hydrangea macrophylla mariesii 'Grandiflora' 'White Wave' Hydrangea macrophylla mariesii 'Perfecta' 'Blue Wave' Hydrangea macrophylla 'Masja' Hydrangea macrophylla 'Mathilda Gutges' Hydrangea macrophylla 'Merritt's Beauty' Hydrangea macrophylla 'Merritt's Supreme' Hydrangea macrophylla 'Merveille Sanguine' 'Brunette' 'Color Fantasy' Hydrangea macrophylla 'Multiblue' Hydrangea macrophylla 'New Wine' Hydrangea macrophylla nigra Hydrangea macrophylla 'Nikko Blue' Hydrangea macrophylla 'Parzival' Hydrangea macrophylla 'Peppermint Twist' Hydrangea macrophylla 'Pia' 'Pink Elf' Hydrangea macrophylla 'Pistachio' Hydrangea macrophylla 'Revolution' Hydrangea macrophylla 'Rotdrossel' Hydrangea macrophylla 'Selina' Hydrangea macrophylla 'Sister Therese' Hydrangea macrophylla 'Snowstorm' Hydrangea macrophylla 'Sumida no Hanabi' 'Shooting Stars' Hydrangea macrophylla 'Taube' Hydrangea macrophylla 'Tivoli' Hydrangea macrophylla 'Tokyo Delight' Hydrangea macrophylla 'Twist & Shout' Hydrangea macrophylla 'Wedding Gown' Hydrangea macrophylla 'White Robe' Hydrangea macrophylla 'You and Me Forever' Hydrangea macrophylla 'You and Me Romance' Hydrangea macrophylla 'Zaunkoening' Hydrangea macrophylla 'Zebra' Hydrangea paniculata 'Bobo' Hydrangea paniculata 'Fire & Ice' 'Wim's Red' Hydrangea paniculata 'Limelight' Hydrangea paniculata 'Phantom' Hydrangea paniculata 'Pinky Winky' Hydrangea paniculata 'Strawberry Sundae' Hydrangea paniculata 'Vanilla Strawberry' Hydrangea quercifolia 'Alice' Hydrangea quercifolia 'Ruby Slippers' Hydrangea quercifolia 'Snow Queen' Hydrangea quercifolia 'Snowflake' Hydrangea serrata 'Beni Gaku' Hydrangea serrata 'Blue Billow' Hydrangea serrata 'Blue Bird'' Hydrangea serrata 'Blue Deckle' Hydrangea serrata 'Chiri-san Sue' Hydrangea serrata 'Golden Sunlight' Hydrangea serrata 'Preziosa' 'Lanarth White' 'Revolution' 'Hot Red nigra (acidic soil) black stems 'Let's Dance Diva' 'New Wine' 'Zaunkoening' 'Blue Wave' (past prime) (unknown blue/pink) w huge heads

Hydrangea macrophylla 'Enziandom', sometimes sold in this country under the name 'Gentian Dome'. It is usually referred to as the "deepest of all the blues". Not only is it a true, deep blue in my acidic soil, but it produces very large flowerheads. This was the very first hydrangea I bough for my new garden this spring - it was a "must have" now that I have a sufficiently shady property!! Mine is still just a baby and not the one shown in the linked photo but I expect it to look like that soon :-)