unheated garage is the perfect place to overwinter a young hydrangea...you can just stick your plants in there, need no wrappings or anything...you do need to water them...say you put the plants into the garage in mid-nov (after the plant has gone dormant, it will lose all the leaves by then), water it once good...then stick it into the garage, preferably in a very dark location...once a month, check the soil's dampness, should be dried for the top 2 inches or so...then you need to water them...do that check and water routine again a month later...the hyrangea plant will wake up from the dormancy if the temps stay above 55F for a sustain period - i have seen these sleeping beauties waking up in march...if they do, do not stick them out in the gardens..in zone 5, the late frost danger wont be over till end of may...you can keep these early wakers in the garage, with lots more light, and water them once a week or so...a weak fertilizer application will help...have fun..btw, FE is prone to powdery mildew problems, so the leaves will look pretty sick, ugly later this fall..right before they lose them all for the dormancy...

I am in a slight warmer zone (5b/6a) in NE Ohio, and I have overwintered a few potted hydrangeas (namely the macrophyllas) simply because I ran out of flower beds to plant them in. My garage is not heated but quite well insulated, so even during the coldest weather in winter, it would never drop below freezing point (even when it is in the single digits outside). Anyway, what I have noticed was that, my hydrangeas never really completely went dormant. There was still some foliage growth. Maybe I just brought them in too soon. Anyway, my point was that since they never went dormant completely, the following year, they just did not bloom so well. After a few years, I gave up overwintering these things because the end results were not that satisfying. In short, too much work for too little results. In the end, I gave them all away to friends who could plant them in the ground.

I guess someone else who really overwintered their hydrangeas properly would disagree with me on this, but this is just my own limited, personal experience on this matter...

the biggest problem i have and others have as well for overwintering in the unheated garage is early awakening...i have had plants awaken in middle of march - when there was a warm spell outside...since late frost danger really wont be over till second week of may here, i then have to care for the plants as a semi-indoor plant for eight weeks or so...that can be difficult for some people...

i do wait to have the plants gone completely dormant before i put them into the garage...when the plants are ready to sleep, they will lose all the leaves naturally...if you have to pluck them off before their beauty sleep, they are not quite ready for the deep sleep yet...

as for the next yr blooms, i'd bet you have the same problems i have seen with some of my hydrangea growing buddies...they have these great blooming specimens, took good care of them, but the plants became pot bound, so much roots in the pot that there is hardly any loose soil left...

this is my recipe..for all those 3 gallon plus plants getting from the nursury, the minimum pot size for overwintering, growing in a pot is 16 inches...a nice 16 inch cylinder or round drum like pot is perfect...should hold 7-8 (30 liters or so) gallons of soil in full capacity...then in late Feb or march, before the awakening, when it is still cold (and you dont have much to do with the outside gardens)...you take the plant out of the pot...the soil should be very dried then, and it should be a breeze to take the plant out..the whole rootball with stick..be gentle with the sticks with the tender buds...take a look...if the rootball is so big and have pretty much taken over the pot, you have to get a sharp utlity knife...be absolutely brutal about it...cut about 2 inches of roots off all around (the edges and the bottom...dont touch the top)...stick the nicely trimmed plant back to the pot, put in new soil - fresh potting mix or whatever soil mix you like...and water the plant then...and wait for another month for the soil dryness check...then the awakening...

the lack of vigor in blooms has to do with the plant being so pot bound...more than the overwintering procedure itself...i know it is a lot of work...but for some situations - say the only shady spot you have is under that nice japanese maple (but do you want to plant an endless summer there fighting for water against the maple roots - endless summer will lose)..or if you have soil that's very limey..and you really wanna have gorgeous blue blooms...or wanna have a hyrangea loaded with blooms placed in a big planter outside the front door...then you will need to grow these plants in a large pot...i have a few pot bound plants for 3 yrs now, never have any problems with blooms..these plants - endless summer and domotoi - are generally happy as long as I give them a nice root trim...their roots will fill up the pot in one season..so the trimming is an annual chore...i suppose that's the tradeoff chore vs the wrappings, the leave heaping chore for the ground grown plants/...jmho.