Hydrangeas do not need much fertilizers because they are not as hungry as roses are so on a newly planted hydrangea, add about 1/2 cup or 1 cup of manure, compost or cottonseed meal once -in June/July- if you live in the northern half of the country or twice -in May & July- if you live the southern half. You can also use a general-purpose slow-release 10-10-10 chemical fertilizer. Sprinkle weak fertilizers like coffee grounds, liquid seaweed or liquid fish during the growing season but stop all fertilizers by July-August so the plant goes dormant in time for winter.

Amendments, such as those that acidify the soil, can be added according to the frequency stated in the product label.

See the link below for more information.

Here is a link that might be useful: Fertilizing Hydrangeas

I use Hollytone in early spring and a generous ring of composted cow manure around the root line. I also apply aluminum sulfate mixed with water at that time to intense color of the blue plants. I keep a big empty Folgers can by my coffee pot and dump my coffee grounds in that all week- I sprinkle the grounds around them all year. If I am feeling industrious, I try to fertilize them again mid-summer again w/ hollytone and manure.

lauriewood: How do you know where the rootline is? Or are you kind of guesstimating?

I guesstimate. I have transplanted so many hydrangeas I guess I just kind of know. Generally it is around the size of the diameter of the canopy- maybe a little larger. Precision doesn't really matter anyhoo. Hydrangeas love organic amendments. There is no science. I just try to not put them right up against the stem. I see you are in a trickier zone (5) than I am (8)- so you may want to call your local gardening extension to verify. Mehearty said something about them just going green- that is not an issue here in South Carolina and I would hate to steer you wrong! However, my goal is to feed the earthworms as much as the hydrangeas.

I just add some heaps of composted manure around my hydranges in early spring. They really seem to like that. I try to keep all other fertilizers away from them, because I don't want them to just get green.

Oh! I almost forgot. Every year in fall a ton of oak leaves blow into my beds. I'll clean out most of them in the spring, but I like to leave some gnarly looking ones under the mulch. Leaf mold is nice for the soil, and my hydrangeas seem to like it. In fact, I've been building up the back of a bed with leaf piles, and the plants near that pile grow like gangbusters. When I plant hydrangeas, I toss a bunch of old oak leaves in the hole.