I think it's cottony scale. (I've seen it called cottony cushion scale, cottony camellia scale, and cottony hydrangea scale--not sure if these are different things that can all affect hydrangea, or different names for the same thing.) I found some on my hydrangea last year but it was a limited infestation so I just pulled off those leaves. Hope someone else has a suggestion about how to treat it.

I'm going to go check my plants now!

You can always use a spray oil that smothers them. I have gotten that on holly and it worked. Do yourself a favor and get a jar sprayer for the hose, the oil is a concentrate. I used a pump garden sprayer and I got more on me than on the plants.

I did find some scale on my hydrangeas too! bummer.

And I guess my memory is not so great, because I asked a question about it on the hydrangea forum last year and got a good explanation. Here is the thread:

Here is a link that might be useful: Hydrangea Scale post

Thank you all for suggestions. After all the reading on scale I think I'll try washing today with some soap and oil. It may not work, but it'll make me feel better than watching it grow!

Interesting, I have two holly trees between two of the hydrangea plants and I've read that holly is susceptible to scale. I pray to the garden gods that they're safe.

i just removed a holly bush that was covered with the stuff. I was wondering what it was.

I have been getting it on hollies also. I sprayed one last year with the oil and it worked but the other 2 I had to respray this weekend. I also get scales on euonymus. I fina;;y got a hose end sprayer and it works much better than the garden pump srayer.