My hydrangeas have had a very stressful summer. I've lost a few and some are struggling to survive. I'd like to add a mild dose of fertilizer to give them a little boost but not sure if it's too late in the season.
I normally suggest not fertilizing after the July-August time period if you are using a slow-release fertilizer. Something like liquid fish or liquid seaweed would be ok since they contain small amounts of nitrogen but I would rather prep the plants for dormancy and give them a boost in Spring 2013.
You could add a little of muriate of potash if you want. It normally has no/little nitrogen and is a good source of pottasium, which is good for the roots. I would not worry about the leaves since the growing season is close to being over in cold zones like yours. Not recommended if your soil has a high chloride content.
The goal is to make sure that the plants go dormant when they are supposed to be dormant and nitrogen prevents that if you apply large quantities of it.
Do remember that plants that are stressed should not be fertilized so unless your soil is defficient in some minerals, do not worry about fertilzing again. I have had years where I did not add fertilizers at all and yet could not tell the difference; the only thing I had to do was amend the soil to keep the soil pH in check.
Fish emulsion would be ok, then.....have that. Just want to nourish/strengthen them a bit before they go dormant. I have a couple that have almost died but have little sprouts coming from the soil surface.