|1. What's the best fertilzer for them?
2. When can I prune away old canes that do not have any growth on them this season?
|As far as fertilizer, an annual top dressing of compost and/or a general purpose granular or slow release granular with an analysis of 10-10-10 or similar is fine. If you have a blue flowered hydrangea and want to enhance the blue, then use a slow release fertilizer for acid loving plants such as Hollytone. Follow the label directions for how much to use. For a more precise recommendation, run some basic soil tests to check fertility and pH and then fertilize based on the test results. Also, using an organic mulch year round will help to feed the soil slowly as it breaks down over time.
Once the plant has started to leaf out and you are certain a cane is dead, go ahead and prune it off. It is usually a good idea to wait until after the last frost to do this. (The old canes, even if dead, give some protection to the lower part of the plant.)