I provide compost to our many hydrangeas but I am wondering if they would benefit getting an additional 'chemical' fertilizer. If so what do you use and when during the season do you apply it?
Benefit from 'extra' fertilizer?
Hydrangeas do not react to "extra" fertilizer like roses do. They react more like how azaleas would react: ho hum. The best results are obtained using a slow-release general purpose fertilizer like organic compost, cottonseed meal or composted manure (!!) in Spring. That is it. Then throughout the growing season, you can add coffee grounds, liquid seaweed or liquid fish. Then stop all forms of fertilizers at the end of June so the plant will not get too much nitrogen; this way it will be able to go dormant at its appropriate time. A 1/2 cup to 1 cup of those items is enough for a shrub about the size of a newly purchase hydrangea. You can also use a general purpose, slow-release chemical fertilizer like Osmocote; look for a NPK Ratio of 10-10-10 but add the amounts specified on the label in Spring. By the way, nothing bad will occur if you forget and skip fertilizing for a whole year (assuming your soil has no mineral defficiencies).