Hydrangea macrophylla, the pink and blue hydrangea are commonly known as Big Leaf or Mophead. Most of these bloom in July or August in either blue or pink, although a few varieties
If you have pink blooms and would like them to be blue, simply amend the soil with aluminum sulfate. Your local garden center will have aluminum sulfate.
Apply it around plants that are at least two years old and be sure to water the plants well before applying. One tablespoon of aluminum sulfate per gallon of water is recommended, and can be applied throughout the growing season. Don’t overdo it though. Too much can burn the roots and harm the plant.
If your soil is naturally acidic but you want pink hydrangea blooms, a fertilizer high in phosphorus will prevent the plant from taking up aluminum and the blooms will become pink.
Prune your macrophylla hydrangea right after they bloom. They start producing flower buds for next year as soon as they finishing blooming, so if you wait too late to trim them you’ll cut off the flower buds for next year.
PeeGee hydrangeas bloom white then turn pinkish. PeeGee’s bloom on the current years growth. Therefore you can trim them from the time they quit blooming until mid spring. PeeGee is
hardy up to zone 3 and can also be trained into tree form.
Annabelle hydrangeas have white blooms that can reach up to ten inches in diameter. They also bloom on new growth so trim them just as you would PeeGee.
Oak Leaf hydrangeas like sun but they do not like wet feet. They also bloom on old wood, and should be pruned after blooming but before they start to make new buds in August.