Care for Blooming Plant
The bigleaf hydrangea needs strong illumination but may burn if subjected to midday or afternoon sunlight. While it is in bloom, position it in bright, indirect light.
Keep its soil evenly moist, as this plant wilts quickly when allowed to dry out. If your tap water is hard, water the acid-loving hydrangea with rainwater or spring water instead to avoid raising its soil’s pH. Don’t fertilize it while it is flowering.
Grow Plant After Bloom
Once the hydrangea has finished blooming, cut it back to the bottom two leaf nodes on each stem, and repot it in a slightly larger pot filled with a mix of 1 part potting soil and 1 part peat moss. Place the pot on a cool east-facing windowsill – preferably with temperatures between 60 and 65 degrees Fahrenheit -- where the plant receives sun in the morning and shade during midday and afternoon. Alternatively, you can set it on a sunny south-facing windowsill with a sheer curtain between it and the glass to diffuse the light.
After the plant begins to grow again, fertilize it once every two weeks until the end of September with a plant food intended for acid-loving plants such as 30-10-10. Mix 1/2 teaspoon of crystals into each gallon of water or whatever amount the instructions indicate.
Move Plant Outdoors Temporarily
In late spring, move the hydrangea outdoors if possible, placing it in a shaded position first and gradually shifting it to where it again receives morning sun and afternoon shade. Pinch back the plant’s shoots as necessary to maintain a bushy specimen, but stop pinching those shoots after the beginning of July. Leave your hydrangea outdoors for as long as possible in autumn, protecting it from light frosts by covering it with a frost blanket or taking it indoors briefly.
Force Spring Blooms Again
Just before your first hard freeze, bring the plant indoors to a dark, chilly place such as an unheated garage or basement, where the temperature remains between 35 and 40 degrees. Reduce the amount you water the hydrangea and allow its soil to remain barely damp for six weeks, during which time its leaves should fall off.
After that time period, move the plant to a location where it receives bright, indirect light and temperatures around 50 degrees Fahrenheit. Resume watering the plant as usual and leave it in that position for two to three weeks.
You can then return the plant to its previous east-facing or curtained south-facing windowsill where it receives partial or filtered sunlight. The hydrangea should flower about four months after it was brought back into the light.