How to Get Potted Hydrangeas to Rebloom
Potted hydrangeas add a burst of spring color to your winter and early spring arrangements. The florist hydrangea (Hydrangea macrophylla), which grows in U.S. Department of Agriculture plant hardiness zones 5 through 11, is the variety usually grown indoors. Although you can transplant these hydrangeas outdoors after they stop blooming, it's also possible to force them into bloom a second time as a potted plant. Reblooming hydrangeas requires proper pruning and care, along with a dormant period, to produce attractive flowers.
Cut back the hydrangea after the last blooms wilt and die. Trim each stem so only the bottom two sets of leaves or leaf buds remain on the stem, using clean pruning shears.
Mix together equal parts potting soil and peat moss. Fill a pot one size larger than the old container with the soil mixture. Lift the hydrangea out of the old pot, and brush off as much of the old soil as possible. Replant it into the new container at the same depth it was growing at previously.
Set the pot in a location that receives all-day sunlight. Move the plant outdoors after all frost danger has passed in spring, and place it so the hydrangea receives full morning sun and light afternoon shade.
Water the hydrangea when the top 1 inch of soil begins to feel dry. Allow the excess water to drain from the pot. Dilute 1/2 teaspoon of 24-8-16 fertilizer in 1 gallon of water, and irrigate with the fertilizer solution every two weeks.
Pinch back the stems to the topmost leaf at four-week intervals beginning when you repot and continuing until early July. Prune the hydrangea so it only has three or four main stems if you want larger flower clusters. Remove the interior stems back to their base and leave the exterior stems to grow on.
Bring the pot indoors in fall before the first frost. Continue to water the soil as needed until the beginning of November, then remove any remaining foliage on the plant. Move the hydrangea to a dark, 35 to 40 degree Fahrenheit location and stop all watering for six weeks.
Move the hydrangea out of storage at the end of the dormancy period. Supply it with all-day sunlight, and resume regular watering and fertilization. Hydrangeas typically begin to bloom within four months after dormant storage.
Soil acidity determines flower color. For blue flowers, mix 7 teaspoons of aluminum sulfate with a quart of water, and water the hydrangea with the solution every 10 days from midsummer until storage. Make four more applications of the solution at 10-day intervals after storage. For pink flowers, mix 2/3 teaspoon of hydrated lime in a quart of water, and water the hydrangea with this solution weekly.
Hydrangeas are highly toxic if ingested, and the plants may also cause minor skin irritation.