Answer: Hydrangeas are popular houseplants with their showy blooms in pink or blue! There are steps you can take to prolong their current bloom time; however, you should know that these plants can be a challenge to grow as a permanent house plant, and that you'll need to take some extra measures to get them to bloom again.
Although there are many hydrangeas, the common houseplant hydrangea is Hydrangea macrophylla, which can be grown as a large shrub in temperate zones. Unfortunately, your climate is a little too cold for this plant to really thrive outdoors. Treatment as a house plant is, as you can imagine, slightly different than if you were to plant it outdoors.
The following is an excerpt from a lesser known but wonderful book on growing plants indoors, "Crockett's Indoor Garden" by the late Jim Crockett, well known for his Victory Garden.
"(House hydrangeas) are usually sold when they're about 18-24 inches tall and just coming into flower. They'll retain those commanding blooms for 6 weeks or more if they're given the right conditions: bright indirect light, and constant attention to moisture. The leaves of hydrangeas are so large and thin that they lose moisture very quickly. It's not uncommon to have to water them more than once a day, but setting the pot into a humidifying tray helps the gardener stay ahead of things. If they do begin to dry out, and the foliage wilts, plunge them, pot and all, into a pail of water for a few minutes and then set them in a cool shady spot; they usually revive in an hour or so. They do well in average household temperatures, with the nights in the high 50s and the days around 70F degrees. They shouldn't be fed while they're in blossom. Bright sun will bleach the flowers, so keep the plants in a lightly shaded spot when they're in bloom.
Hydrangea plants can be kept over for years if you have an intermediate or cool greenhouse, or live in an area where winter temperatures usually stay above zero.
If you don't have a greenhouse, if you live where the winters are fierce, and if you have the patience of Job, try this technique to see if you can bring your plant into flower a second time. After it has finished flowering, cut the plant back to two nodes from the soil level and repot in ordinary potting soil. Set it outside for the summer, keep it moist, and feed it twice a month with any standard houseplant fertilizer. In the fall, while the leaves are still on the plant, give it at least 6 weeks of night temperatures below 65F degrees. Then after the leaves drop, give it another 5 weeks of night temperatures between 35 and 45. Then set the plant into a protected cold frame-one that's been hilled up with soil and leaves and covered with a tarp to prevent repeated freezing and thawing of the plants inside-until January. Bring it inside where the nights are in the low 50s, feed it every other week, and it should blossom in about 2 months. Good luck!"
Crocket goes on further to say that cuttings can be taken in the spring from the stems of flowering plants in order to have continued new young plants. The older hydrangeas will continue to need to be repotted to a larger size.
I imagine your plant is trying to take its winter's rest, and it may not be too late to allow it to go into dormancy if you can find a cool, then cold spot for it. Have fun! I guarantee that if you get the plant to rebloom yourself, you will enjoy those blooms far more than the original! Even if it fails to rebloom, try taking some cuttings this spring and start new plants.
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