I was doing a Zoom presentation for a group last week, and one of the questions from an attendee was about his blue hydrangea. “It never flowers,” he said, “no matter what we do.” Since this group was from all over the country, I asked him where he lived. It turned out that he was in Illinois, so I knew why his blue hydrangea wasn’t flowering: it gets too cold in the winter, and the flower buds get zapped. This question reminded me that people all over the world wonder how they can grow blue hydrangeas. How to get them to flower, keep them blue and not pink, or prevent the flowers from browning quickly in the summertime. So here are the basics of how to grow these lovely shrubs.
- The blue mophead hydrangeas are all varieties of Hydrangea macrophylla. This is important because of how and when their flower buds are produced. The photo of the shrubs above was taken in late August, and although the flowers are what capture our attention, know that the buds for next year’s flowers are already on these plants. Knowing that the flowers for next summer are formed in the late summer the year before is the key to knowing why your plant does or does not bloom. Flowering depends on those buds making it through the winter, and not getting cut down by pruning at any time of the year.
- If the winter temperatures fall much below 15° f. those buds are likely to be killed. Some varieties are more prone to this bud death than others. Some Hydrangea macrophylla varieties have buds that swell and almost start to break dormancy in a warm fall, and these are more likely to get killed by winter temperatures. If both day and night-time temperatures fall into the single digits, especially for days at a time, the buds are more likely to die. And if your winter temperatures fall below zero degrees Fahrenheit those buds will be zapped for sure. And no, protection by wrapping or mulching does not work. Sorry. If you live in a part of the country where your winter temperatures are so cold, see below for a way you can still grow these plants.
Another reason your hydrangea might not flower is if someone cuts the canes back in the fall or spring. Since the flower buds for next year are formed the summer before, you can understand that “neatening the plant up” in the fall or spring will remove those flower buds. The general rule of thumb for these plants is to wait until late May and at that point remove any canes that do not have green buds or leaves on them. If all the canes have died to the ground, you can remove them but that plant will not flower in the coming summer. If the buds don’t freeze in the winter, but the canes are “neatened up” in the fall, you’ll find a dome of green on the top of the plants and all the flowers will be on the bottom. See the photo below:
- To keep hydrangea flowers blue the plants must be growing in acidic soil. It’s the aluminum in the soil that turns the flowers blue, but that can only be absorbed in acidic conditions. In some areas, like on Cape Cod, the soil is naturally acidic and if you do nothing your flowers will be blue. But in other parts of the country the soil is alkaline, and so sulfur should be applied around the plants in order to turn the flowers from pink to blue. You should apply sulfur according to directions in spring and fall, and be sure that it’s spread well beyond the drip line around the plants.
- Blue hydrangea flowers last longest when they are protected from wilting in hot weather or in full sun. Even when well watered the blooms can wilt when temperatures are above 80 degrees or when the plants are growing where the noontime and afternoon sun hits them. So for the longest lasting flowers, grow these shrubs where they are in sun in the early morning but in the shade from ten AM on. If you live in a region where the summer temperatures are routinely above 90°, this is not the plant for you.
- If your winter temperatures prevent your plants from flowering, grow these in pots! There are many shorter varieties such as Blue Jangles that do well in containers. Pull them into an unheated garage, bulkhead or cold basement once they have lost their leaves in the fall but before the temperatures have fallen below 30° for any length of time. Water them if the soil is dry, and don’t worry if these plants start to break dormancy in the early spring. Pull the pots outside once all danger of frost is past, fertilize, and place the pots in a shady location.
- So to wrap up: for the best blue hydrangea blooms, grow where the winter temperatures stay above 15° F, never cut them back unless the canes are empty and dead in late May, grow them in acid soil and be sure the flowers are protected from hot, mid-day sun.