We get more questions from people about why their hydrangeas aren't blooming than about almost anything else. When hydrangeas bloom there's nothing prettier. When they don't, there's almost nothing more frustrating.
The secret to success with hydrangeas is fairly straightforward:
Know what type of hydrangea you have and care for it accordingly.
There are many different types of hydrangeas (23 species), including hardy, mophead (also known as bigleaf), oakleaf, smooth, lacecap etc. and so on. It is helpful if you know what type of hydrangea you have so that you will know whether it blooms on old or new growth.
Old growth: Last year's growth (that survived the winter). In the picture, right, the old growth is brown and looks like dead twigs. You can just see the green leaves of buds breaking at the tips of some of the stems.
New growth: This year's growth. The new growth is green. You can see entire new stems sprouting at the bottom of the shrub near the ground.
A shrub that blooms on old growth will not bloom if you cut off the old growth in the winter or spring prior to bloom.
When a shrub looks like nothing but sticks (and everything else has already leafed out), the temptation is great to hack it all back to the new green leaves sprouting from near the ground. DON'T DO IT! Let the plant leaf out. Once it is nice and full and green, THEN you can assess damage.
The twig pictured, left, most likely has some winterkill on it. The buds on the top of the twig have turned brown and/or fallen off. About four inches down, the buds have started to break.
I won't prune the dead out of this shrub until is is nice and fully leafed out.
So, just because you managed to resist the urge to prune your hydrangea back to the ground when the weather warmed up doesn't mean you're in the clear.
Even if the entire plant isn't killed back to the ground during the winter, the flower buds can be damaged by late spring freezes. Bigleaf hydrangeas (mopheads), in particular, suffer from this type of damage. If the plants break dormancy during a warm spell that is followed by a cold snap, flower buds can be damaged, which will cause the plants to have fewer flowers. (Or no flowers, depending on how significant the damage is.)
Other Issues Disrupting Blooming
In addition to cold damage, improper pruning and lack of light can affect hydrangea blooms. The best time to prune any plant is right after it flowers. If you prune right after flowering, you can't cut off the flowers. Hydrangeas are no exception.
Even hydrangeas that bloom on old and new growth (such as the Let's Dance® series-Blue Jangles pictured, right) are safe to prune right after flowering. If a hydrangea blooms on new growth, it's possible that you could prune in early spring, but it is safer to prune after blooming.
Sunlight is another contributing factor in lack of bloom. Most hydrangeas grow best in partial to full shade, and need more shade the hotter the area where they're planted. Panicle hydrangeas, such as the Limelight Hydrangea, need more sun in order to flower. They don't like full hot afternoon sun, but plant them in the shade flowering will be greatly reduced.
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