The large blooms of Hydrangea macrophylla, and other Hydrangeas such as H. quercifolia, H. paniculata and H. arborescens, can be a great addition to a garden. But it is important to understand how to care for your plants. Sub-zero temperatures can severely affect how your hydrangea look. Fortunately, most damage will be superficial, and you can help your hydrangea to recover from frost damage.
Here in the UK, of course, winter freezes can be a regular occurrence – even in the most southern reaches of England, a few frosts are par for the course. But the problems to do not tend to arise in the depths of winter, but rather when new grown forms in the early spring. Hydrangea eagerly put on new growth at the first signs of spring sunshine. But a late frost can damage tender new growth.
Why Does Frost Damage Hydrangea?
A few nights below zero are unlikely to kill the whole plant, or do any serious long-term damage. Hydrangeas (H. macrophylla and H. quercifolia) that bloom on old wood are root hardy down to around -28 degrees C. New-wood bloomers are even more cold hardy.
However, frost can damage new, more tender growth on the plants. It damages the new growth of hydrangeas because the air temperature is cold enough to freeze the water inside plant cells. When the water in the cells freezes, the ice expands, damaging the cell walls and killing parts of the plants. Tender fresh growth is more susceptible to this sort of damage.
Preventing Frost Damage in the First Place
Prevention is generally better than cure. If you are placing a new hydrangea in your garden, where you choose to site it will have a bearing on whether or not it will be frost damaged. For example, if you have an exposed site, placing your hydrangea in a sheltered spot (in the lee of your polytunnel, for example) can prevent frost damage. A windbreak hedge may also help. One option to consider is planting your hydrangea against a south facing wall. You should also take care not to place your hydrangea in a frost pocket.
Identifying Frost Damage
Foliage that has been frost damaged will turn purple/reddish. It may wilt or collapse. In a freeze, the stems, buds and foliage can turn black and dry looking. If you have experienced a frost or unexpected late freeze after your hydrangea have put on fresh new growth, it should be obvious that this is the problem.
Assessing the Damage
Fortunately, even if a large proportion of the new growth has been affected, you can still usually help hydrangea recover from frost damage. You can work out how much of the new growth has been killed by scraping your fingernail across damaged stems. Scrape back the bark with a fingernail at various locations. Move down from the tip towards the base of the plant until you find a location where the stem underneath the bark is still green (and living). This allows you to ascertain the extent of the dead/ damaged material.
Frost damaged hydrangea can look rather unsightly. You may be tempted to act straight away. But it is important not to act too quickly. Do not be tempted to prune away the dead or damaged material until all risk of frost has definitely passed. If you prune your plants too quickly, and another freeze strikes, you risk exposing new parts of the plant to damage. Pruning will also sap the plant’s energy and may make it more difficult for the plant to recover.
Wait until warmer weather arrives, then prune back to the next healthy bud or set of leaves. Hydrangea that bloom on old wood may still bloom at the bottom of the plant. New-wood bloomers can be pruned as usual to a few inches off the ground and should flower just as they would have without the frost.
To help hydrangea recover, add a spring mulch of organic matter to the soil around the plants, and consider watering with a suitable organic fertiliser.
Have your hydrangea been damaged by frost? Have you successfully helped your plants to make a full recovery? Share your tips or suggestions in the comments below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.