Wrapping up the garden for winter

Compost mulch on borders © Isabelle van Groeningen

Compost mulch on borders

As cold, damp November weather creeps into our lives, the Garden Academy enters one of its busiest phases of the year. Inside the preparations for the big annual advents’ display are well under way, outside the gardeners have started preparing the place for winter. Since moving to Berlin 10 years ago, I have learnt to respect the winters here. We’ve had the full spectrum from mild, very wet ones, more akin to what I grew up with in Belgium to long very cold ones with three months of thick snow -15° to -24°C, or worse still, weeks of -15°C without snow and bright sunny skies.

Plant Hardiness

Perhaps not the coldest, but the meanest was 2013. By the end of January I really thought we’d got off lightly. There had been no hard frosts, so that in the mild weather all the winter-flowering shrubs like Hamamelis were in full flower, snowdrops had poked up their noses spreading their little white wings. Suddenly the frost came, long and hard. Almost 6 weeks of about -15°C.

The result was devastating. When it finally warmed up, some of the damage showed straight away, some took months to appear. Shrubs coming into leaf, or at least trying to do so, then died. Even the Hamamelis, which is normally a tough plant coping with anything our winters throw at it, died shortly after forming leaves. Some roses were outright dead, others had frozen back to ground level. ‘New Dawn’ started the season as normal, turning completely chlorotic and withering as the first flowerbuds appeared in early summer. Interestingly, our David Austin roses which I had planted in the border in 2008 to test them in the German climate, all survived beautifully without any frost damage, confirming their hardiness once more. We do earth up all our roses with a small heap of well-rotted stable manure.

Old local gardeners tell me they never witnessed such an unusual winter, causing such frost damage. I do hope they are right, that we never have to witness such devastation again.


Growing up in Belgium, where we have an intermediary climate, halfway between the moist, mild British maritime weather and the cold, dry continental weather for which Berlin is know, I have realised I have lost more plants to humidity than to cold. Both in the sense of not enough water as too much water. In Belgium it was the former: Plants like lavender would quite often rot away during winter months, under damp grey skies, little wind and cold temperatures little above freezing. Likewise, in our English garden, on our heavy clay soil, certain plants like Achillea or Echinacea would struggle to survive.

Following recent mild winters, many customers lost container plants, even though it had not been that cold. In this case it was most likely the lack of water that killed them off. Few people think of checking their plants for water during the winter months, when they appear dormant and inactive.

Reviewing Plant hardiness

In recent years the Royal Horticultural Society reviewed the hardiness rating for many plants. there are now a number of plants rated as not hardy in the British Isles, which seem to survive the Berlin continental weather. Hardiness seems to be affected by so much more than just winter temperatures.  As the winter of 2013 has shown, much depends on when the frost arrives, at which stage of growth it hits the plant. This spring severe night frosts in the second half of April caught advanced leaf and flower buds, devastating apple blossoms and vines, and causing severe damage to Cornus and Viburnum for example affecting their growth and flowering throughout the season.

Frost protection of trees and shrubs

Although the large majority of woody plants are hardy, some of the evergreen ones are more susceptible to frosts. During periods of frosty sunny weather evergreens are endangered from drying out, as the leaves warm up in the sunshine, want to transpire but can’t access any water as the rest of the plant is “in the freezer”.

The last three winters have been mild, making us all bold, daring to plant plants which will not survive the next hard winter. Providing some shading is important by covering the plants with fleece or jute. These still allow light to filter through, but keeps the sunlight off, so the whole plants stays frozen. It will also provide some wind protection to stop it from drying out.

Leaf mulch © Isabelle van Groeningen

Leaf mulch

Container plants

Trees and shrubs in containers are always more endangered in cold weather. Ensure they are well watered in the autumn, and keep an eye on them in the course of the winter that they do not dry out during mild weather. Elevate tubs slightly off the ground on small feet to ensure excessive water can drain off to prevent the container from cracking. Provide shading on the eastern and southern side to keep them cold. Containers can be wrapped up, though I do think this is more likely to be good for the owner’s conscience rather than the plants. The feeling one is doing something to help makes us feel better, but if we get 6 weeks of -15°C, the whole rootball will be frozen however thick it’s been wrapped up. On the other hand, if they are wrapped beautifully, it makes them more attractive at a time of year where little is going on in the garden.


Our large olive trees at the garden academy spend the winter in the shelter of large lime trees and buildings, wrapped in fleece. So far so good, though we have not yet been through a hard winter with them.

At home we have two large Magnolia grandiflora trees in our Berlin courtyard garden. After 5 years, I hope they are well enough established and mature enough to make them more robust. I also hope they are sheltered enough, that the wind protection and warmth of surrounding buildings will help them. To help, I have put a nice thick blanket of leaves on their feet.  Likewise, our palm tree, Trachycarpus fortunei, the hardiest of all palms, has its feet cosily blanketed by leaves. In past years we have also enclosed its stem with chickenwire, stuffed with leaves. The first year we had also tied together the branches and wrapped in hessian and stuffed with straw. Subsequent winters he survived well without this not so attractive contraption, so we left it. Only the Japanese learn the art of packaging plants for the winter beautifully! Theirs always look like gift-wrapped masterpieces.

Grasses and perennials

As most perennials and grasses have their period of rest in the wintermonths and withdraw themselves into their rootsystem underground, they are far less bothered by the cold. Traditionally small conifer twigs are often used to provide protection, by laying them on top of plants, or covering the base. The problem with these is that they do not provide much wind protection, and in the springtime when plants rush into growth, it become difficult to remove these twigs without damaging delicate new shoots. We scatter a thin layer of compost on the borders in the autumn, which at the same time acts as a soil improver. Especially in shady areas, I rake the leaves onto the borders. After all the woodland plants are used to spending their winters under a thick blanket of leaves, and will push their way through in the springtime when the time is right. A thin sprinkling of compost will keep the leaves in place!

Berlin winter - Leaves held in place by compost © Isabelle van Groeningen

Leaves held in place by compost

If only somebody had a crystal ball and could tell what’s in store for us, we could take appropriate action. Like this, we just do what we can and hope for the best. If it fails, and a plant dies, we just need to see it as a good opportunity to try something new!

Conifer twigs with Crocus in spring © Isabelle van Groeningen

Conifer twigs with Crocus in spring

Isabelle Van Groeningen

3rd November 2017