Preparing Your Bonsai Trees For Winter

The inevitable onset of winter, complete with freezing temperatures and prolonged periods of wet weather can without proper preparation cause a range of care issues for the bonsai enthusiast. Lets have a think about preparing your bonsai trees for winter.

I’ve written some notes here to help beginners in bonsai get their trees through the first couple of winters.

Know which species of bonsai tree you have

Some species of bonsai tree are hardier than others. Winter hardiness often depends on whereabouts in the world your tree is native to.

Trees from temperate parts of the world such as ours have evolved to cope with the cold weather.. Photosynthesis ceases in deciduous trees and slows to a trickle in evergreen species. Tropical and sub-tropical trees can be more tender to the cold.

Proper fertilisation help to protect bonsai trees in winter

Proper preparation for winter starts during the warmer months. Trees produce sugars and carbohydrates which they use to stop themselves from freezing.  It is important therefore to make sure that your trees are fed throughout the growing season in order to give them everything they need to be ready for the cold.

Cold winter weather can dehydrate bonsai trees

Bonsai trees can be very hardy and able to cope with cold conditions. Generally it isn’t the cold which harms the tree. More damage is often done through dehydration. If the soil around the roots of a bonsai freezes, the roots cease to function. They stop drawing in water. The top of the tree however continues to release water. It is for this reason that prolonged spells of freezing weather cause harm. This is can be made worse by freezing weather coupled with strong winds  or strong winter sun as it dries the trees out even faster. A lot of the potential harm can therefore be removed simply by placing the trees into a more sheltered position out of the wind or sun.

Size Matters

Larger bonsai trees are more robust generally and this certainly applies to the cold. Smaller bonsai trees, perhaps small enough to pick up in one hand, will need protecting earlier than large, heavy trees.

The wet can be as bad as the cold

Some periods of winter are not especially cold. Instead, they are full of heavy rain. This can be just as bad for the trees, as it leads to their roots rotting. The can be combatted by potting your trees into good quality, free-draining bonsai soil. Compost-rich soil mixtures are not helpful, so when you repot your bonsai, this can be removed.

You might also move your bonsai out of the rain, even though the weather isn’t especially cold. This means that you can control how much water they get.

Don’t rush hardy bonsai trees into winter protection

It is quite common for beginners in bonsai to want to over-protect bonsai by bringing then into the house or into a heated conservatory. This may result in the trees not actually entering the dormancy period – It can encourage them to keep growing. If this happens the trees may carry out growing throughout the winter. This distortion of the natural cycle does not help and can harm the tree. The dormancy period is essential to the health of trees from a temperate climate. Trees from tropical parts of the world do not need this dormancy period as they do not experience it in their natural environments. They can therefore be allowed more heat.

A little cold is also good for killing off pests. It can also help keep other tree infections at bay.

When does the cold kill bonsai trees?

Generally speaking most trees from temperate parts of the world can be left unprotected until the weather gets down to -10. When it gets colder than this, protection is required.

Tropical trees will be damaged by even light frosts and should be moved before the temperatures really start dropping,

It’s important to research the species of bonsai that you have to find out the degree of cold that they can withstand – the problem is that you will get conflicting answers!

The duration of the cold spells should also be considered

As I mentioned above, it isn’t always the low temperature that is the problem. It can be more the duration of the cold spell which is the issue.

For example, say that minus 10 will damage Japanese Maples. It’s clear then that exposing them to minus 12 is not a good idea, However, if it is minus 5 for a fortnight, then this is also damaging, as dehydration causes harm.

Winter quarters for your bonsai trees

A greenhouse is probably the best solution for protecting trees from the worst of the weather. A cold frame is also very useful. Easy to build too!

An unheated outbuilding can be used to protect deciduous trees. Since deciduous trees don’t need  light when not in leaf, they can be stored in shed, outhouses or garages when the weather gets cold.

Bonsai growers often construct benches in order to display their trees to best effect in the garden. Dropping the trees under the benches and then covering the bench top and sides with bubble wrap or something similar can be enough protection. This is great for stopping the wind getting to them.

Another solution is to take the trees out of their bonsai pots and plant them out into the garden. This can provide the protection from the cold that the roots of the trees require. When the spring comes they can simply be potted back into the same pots.

As with planting bonsai out in the garden over winter, you can take the trees out of their pots and plant them into larger wooden boxes filled with garden soil. I would find this too much work!

The experience of real-world bonsai growers

As part of running the business, I often come into contact with very experienced bonsai growers.

Some of them move all their trees into greenhouses, while others don’t move them at all.

For me, peace of mind also comes into play. Its far easier to relax on a winters night knowing that your bonsai collection has been prepared for winter. Its saves worry on your part.

Possible Timetable to think about:

Autumn is here: Move your tropical species before the first frost. this includes Ficus, Serissa, Sageretia, Ligustrum and Carmona,

Light frosts: Move semi-hardy bonsai such as Azaleas, Trident Maples, Crepe Myrtles, Buddhist Pines and Japanese Holly. Don’t panic about the others.

Heavy frosts but ones where it thaws during the day: Don’t panic! Consider moving smaller bonsai into winter quarters.

Extensive cold, where the temperature doesn’t rise above freezing for days on end: Move more trees.

Deep Cold, minus 10 degrees: Panic and move as much as possible! I tend to get a bit twitchy when weather forecasters mention minus 7 degrees.