Avoid winter damage in bonsai: Reduced winter hardiness

Everybody who has bonsai knows the worry at the end of winter. A tree is a little slow in responding to warmer weather. No bud is swelling. No green line is showing. Has the bonsai died over winter? This post is part of a series of posts exploring why bonsai die in winter. In this post: The effect of reduced winter hardiness, and how to avoid problems.

Why does winter hardiness in bonsai vary over time?

Each plant has a specific temperature it can withstand before cold related damage occurs. This depends on the species, and the location where the specific individual came from. Is has been found that a lot of local variation in cold resistance occurs, probably due to selection for individuals with better cold resistance in some locations where winters are colder.

The maximum frost resistance in plants in general and bonsai in specific is now available at all times of the year. You would not be able to take an oak from the garden in summer and place it in the freezer and expect it to live. Plants need to prepare to become ready for frost. We see this in deciduous trees very clearly: As days shorten, and night time temperatures drop, plants start pulling nutrients and stored energy out of the leaves. The leaves show a magic array of colours, we call fall-colours. This is the first step in preparing for winter. As temperature keeps dropping, plants move energy and nutrient stores towards the roots, and many reduce the amount of fluids in branches and metabolism drops. At the same time the concentration of dissolved elements In the sap increases. This causes a better frost-resistance of the plant. Every time the temperature drops, the plant increases its defences. As such, peak frost-resistance is not reached until in late winter, after a number of hard freezes have occurred.

When is winter hardiness in bonsai reduced?

In order to avoid plants waking up too early they have, just like seeds, a minimum period they stay dormant. Over time the dormancy hormones in the plant reduce in concentration, and after some 3 months of cold, plants are ready for spring. This is the moment that it gets tricky: Increased temperatures at this point can cause plants to wake up from dormancy and start building sap pressure. Normally this point is reached already in January. Now, if you keep your plants in greenhouses, this is the point where a sunny day warming up the greenhouse may trigger the plants in there to reduce frost resistance and start preparing for spring. A drop in temperatures at this point may result in damage with temperatures that are a lot milder than earlier in the year. The same goes for plants outside: if temperatures in late February rise, plants may be triggered into thinking spring has arrived. A sudden onset of frost in March can then do serious harm to your bonsai.

How to avoid damage?

The best way to avoid damage is to give your plants maximum winter rest. Place your trees in a spot where they will go into deep dormancy. So, place them in a spot sheltered from sun and wind, where they will cool down to close their maximum frost resistance. If you want to move them into a more protected space indoors or in a cold frame, make sure you move them to a space that stays cold, say 5-7 Celsius, and does not warm up during the day. Note that a greenhouse that receives sunlight can warm up to over 20 degrees Celsius even though outside it is well below zero. Once spring starts, keep an eye out for late frost. For many species a bit of spring-time frost (2, 3 degrees below zero for a night) is not a problem, even if buds are moving. But if temperatures are expected to drop below -5, protect your plants.