What kills bonsai in Winter?

Springtime is exciting for bonsai growers.

Spring is arguably the most stressful time for people keeping bonsai. The trees need repotting, wires are placed –if not done during winter- stands need cleaning and the watering setup needs to be dusted and brought outside again. It is however, also a time of excitement. The trees, some of which may have become friends over multiple decades of good care, are waking up after their half year slumber. That is at least, what is supposed to happen. In some cases however, after bringing the tree outside, exposing the tree to the first rays of spring sun.. The tree refuses to show any swelling of the buds. Repotting stays out. The tree ends up on the compost heap. Most have had it happen. Bonsai that do not survive the winter. The big question now is.. What kills bonsai in winter, and how do we prevent it from taking place.

Causes of bonsai / tree mortality in winter

There are of course many things that can kill a bonsai in winter, from rodents ring-barking your tree to wind blowing over your trees without you noticing it. However, some of them are so common that they warrant some descriptions.

Temperature

All trees have different levels of frost resistance. Real tropical species may die after a single night of frost, or at least suffer serious damage. Other species will happily spend a week at -20C without suffering. It is important to realize that the normal ranges for an individual species in the wild does give an indication of frost resistance, but no guarantee: Often the above-ground part of a plant might deal with the big dips in temperature, but the roots cannot. This is simply because the ground never cools down as much as the air does, and the roots are there for protected by the biggest drops in temperature. It is safe to say that not many plants like their roots to reach temperatures below -5 to -10 Celsius. (Continue reading)

Sun & Wind dessication

Perhaps not the main killer of trees in winter, it certainly is one of the main reasons to loose finer branching on your tree. Wind and sun during cold conditions. Even though it may be -5c outside, if your bonsai is exposed to wind and sun, it will evaporate water. Actually, the aboveground parts of your tree may completely defrost. It loses water, which cannot be brought up from the roots, causing the branches to dry out, and die off. (Continue reading)

Moisture

In winter, trees use very little water. Unfortunately, in fall and winter often come with lots of precipitation. Long-term constant wet conditions weakens the roots, making them susceptible to CHECK FOR FUNGUS NAME, and in general, die-back of smaller roots. This in itself opens up the tree for further fungus infection leading to so-called root-rot. This is however the result of roots dying, not the main cause. (Continue reading)

Insects

Bonsai pots are not devoid of life. In the pot, a myrad of insects find their home, and most of the time without causing a lot of trouble. However, some larvae do cause problems. In particularly the bugs of certain beetles eat roots. In the small pots, a dozen or so beetle larvae may eat away all smaller roots. In spring this results in a bonsai that is just not really waking up.Next to this, some insects, most notably scale aphids, like to keep sucking the sugary juices of our bonsai in winter. If not detected in time, these can over the course of a winter suck your tree dry, with complete branches dying off. (Continue reading)

Reduced winter hardiness

As winter progresses, slowly the trees start waking from deep sleep. A short period of warming causes the trees to prepare for spring. Juice flow increases. Buds start swelling. And then winter comes back. Over the years, every few years late-frost surprises bonsai fans. A spring frost may kill trees that have been around, some of which can easily withstand the temperature itself. But because the tree was not dormant anymore, just a few degree of frost are enough to kill. (Continue reading)

These were a few common causes of trees failing to wake up in spring. More short specific articles about each problem are to follow soon, and will be linked from this article.

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