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Avoid winter damage in bonsai: Rain

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 the rain, and how to avoid problems.

Why do bonsai get damaged by rain in winter?

Three factors play a role here. Suffocation of the roots, infection by certain fungus (Most notable phytoftera), and finally, damage done by ice formation.

Sickly olive, note dead roots

The first cause, suffocation. In winter our trees use very little water. However, fall and winter in some countries come with long term rain, soaking the roots of bonsai. Roots that are permanently in water may die because of a lack of oxygen. Especially in shallow, wide bonsaicpots, water drains poorly, and the risk of standing water in a shallow pot is high (Higher than in tall slender pots).

The second cause, infection by fungi. Cold wet roots are especially susceptible to infection by fungi. Especially phytoftera thrives under these conditions, and is able to infect even healthy roots. However, unhealthy roots, for instance those damaged already by prolonged exposure to overly wet conditions, are prone to be infected by a myriad of fungi, and eventually lead to something call root rot: A widespread rotting of roots that have died.

Ice crystals

Ice crystals

The third cause takes place in combination with cold. When a root ball is soaked with water, and freezes, the water becomes ice. Ice takes up to 30% more space than water. This means the root ball expands. In a good pot, the root ball has only one way to go: Up. This process of freezing and expansion can put a lot of pressure on the roots, which can easily get damaged this way.

What to do to avoid bonsai damage due to rain in Winter

Under normal circumstances, plants growing in full ground do not have to deal with these problems. Normal soil drains well enough to allow oxygen to reach the roots, giving infection by all sorts of fungi no change (Although exceptions to occur!). In order to avoid these problems te your bonsai ensure your bonsai are:
– Protected against prolonged rain and/or
– Planted in a well-draining substrate and/or
– planted in a container with very large drainage holes and/or
– You place the container tilted, allowing better drainage

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