Cold and frost in bonsai

The cold and frost are just around the corner, and they can affect our bonsai. Worries grow if we have our bonsai located outside, especially if the cold comes suddenly and very intensely and is accompanied by frost. How can the cold and frost in bonsai? What should we do at that moment?

There are three ranges of frost and in each one different things happen to our bonsai. In this article, we are going to look at them and see what happens to our bonsai in each of these ranges.

Here we show you how to protect your bonsai from frost and what to do if your bonsai has frozen. Take your coat and cover up well, we are going to see what we can do in each case.

How to protect from the cold and frost in bonsai

We can anticipate the arrival of frost and apply some simple actions that help our bonsai to better resist these inclement weather conditions.

1. In autumn, we have a great opportunity to strengthen our bonsai by fertilizing. Remember to use fertilizers that are rich in Phosphorus and Potassium. Potassium strengthens plants and makes them more resistant to low temperatures.
2. Place tropical and/or subtropical species in a greenhouse or indoors.
3. For bonsai located outside, you can place a barrier to prevent the harmful effect of cold winds.
4. It is preferable to water in the morning since, if you water in the afternoon, the water could freeze due to the drop in temperatures that usually occurs at night.
5. Avoid pruning at the end of autumn, because by doing it at that time you do not give time for the wound to heal, and it would remain open and exposed to cold air, fungi and other pathogens.
6. If you live in a very cold climate, we recommend not transplanting in autumn. The roots will not grow due to the cold and with the first frosts they will split, losing their ability to feed the tree.
7. About your outdoor Shohin bonsai, in this blog we tell you all about this category of bonsai, you can put them in bigger boxes with soil. This way you give them a better protection against the cold.

When the frost occurs between 0º and -5º Celsius

When we reach this temperature range is when the bonsai substrate can freeze, forming a block of ice.

This only happens when the temperature range is between 0º and -5º Celsius, since the composition of the substrate (water, earth, mineral salts, etc.) means that the temperature has to be below 0ºC for the substrate to freeze. This freezing is called external freezing, that is, it does not affect the tree, only the external soil.

As a result, the water freezes, forming visible ice crystals on the surface of the growing medium, pushing the growing medium upwards and pressing on the root tissues.

This pressure is well-supported by the main roots, but the capillary hairs or absorbent roots will suffer to a lesser extent, since some of them can break due to the upward movement of the substrate. But nothing that could worry us or seriously damage our bonsai.

The roots have a certain turgidity and withstand the pressure perfectly.

What should we do with our bonsai in frosts down to -5º Celsius?

1. We must not water with hot water to thaw the substrate, since the thermal shock will negatively affect the roots.
2. We can place the bonsai in a greenhouse and wait for the ice on the substrate to melt naturally. If we do not have a greenhouse, we can create a space protected from the cold with plastic or canvas.
3. Once melted, do not water until the soil is slightly dry.
4. If they are Shohin bonsai, we can bury them in a box with substrate so that it melts faster and creates protection against future frosts.

When the frost occurs between -5º and -10º Celsius

At this temperature, not only the substrate freezes, but also the water in the cells. Frost-resistant trees increase the concentration of sugars and proteins in the cells and thus prevent tissue frostbite. It is not harmful since the affected water is the one between the cells and not the one inside them.

Pines, for example, in extreme cold conditions, move water from the cells to the intercellular spaces. We notice it because the needle stays stiff as a stick because the spaces are frozen, but the cells are not.

When the frost occurs between -15º and -40º Celsius

At such extreme temperatures, intracellular water freezes, and tissue death occurs.

In spring we will see the damage caused as the tree will lose its branches. And if the frost affected many tissues, the tree simply will not sprout.

If this freezing is rapid, that is, the sun rises and the tissues thaw quickly, the tree recovers and does not suffer visible damage, but if such low temperatures persist and the tree does not thaw quickly, the tissues die.

If the soil is frozen for a long time, the problem we may have is that the roots cannot absorb water. In this case, they will not be able to supply water to the newly emerged buds and these will dry out as they lose water through transpiration.

What to do when our bonsai has frozen?

1. Do not water the bonsai to defrost the substrate.
2. Wait for the substrate to thaw.
3. Place a protective layer of Sphagnum moss on top of the soil. It will serve as an insulating layer.
4. Placing the tree in a greenhouse will help it thaw.
5. It is not advisable to transplant into a new and dry substrate.
6. Optionally, add an invigorating agent at the first watering to help the bonsai recover.

What to do to defrost our bonsai?

As you can see, there are not many measures to take in the event of freezing. We must avoid at all costs that this happens. But if it does, remember the basics:

• Protect the bonsai in a greenhouse or prepare a space with protective plastics or tarps to help speed up thawing a bit.
• Let the bonsai soil thaw naturally.
• Do not water until the soil dries out again.
• Below -10º C it is advisable that the first watering after freezing brings a bit of invigorating water, such as Vitabonsai.
• We will wait for spring to be able to assess the damage that the bonsai may have suffered.

In conclusion, we can advise you to be aware of the weather in winter and to be alert to protect your bonsai from the cold and frost. As the solution is not always in your hands.

Bonsai care tasks during February