Frost resistance in bonsai pots is an important factor for those who prefer to leave their trees outside in cool weather. Ensuring that my work will withstand frost has been a critical element as an artist who creates pottery specifically for bonsai tree use.
In the following technical article you will find insight into three factors of frost-resistant bonsai pottery: moisture absorption rate, bonsai soil and pot design.
Moisture Absorption Rate
There are different types of clay a potter can choose from to create their work including porcelain, stoneware and earthenware. These clay bodies are differentiated by many properties including clay density, porosity and workability.
When discussing frost resistance in bonsai pottery, one of the most significant elements is the amount of moisture the pot can absorb after it has been fired to full maturity. When clay absorbs water it can easily crack when frozen due to room between the clay particles which allows water to gather and expand.
My bonsai pots are all created from stoneware clay bodies which are fired to cone 6 (approximately 2250 degrees Fahrenheit) and have moisture absorption rates of 1% to 3%. This is an important difference from imported, mass produced pottery which is made with earthenware clay and can have a 10-15% moisture absorption rate or higher.
Testing for Moisture Absorption Rates
Moisture absorption rates can be determined in several different ways however I prefer the boiling method to test discs of new clay bodies which have been through the full firing process. These discs are weighed when dry, boiled for two hours, dried quickly with a towel and reweighed.
The moisture rate is then calculated by determining the saturated weight minus the dry weight and that result is then divided by the dry weight. A simple example would be:
(110 grams when saturated - 100 grams when dry) = 10 / 100 grams when dry = 0.1
The result of this calculation would show a 10% absorption rate.
Another significant cause of pot breakage is overly saturated soil in the bonsai pot during cold weather. When the bonsai substrate is wet and begins to freeze, the substrate will expand and the pottery will crack from internal pressure.
To prevent broken containers, it is essential to use a professional quality bonsai soil mix that promotes excellent drainage. A pot that contains completely frozen substrate is likely to crack regardless of the type of clay it was made from.
Bonsai Pot Design
Experienced bonsai artists will tell you that the best bonsai potters are those who actually use their own containers. As someone who found their way into creating bonsai pottery after being dissatisfied with the limited options available for my trees in Canada, I quickly learned what works well for my trees in pot design and what doesn't.
One of the last factors of frost resistance to consider in bonsai pottery is the overall shape of the container. It is essential that a bonsai pot tapers and angles outwards from bottom to top in order to truly resist frost. This subtle design element can allow freezing water to expand upwards which limits pressure on the inside of the pot.
If a pot were to taper inwards at the top it would essentially trap expanding soil during freezing which would create significant internal pressure and likely result in container breakage.
While technical in nature, I hope this article helped to explain the significant factors involved in frost-resistant bonsai pottery. As always, if you have any additional thoughts or questions, please contact me.