There isn’t a straightforward answer. Generally, unglazed pots are preferred for conifers, while glazed pots are often chosen for deciduous trees. The ideal pot size varies significantly based on the species and size of the tree. I’ll be writing a detailed blog post covering these factors in depth and will share the link here once it’s ready.

A ceramic pot is considered frost-resistant if it can withstand the freeze-thaw cycle without cracking. This frost resistance primarily depends on the pot’s material composition, firing temperature, glaze, and overall density:

1. Low Porosity: Frost-resistant pots have very low porosity, which means they absorb minimal water. This prevents water from seeping into the clay body and expanding upon freezing. High-quality, dense clay bodies, such as stoneware or porcelain, naturally have lower porosity and are better suited for frost resistance than more porous materials like earthenware.
2. High Firing Temperature: Ceramics fired at higher temperatures (often over 2200°F/1200°C) are denser and less porous, making them more frost-resistant. Stoneware and porcelain clays are typically fired at these higher temperatures, resulting in a vitrified, glass-like structure that absorbs less water.
3. Frost-Resistant Clay Bodies: Certain clay bodies are specifically formulated for outdoor and frost-resistant use, with ingredients that reduce porosity and improve durability. For example, grog (fired clay that’s ground up and mixed back into the clay) can be added to create a more robust structure.
4. Foot Drainage or Raised Design: Pots designed with feet or drainage holes allow any collected water to drain out, preventing water from pooling at the base and freezing. The raised design also reduces contact with frozen ground, which can reduce thermal stress.

5. Design of a Pot:A pot that prevents the soil from rising through the top when freezing will apply pressure to the walls. This can cause a pot to crack. How compact the roots are and soil size can have an impact on the expansion within the pot.

It is unnecessary to glaze the inside of a pot.

Bonsai pots develop a patina over time due to natural processes involving mineral deposits, organic matter, and environmental exposure. Here’s what contributes to this effect:

1. Mineral Deposits: Water used for watering bonsai trees often contains minerals like calcium and magnesium. Over time, as water evaporates from the pot’s surface, these minerals accumulate, leaving a thin, whitish layer. This mineral buildup contributes to the aged, textured appearance of the pot.
2. Soil and Fertilizer Residue: Fertilizers contain various nutrients and salts, which can also deposit on the pot's surface as they are washed through with each watering. These residues gradually build up, especially on unglazed or semi-porous surfaces, giving the pot a unique, earthy look.
3. Organic Matter: Tiny particles from the soil, roots, and decomposing leaves can become embedded in the pot’s surface, adding to the patina. Algae, moss, or lichen may also grow on the pot, encouraged by the moist environment around the bonsai. This growth gives the pot a more natural, weathered look, enhancing its aesthetic appeal.
4. Environmental Exposure: Exposure to sunlight, rain, and varying temperatures slowly alters the pot’s color and texture. This process, combined with accumulated minerals and organic matter, creates a more “lived-in” appearance that many bonsai enthusiasts appreciate, as it symbolizes age and character.

These factors together contribute to the patina, which is often viewed as a desirable feature in bonsai aesthetics, reflecting the natural aging of the pot alongside the bonsai tree itself.