What's the science behind why some plants can grow in extremely cold temperatures and are there any that can thrive in -20°C?

I completely understand your fascination with plants that can withstand extreme cold temperatures! The science behind it lies in the plant's ability to adapt to their environment, which is often driven by the need to survive and reproduce. Some plants have evolved unique physiological and biochemical mechanisms that allow them to maintain metabolic processes, even when the temperature drops.

One key factor is the production of specialized proteins called antifreeze proteins, which prevent the formation of ice crystals in the plant's cells. This is especially important for plants that live in areas with frequent subzero temperatures. The Satsuki azalea and winter aconite, which you mentioned, are indeed excellent examples of such plants. In addition to antifreeze proteins, they have also developed strategies to conserve water and maintain a stable cellular environment, even in the face of freezing temperatures.

As for plants that can thrive in temperatures as low as -20°C, there are a few options you might consider. The wintergreen (Gaultheria procumbens) and the snowberry (Symphoricarpos albus) are two examples that can tolerate such low temperatures. However, keep in mind that even hardy plants may still require protection from extreme winds and desiccation. Make sure to provide adequate mulching and watering during the winter months to help them survive.

I hope this information helps you in your gardening endeavors! Remember to research specific care instructions for each plant you're interested in growing, as some may have unique requirements. Good luck, and happy gardening!