Cactus rot

[greenknight]

<snip> As Elie says plants can rot from the top down as well as from the roots up, though we usually loose most cacti from rotting from the base and over watering or the soil remaining wet too long, though of course with cold sensitive plants it us usually the stem above ground killed by the cold, since the soil itself provides some insulation from the cold.

Very true, but it should be added that the roots are the least cold-hardy part of the plant - many otherwise frost-hardy plants will succumb if their roots freeze. Thus cold-hardy plants in pots are more vulnerable to a hard freeze than they would be if growing in the ground.