As someone who is a die hard aquarium person and worked with various aquarium heaters for the last 20+ years I DO NOT recommend them for outdoor use, especially in a watering can. They are wasteful of energy, if they short they can over heat and melt things, they would need to run really hard to keep up with the cold, most can only go down to 65 degrees if adjustable or are set at 78 degrees if not adjustable. If the water level goes down enough that the glass and heating element is exposed, they can crack or break (most are glass). They also have a regular household cord. Plus they are almost the same price as a stock tank deicer!