I am growing begonias from corms for the first time. I have mixed all purpose compost with sharp sand, watered them and put in propagator. How often should I water?
You'll only need to water, lightly, if the planting medium gets dry, so you should feel it with your fingers daily. If it gets to the point where medium pinched between your fingers doesn't stick together, you can spray it gently with water from a spray bottle. No way to tell you a "how often" rule, it depends on so many variables. Here's an article on growing begonias that you might find useful: https://www.gardeningknowhow.com/ornamental/flowers/begonia/growing-begonias-from-corms.htm