Can anyone advise how to successfully bring on plug trailing begonias?. I have a heated greenhouse.
Just keep potting on into bigger cell trays, you will probably see roots coming out of the bottom of the cell tray, or pop them out and have a look at the root system using a pencil.
Begonia hate being over watered, never water from the top it will increase the chance of fungal disease, water from the bottom using a tray. I find it easier to tell how dry/moist the tray is by how heavy it is.
Young plants need to be kept just on the moist side. When they are larger I tend to keep them more on the dry side. After watering they should be ok for 5-7 days at least pending on GH temperatures.
This is what I do and it works very well for me.