I have a beautiful begonia plant that I would like so save and replant next year. How do I store the corms?
This article will help: https://www.gardeningknowhow.com/ornamental/flowers/begonia/wintering-begonias-overwintering-a-begonia-in-cold-climates.htm
When overwintering potted begonias, should they be trimmed back?
If you are allowing the plant to go dormant, it may be a good idea to trim it up and stick it in a dark location. Otherwise, you can allow the plant to continue its growth, treating it as a houseplant. Of course, it may suffer less stress if trimmed as well. This is up to you and dependent on the type of begonia you have. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/begonia/wintering-begonias-overwintering-a-begonia-in-cold-climates.htm
When the growing season is over, do I let the leaves die back before lifting and where's the best place to store them in winter?
Once the foliage dies, you can cut it off. Place in a dry, dark place such as your crawl space or basement. This article may also help you: https://www.gardeningknowhow.com/ornamental/flowers/begonia/wintering-begonias-overwintering-a-begonia-in-cold-climates.htm
Can you put begonias in a cool, dark place until spring?
Yes, you can. Here is more info: https://www.gardeningknowhow.com/ornamental/flowers/begonia/wintering-begonias-overwintering-a-begonia-in-cold-climates.htm
Our begonia corms got mixed up, so is there a way of telling the trailing ones from the upright?
They should be properly labeled at the time of purchase. Otherwise, you may need to wait until the plant sprouts before knowing which type of beginia you have.
I overwintered container begonias in a sunny room, and now they are very leggy. Can I cut them back, and if so how much?
They can be cut back in spring with leggy growth being trimmed back to about pot level (or 2-3 inches), which will promote new growth and a more compact form.
Every year I successfully manage to store and bring on my non-stop begonias. This year because of the exceptionally warm weather they were coming on so well I put them out in a covered cold frame. Temperatures rerturned to normal and we had an overnight frost - I thought they would be ok as they were under cover, but obviously not though as they have now gone brown and yucky! What should I do to rescue them? - I have about 100!
There isn't a whole lot you can do but this article may help: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm