I have geraniums overwintering in greenhouse (without heat). They were cut back before storage. The tips of quite a number of them have turned black and are obviously starting to rot. I have cut these bits away – is there any way to prevent this recurring, please?
Geraniums are tropical plants and can't take the cold; many people bring them indoors, cut back somewhat, and keep them in bright light indoors, or they root cuttings. Another approach is to keep them in a cool (not freezing) spot and let them go dormant. This article has more information: https://www.gardeningknowhow.com/ornamental/flowers/geranium/how-to-save-geraniums-over-the-winter.htm I suggest that with yours, take them into the warmth and light, cut off the rotted parts, and see if they will sprout any leaves for you.