The thing about Agapanthus preferring pots is based around two things, one of which makes sense, the other is a myth. The first and good reason to keep them in pots is if they are tender types, particularly evergreen varieties, then the pots can be moved under shelter for the winter. The second, mythic reason is that agapanthus flower better if they are cramped at the root, and this just isn't true. It arises from the original discovery of this plant, when it happened to be found growing from a ledge in a rock face, so the assumption was made that that's what they prefer. In fact, they flower freely and do well in open soil in the ground.

In a pot in a hard winter, they are vulnerable to being frozen, so you can either plan to put the pots in a cold greenhouse or a cold frame if it gets particularly cold, or insulate the pots and use fleece over the top, again only if its really cold, below freezing for a few days. Otherwise, find a spot in the ground and plant them, now, unless you know they are not hardy ones.

That's very useful, Bamboo. I had no idea that some aren't hardy . . . thank you.

The Headbourne Hybrids are the hardiest, but with climate change and our recent mild winters, even the less than hardy ones might survive in the ground outdoors.