@Nollie ..yes that's correct for the most part... they are potted up during the winter and shipped to garden centre's and suchlike for Spring and summer planting... it's best to plant these from end of May onwards in the UK, as the root systems won't have developed sufficiently in the pots to be planted before then... although there are ways and means around that..
I'd be more concerned about what perennials to buy. I have bought many over the years some great some not so great. Jacobs ladder is great for the bees but is a pain for self seeding and on the plus side I started off with one 2ltr geum which is now 30 plants strong. Can't get on with lupins for some reason and I know not strictly a perennial but wherever I grow lavender it never ever grows
One of my gripes is this. '2l pot' means nothing if you want to know how big something is. Litre is a measurement of volume. You can have one 2l pot, and if you have another pot that is half the diameter but twice the height it is still a 2l pot. But totally different. Why cant they just give us pot diameter then we will take an educated guess as to the size of the plant it contains.
Gardening. The cause of, and solution to, all of my problems.
Small pots are measured by diameter of the top, large ones by volume. Don't know why... I've just found this, which gives the size in litres AND in inches/centimetres:
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https://www.jacksonsnurseries.co.uk/plant-pot-size-guide.html