It all depends on what you want. if you would like a large swathe of the same plant then plant altogether or if you want it dotted around the garden (cottage garden style)then plant them singly. Or you could plant two groups in difeerent places. Hope this helps.Lemondog.

Please be aware that this plant can be invasive - it spreads like fury in my garden, and I have to dig some up every year, so don't plant it where you won't want it to overwhelm other precious plants!

I agree with both comments but Spritz's warning is very important if you dont want to have to dig it up and pick through other plants.