Posted: 21/02/2014 at 18:45
Not a stupid question at all pariate
In theory you would look at the ultimate spread of the plant as you say, but I'd reckon most of us here probably don't do that! It also depends on when each plant flowers - you may want a succession of flowering so, for example, you might put two clematis which flower at different times quite closely together so that the same space is covered for a longer period of time. Like shrubs - or any other plant - the conditions in your garden can affect the ultimate size as well, so you need to consider that as well. What grows like mad in one part of the country could be well behaved in another. I tend to plant closer than is probably ideal, but I always reckon I can prune back a little, or just remove something if it gets too big for the area it's in. Just remember that the closer you plant, the more quickly nutrients will be taken up, so you will probably need to feed more often.
Not sure I actually helped you much there!