Stuck with a problem, I built a 6ft high trellis fence 'windbreak'. Essentially it's 3 6ftx6ft panels attached to posts. On each I have planted clematis. These are plants a number of years old, and having been shifted from cramped large pots, have gone beserk! They are all type 2, and I stuck some wire fencing over the trellis to give them something to grip to. The Nelly Mosa and old man's beard dahlia type, have been quite good staying quite compact and I'm gradually training them to cover the whole panel. In the middle I have 'The President', again a plant that's maybe 5 years old, but it has insisted on only growing 4 or 5 main shoots. These have now reached the top of the trellis, and compared to the other 2, looks very threadbare. So now, do I sacrifice flowers this year, and prune back these 4 or 5 main leaders down to say 18 inches? Hoping that each will then throw maybe 4 or 5 more shoots and thus cover the trellis? I have two presidents, they both have always grown quite lanky and spindley, I'm thinking maybe I'm missing a trick here, and not getting the flowers up the plant I should, just at the very tops..
Looking for advice from folks who have been successful with 'The President' in particular.