Answer: Your spacing will depend in part on how densely covered you wish the fence to be and also how tall the fence is. These vines will be cut back very hard each year so they will become denser from the base over time regardless. They can each grow as long as 18 feet. If your fence is four feet high, then about 6 to 8 feet apart should allow for ample coverage but you could put them as far apart as 12 feet if you do not want them to overlap too much.
You can try to encourage them to grow sideways by directing the new growing tips horizontally, but they will naturally try to climb so this may be a daily task. Once they reach the top of the fence the vines will stretch horizontally and also tumble back on themselves and drape or cascade and billow. So the effect will be quite full at the top, less so at the ground level.
Clematis tend to take several years to become rooted and established and really grow and bloom lustily, so be patient with your new vines. It will be worth the wait!
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