Answer: What you've read is commonly recommended. If you cut the stems back to 12 inches in height, it will help the plant branch as it begins to grow, and will reduce the chance of stem breakage during the planting process. Clematis are normally planted with the crown one to two inches below the soil surface (this enables the plant to recover should it be mowed off, damaged by animals or infected with clematis wilt). Once the plant is in the hole at the proper depth, fill in with the backfill soil, firm and water well to settle soil around the root system.
If you didn't plant it deeper, don't worry about it. Just be careful to keep your weed whacker away from the plant! It's still early enough in the season to prune your clematis back to 12" above the soil. It will produce several new stems and the result will be a more densely branched plant. If you cannot force yourself to cut the plant back. allow it to grow as is and plan on pruning it back next year, after you're sure it has established itself and is healthy.
Best wishes with your new clematis.
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