I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Keeping any plant that narrow is very difficult. You might consider building a vertical trellis and planting a vine on it. I would consider a clumping type of bamboo such as Bambusa multiplex (Alphonse karr is one variety). Then cut it off at the desired height.
If you want a free standing plant the barrier will need to be a bit thicker.
Some plants include 'Bright N Tight' Cherry Laurel or 'Little Gem' Magnolia.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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