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Mine always grew horizontally. at one point I had 2-3 cubic feet of the stuff.. Could never get it to stop spreading horizontally. So I'm not sure where you heard that rosanervig grows straight or vertical, but mine should have read that part.
That is all rosenervig and was started with 3 stems. It's also about 1' thick (so 1x1.5x1.5' chunk) and that was the way it grew in. Filling in as much as it could, rising when it must.. But in tropica's defense, each stem is no more then 10cm from leaf tip to leaf tip.