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No matter how much you're trimming the tops the roots will grow to support an 8' to 10' shrub and therefore compete for soil space and nutrients if planted too close together. Having said that, I've seen mature ones that were planted 4' apart and they hadn't killed each other, they just looked like a thick mess of stems. The spacing of the hardy hibiscus will depend on the cultivar you use. Dwarfs like the Luna Series could be 24" apart but the largest types are better off if spaced 4' apart or more.