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Rosemary will branch root. The reason the main mass is so big. The branches lay on the ground and then grow roots at nodes which actually increases the size of the plants as a whole. The main mass of plant is really that big of a root mass beneath it. Given a conducive environment these things can get huge in the southern tier states.

I worked at a nursery between Joshua Tree and Yucca Valley once and we would go take tip cuttings of the massive rosemary along the fence line. I would then trim the ends, dip them in rootone, and put them into 12x12 flats. We would end up with a 95%+ survival rate on the cuttings. These plants WANT to root.

LOL - Very cool! I just did that and was working on a post about it now!