Answer: Probably the fastest topiary plant is English ivy grown on about a nine or twelve inch frame. This can look quite charming in a hurry (in topiary terms--a hurry is a season!). If you fill the frame with filler concealed with Spanish moss it will look presentable even if the ivy hasn't filled in completely. Other possible plants are creeping fig, also on a frame, and rosemary which would be grown as a standard and then clipped into the globe shape. The easiest, in my experience, would be the ivy although it is possible to purchase partially trained rosemaries in the spring and then grow them on over the summer to finish out the shape. Good luck with your project!
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