Although Martha knows there is no such thing as an instant rose garden, experience has taught her that structure is the catalyst. "With a really good plan and good architecture, you can make a garden happen," says Martha, who began planting her ambitious rose garden in her East Hampton weekend place in 1991. By the end of the first season, the climbing roses had scaled their supports, and within a few years, they had achieved a mature look. By comparison, it had taken 16 years for…