MARBLEHEAD – From across the garden, it looks like an evergreen hedge sits atop a stone wall in Ellen Cool’s garden.Only on closer inspection, do you recognize that the whirled and swirling needles are in fact, three individual miniature cypress trees with subtle variations in color and texture.In approximately 2,500 square feet of downtown Marblehead, landscape designer Cool’s garden is a laboratory, studio, and living catalog of biological diversity.”I call it ?The Parable:’ It’s a small, illustrative story of large things that can happen,” said Cool. “I’ve experimented with untold amounts of plants. The best list is what’s here now.”This Saturday, you will be able to see these specimens, as Cool and owners of gardens on Waldron Court and Marblehead Neck open their gardens to the public as part of the nonprofit group, The Garden Conservancy’s, Open Days Program.”It’s part of educating people about types of gardens that are around, get them a little excited about gardening, and raising money for our preservation [efforts],” The Garden Conservancy’s Communications Manager George Shakespear said.Since 1995, The Garden Conservancy has worked with private landowners to open their gardens to the public in order to raise money for the organization, which works to preserve the cultural impact and biodiversity of notable gardens. This season, more than 300 private gardens in 17 states will be opened to the public, Shakespear said.Cool said that tour guides on Saturday will be able to guide visitors through the flower beds, answer questions about plants and provide instructions on how to recreate the displays in their own home.Entry to each garden costs $5 per person and will be collected at each location’s entry n there are no tickets required.Cool said that the proceeds will go to The Marblehead Garden Conservancy’s efforts to maintaining public lands and the national organization’s efforts to preserve unique and significant gardens.Cool added that the gardens in Saturday’s tour represent some of the challenges that local gardeners face.The oceanfront garden at Grey Gulls on Ocean Avenue is an ideal place to learn which plants can grow in conditions with a craggy coastline, salt spray and maritime winter wind, Cool said.The Fettyplace – Bowden House on Waldron Court – incorporates outdoor living spaces into a tight corner of downtown Marblehead, according to the Conservancy guide.Meanwhile, Cool’s garden – and the gardens of her neighbors that she has designed and maintains – is the showroom, laboratory, and nursery for her landscape design business, Stone Garden Designs Inc.She estimates that approximately 1,500 species and varieties of trees, shrubs and plants are growing under constant scrutiny – recorded on her blog A Reasoned Landscape.com – to see what conditions they will need to thrive in client’s gardens.A studio/shed displays her favorite tools, a library of reference books and her computer, and a catalog of the plants that visitors see in the beds and containers.”There are always experiments going on,” she said, pointing out three different-sized varieties of a violet-colored aster. “I’m always looking for the longest blooming plants, which look the best longest.”Currently, experiments involve ornamental onions, subspecies of dwarf cedars and miniature hostas.It’s a perfect environment for Cool.”I’m a field biologist by training, (and) an artist by who I am in life,” she said. “This brought it all together.”The Open Days Program includes open gardens at The Fettyplace – Bowden House, at 15 Waldron Court; Grey Gulls, at 429 Ocean Ave.; and The Parable n Ellen Cool’s Garden, at 19 Circle St.Cool’s garden is open from 10 a.m. to 5 p.m.; others are open from 10 a.m. to 4 p.m. on Saturday, Aug. 13.For more information and directions to the gardens, visit www.opendaysprogram.org or call 1-888-842-2442.