WASHINGTON — You don’t need dirt to have a green thumb. It turns out, you can grow a pretty wild garden in a few cups of gravel.

Richard Campbell, of Ellicott City, Maryland, has been gardening sans soil for more than 20 years — ever since he witnessed a watermelon vine growing out of rocks in his uncle’s yard. Campbell formulated a theory that the rocks were feeding nutrients to the watermelon, and has been researching geological agriculture ever since.

Now Campbell is the author of “River Stones Grow Plants” and founder of To Soil Less, a company that educates schools, communities and government agencies on the science and benefits of gardening in gravel. He showed WTOP how to grow a produce section in a few plastic cups. But before you begin, there are a few things to note:

• Any old rock won’t do. Campbell said river rock, which you can find bags of at Ace Hardware or other home improvement stores, is the type one needs to use to grow a garden. It’s a sedimentary rock, which Campbell said feeds nutrients to the plant.
• Sand is the secret ingredient in a gravel garden. It acts as the irrigation system, retaining water for the plants.
• Outdoor gravel gardens require a different setup and configuration than indoor gravel gardens, including a layer of agricultural fabric between the rocks and the sand. The following steps are just for indoor/windowsill gardens.

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