What is a Rain Garden? Rain gardens are garden beds that are designed to collect water when it rains. The purpose is to reduce the amount of storm water that runs off of your property into the wastewater sewer system or the local rivers and ultimately the Chesapeake Bay. Rain gardens are designed and built in order to allow water to percolate into the ground within 24 hours, inhibiting mosquitos from hatching. We use a lot of native plants in our rain gardens because they are beautiful and are ecologically friendly, providing food for animals and insects.