Rain gardens and bioswales are garden spaces that collect rain from some other surface and let it sink into the ground slowly. Bioswales have the added feature of moving water from one place to another. So if you have a parking pad at the top of your yard, you might install a bioswale to channel water into a rain garden below. What water does not percolate into the ground in the bioswale will collect in the rain garden and sink into the ground there.
Rain gardens are not swamps! They are quite dry if it is has not rained recently. In fact, we recommend planting deep-rooted, drought tolerant plants in rain gardens so you don’t need to water them between rainfalls. Well-designed rain gardens drain within 24 hours of a rain event. That means in order to design your rain garden, you need to know how quickly your soil drains and how much water you are sending to it.