We’ve been design ALOT of rain gardens in Maryland. Right now, the rationale is squeezed into the current Maryland Design Manual by “qualification” for a rooftop disconnect. This will hopefully change in the future to facilitate more opportunities to apply these at-the-source measures. A nice DNR doc on rain gardens can be found here.
However, like most made-for-homeowner docs on this subject, they COMPLETELY MISSED the fact that – unless you have very sandy soils, you really should incorporate an underdrain system and man-made soil mixes. This probably illustrates the difference between what a homeowner can do in their yard and what a professional specifies on a plan for someone else to implement.
Sustainable Stormwater Management 
If you have an underdrain, though, you really aren’t disconnecting the runoff, are you? Most soils should be able to accept runoff except the real clayey ones (as long as you’re staying out of the water table).
Have you seen any of the rain gardens you have designed a year or two after installation? I’d use them more often, but I worry that it will turn into a mudpit in a year when the homeowner neglects it.
Here’s a pamphlet produced by a local conservation service here in Vermont (land of clay and high water tables): http://www.vacd.org/winooski/winooski_raingarden2006_brochure.pdf
Cool site, thanks for another rain garden resource.
Thanks for the comment.
Whether or not the disconnection is actually happening is – as are many other Low Impact Development (LID) and Environmental Site Design (ESD) techniques – the subject of quite a bit debate. This post refers to the regulatory guidance in Maryland. Which, like any regulation, does not always reflect the logic or reality of all cases.
After a few years, rain gardens and bioretention facilities – if properly constructed with sandy soil mix and underdrain systems – turn out fine. I have some that are coming up on ten years old. Usually they are dryer than expected because of the sandy soils. I have one at my home with native silty soils and no underdrain – which is likewise doing fine.
It all depends on the soils if an underdrain and made soils are not used. If a homeowner does normal landscaping maintenance, the rain gardens should be fine.