Thank you for the very helpful info! I actually do grow flower vines up that lattice (was basically put up to give me some privacy from them), so there would be a layer. I also like the idea of grading it a bit...that's doable. I may just play it safe and grow mostly flowers, and some veggies around the edges. Sadly, the dogs will be dropped off at the shelter by the end of summer. They do this all the time, and I hate it. For some reason, they've been annoyed at my various gardens. I don't know why. Times when they thought I wasn't home but I was, I've seen them come into my backyard and dig things up, etc. Nothing esp. malicious, just weird. I'm putting up a fence this spring for a variety of reasons, all to do with these neighbors. :(

If I were in your situation I would check with my local extension service. The link below might be a good place to start. Here is a link that might be useful: Purdue Univ. Extension Service

First ... kudos for thinking ahead on this potential problem. You say that the asphalt area is old, which MAY mean that its most volatile time is PAST. And IF that is true, it means that the wetland/woodland has thrived in spite of the years-long runoff. Will this parking area be repaved in the near future? Are there local/state enviro-regs that will prohibit use of toxic materials to repave? Beyond these questions, if a berm to chanel rain water away from the wetlands is possible, that seems like ONE among MANY things that might prove effective. Also, you comment that this area is a part of your gardening area. That suggests that a certain amount of the water run off is within your control, which may be another thread subject on how to maximize water retention in your garden soils. Well, there are certainly layers of problems and solutions in your question. That wetlands is fortunate to have alert "friends".

Hi R.Harrison - what a nice photo! I know where this is. I like how it shows the waterbody that this rain garden is protecting. "You (pollution) shall not pass!" a la Lord of the Rings.