Excellent questions and comments, Gardengal.

My first impression is that it doesn't look "natural" so much as it does haphazard. Per the "look" itself without regard to other issues that have been mentioned, seems it would be improved with larger groups of plants and less individuals. And fewer, but larger rocks ... in an artistically grouped arrangement

Thanks for the input!! Great thoughts!

I have designed rain gardens before, and yes, typically they are at the lowest point. I've installed two, and both of them were on a flat area designed to receive runoff, but we don't really have that here. The whole hillside is basically one big "rain garden." I did some researched on perched rain gardens and it can be done. In some sense, what I am making are "filter strips" along the hillside. Prior to the "rain garden" idea, the runoff was just dumping onto the hill and it wasn't causing much of an erosion problem because the soil was naturally quite permeable. You can't see the rest of the hillside to the left of the photo, but it is the spot where there is an erosion problem, and we installed drain tile all along the top of the hill to direct the water to this spot. The other alternative we considered is to direct all the water to the third garden spot which is at the far right, which can still be done.

I don't think there is going to be so much water flowing out that it needs to be channeled into streams. But I could be wrong. You can't really see, but there are depressions behind the berms that will be filled with compost and sand, to absorb the runoff, so they are flat. The berms are a terraced look. I do wonder about having to channel the water but I'm not sure yet if that is necessary. Great point about the "zig zag" shape as opposed to the tiered one we have there!

This is one of the manuals I am using. http://www.cdfinc.com/xm_client/client_documents/Chicago_GuideTo_Stormwater_BMPs.pdf

So basically the whole area serves as the "rain garden" because it receives and absorbs the runoff. The biggest issue is that there is often high salt content because of how much they salt the service road above. My main concern is how it will look from an aesthetic perspective. It's a large rock garden that will also serve a runoff absorption function. The aesthetics is where I am not particularly adept. Some of the plants have been chosen due to their specimen value, and others to serve as seed sources for neighboring wild areas. I did leave some haphazard native trees there to work around, so does not have the repetitive style imposed on it at this point. But, IMHO, creativity is working around some givens that are not always ideal. That said, I would like to improve upon the overall design, and I can totally consider more "artistically grouped arrangements" which is what I would like to do, but this knowledge of what that means is not automatic, for me or anyone else necessarily.

P.S. The reason I spread the shrubs out instead of grouped them, is because I imagined it mimicking the way the understory in a forest bursts into bloom in spring. I imagined someone walking up the hillside and coming upon a blooming forest with a rock outcrop covered in wildflowers. However, as I said, I could be wrong, since the setting is already pretty contrived.

P.P.S. I talked to my landscaping colleague and he said, "As per usual, you over-plant!" But he also thought that those outcrops should have more rocks, not less, since rock walls are a motif thoughout the arb. (It's an old gravel pit). Here's my second attempt.

Here's a "before" photo looking in the other direction. You can see a bunch of broken up asphalt that was dumped there and also two telephone poles, which got taken down allowing us to really get in and re-landscape. When the poles were put in is when this area got messed up, and then filled with invasives which we are now removing. It's been regraded, but as I mentioned, the area was already absorbing runoff from the adjacent service road.

Two questions before I add to the conversation:

1. Is this 'rain garden' going to be open to and widely viewed by the general public? If so, from which direction?

2. In an effort to match site to surrounds mention was made of location being an old gravel pit. Possibly that area of western NY, which is a glacial out-wash terrain famous for its drumlin fields? A better idea of location would aid in rock and plant choices.

Southern Michigan, moraine area. I have a real problem because it will be viewed from both directions. There is a path adjacent to the left, and people go up and down the path constantly. The hillside faces southeast. So people will be viewing it from the south and north. The plants tend to tilt east since there is a line of pines on the west side. We'll get the rocks from onsite, there are tons of them, glacial deposits. Attached photo to give you an idea of our style of walls/berms whatever you want to call them. All from onsite.

An interesting learning opportunity. Some random thoughts to consider.

1. Rocks. Through the years this will develop as a wild area with annual detritus collecting, covering small rock areas. Use largest glacial boulders available. Contrary to usual 'rock setting methods' place them on soil surface anticipating future natural composting of leaves.

2. Layer the planting, angling in a manner that places tallest trees/shrubs back from viewing paths and grading growth heights down to lower growth and wild flowers along paths, not close enough to be picked. Chionanthus virginicus (male and female) which grows slowly will in time reach 25-30'. Selections of Viburnum with various heights from tall to short, colorful berries and/or fall leaves, perhaps a few of the fragrant types. As the layered plantings reduce in size, Ilex verticillata (male and female), Clethra alnifolia, heavy on types of ferns, Symplocarpus foetidus (?), planting tapering down to damp loving wildflower groups native to your area.

3. Could winter salting be reduced on the road along this "rain garden" area? An unfortunate learning situation. I have been following some Dutch research re growing vegetable in various densities of saline water. Some, such as carrots and potatoes are thriving. You might trial scattering the contents of 40# bags of gypsum on the spring melting snow along the bottom edge of that upper fence. Gypsum is the standard treatment for land areas that have received heavy salting from neep tides, hurricanes and over zealous salt trucks flinging salt on lawns.

4. Many plant choices available for this project. I mention a few above. Do keep us updated, please.

The plant list is similar to what you have described Nandina! Chionanthus is definitely on the planting list, next to the bench. Did not know it has both male and female plants. Have already planted several viburnums down below--Sargent's viburnum. And dwarf fothergilla (witch hazel's cousin) in the front, and some red twig dogwoods but I think I will transplant them, I don't like them in this setting. The larger shrubs are mostly our witch hazel collection, so Hamamelis species and varieties. We surveyed a local native high quality natural area to come up with some ideas on plants that would naturally be growing in this situation. One we are going to try along with the viburnums is pagoda dogwood, but keeping deer off of it will be a challenge. We found native currants in our reference plot but staying away from them due to lots of white pines nearby. Hence the maple-leaved viburnum choices. Debating on Illex verticullata because they don't do well with the excessive deer browsing problems around here. Am trying to solve that problem as well! I am planning on perhaps wild blueberries but so far haven't found any low growing ones and I have enough high stuff for the back. They would replace the redtwig dogwoods. The ones I got are cute, with small leaves close to the stem, so very ornamental but they look too contrived in this natural setting. I have a more formal place for them.

As for the salt, YES, there are alternatives, but to get the grounds crew to coordinate and think out of the box may take some doing. I have had to re-do the plant list a couple of times. It's sometimes like searching for the needle in the haystack--native, deer resistant, salt tolerant, shade tolerant . . . I mean c'mon!! :) Our soil is limestone base so somewhat forgiving on salt. I have a huge compost stash for ammending and that helps. Also helps to counteract juglone, which thankfully is not really a problem in that area with the one walnut tree. Anyway, I will post the plant list in a bit for anyone interested. Lots of wildflowers, some ferns and grasses, not all for the rock garden, some for the hill down below. Am focusing on shrubs and trees first though.

So far I've shown the two schemes/layouts to a lot of folks for feedback, some like the busier one, some like the more formal one. I'll probably do a hybrid. My MS Paint skills are deteriorating so not sure how many more versions I will play with. Eventually I will map the whole thing. Actually doing a map with grid paper next week with my class.

Would love to find a site where you could download standard graphics for trees and shrubs and forbs. That would save me having to draw or try and find clip art for them.

Just had my rock wall lanscaping guy come out and he immediately said that my biggest problem won't be water infiltration and runoff, instead it will be that the plants will have trouble establishing because it is so dry. We were already intending on ammending with compost but I don't think we have enough. Now trying to figure out how I can get enough compost that is not infested with invasive weed seeds. We have municipal compost but I'm not sure it gets hot enough and I don't want to import a lot more weed seeds. But hot compost is expensive. It's a trade off.

Use your on-site rocks to solve this wet/dry problem. Stake the location of each tree and major shrub. Then bury either a large flat rock or a collection of smaller ones at each stake as a rock garden or just below soil surface. Install staked plant just above or next to planted rock. The rocks collect and store moisture beneath. Old time method.

Also, when planting wild flowers/perennials use the same method. Except work the plant root balls in and under a rock planting them sideways. As they grow they will 'right' themselves. This method also allows one to plant more tender perennials. If a tender plant freezes back it will return to grow again because the roots have been protected from freezing.

Go easy with the compost in your glaciated soils enriched by 'goodies' from the Canadian Rock Shield. Between spreading a bark mulch and falling leaves Mother Nature will surface nourish your plants and that is where plant roots feed, seeking mycorrhizal fungus.

Today I was out digging a hole for a perc. test and I realized that we graded out pure fill . . . filled with broken up asphalt that is. And the soil tests out alkaline (8 for pH) and high P. Not sure why the high P, but luckily low N. From what I've read on restoration, leaf mulch compost which is what I could get from the city, doesn't flood the soil with N which could be a problem. No trees in this area, although the area just below it is fine. It's the rock outcrop that has all the issues . . . This is ending up way more restoration than landscaping!

Interesting tip on the rock plantings, I had never heard of that trick and can't wait to try it!

Had another botanist/rock gardener come out and take a look at it, and he suggested I use the big rocks to create something like a raised bed and fill it with sand, since the fill doesn't perk all that well either. I am thinking maybe to haul out some of the fill, but others say no, a lot of work and expense. But I found out that the fill was hauled in. Probably should not have been graded out, it started out all piled up into a big hill. But it looked ugly. This really is a "restoration" trying to make something pretty out of something ugly.

Pinkmountain, I'm very curious how this project turned out. Can you fill us in?