A rain garden needs to either be in a place where surface water is already going, or it needs to be in a place where surface water can be directed to either by grading or piping. Then it needs to be contoured to hold the water in order to give it time to infiltrate into the soil and available for uptake by plants.

It certainly looks like some runoff heads to that area. You can shape it anyway you want when you contour it (scrape out a bowl). Keep in mind that the edge of the effective shape will all be at the exact same level, so in order to shape the "rain garden" you need to be very aware of elevation. You can check it by running a hose into it, if you have no other devices. Volume of runoff is goig to determine its maximum effective size. The rain garden does not have to handle all runoff as long as the water has the ability to continue past the rain garden once it is full.

Can I ask an opinion on the shape? Will it look odd to have such a long and narrow bed like that?

No, it will look fine, especially if you look at the shape of the lawn going toward the gate. You might want to reshape the left side of that grass (the foundation planting bed as it goes to the gate). This will help make the shape of the grass what is noticed rather than the bed shape.

Actually, it is a logical place for a planting bed and would not be perceived as odd whether it is a rain garden or other type of planting as it is common to see plantings along property lines. I would not hesitate to make the back side of it straight along the property line and using the curve of the "walk" to play off of as you shape the sides that face your property and the street.

Obviously, the raised area around the tree is not going to function as a rain garden, but can be incorporated into the same planting bed.

Does this area currently flood?

Thanks so much for the wonderful suggestions. I like the ideas you gave me.

That area has only "flooded" a few times after extremely heavy rains. The area held water for less then a day. The problem is that when it rains hard, the water washes away the nutrients in the soil and runs off towards the back yard (under the gate). We ammended the soil with compost several times and seeded it but it still keeps washing away. That's why the rain garden was suggested.

DH didn't want me to dig up any more lawn but I think he's OK with this area b/c it's the ONLY spot in our yard that grass won't grow.

I hope someone else reads this because I have another question and didn't want to post these pictures again.

I'm still working on the actual design of this bed but I have started making a possible plant list. The women that suggested this bed for me suggested a 'Sweet Bay' magnolia or a river birch for this bed. I absolutely LOVE both of them but I'm concerned that they would get too tall, shade all the sun plants I'm planning to use there, compete with the large oak tree there and shade my neighbor's house.

Should I stay away from larger trees and maybe use large shrubs instead?

I would suggest incorporating the bed infront of the house and the oak tree so that it is all one large bed. Assuming you have a budget to accomodate it of course. You can always do it in pieces w/ that as the eventual goal. You could then do a flagstone or stepping stone path from the gate to the front yard.

As far as using tree species, I would assume that there is already at least filtered shade from the existing tree??? Most ornamental/native grasses and wildflowers will tolarate some shade as long as they get 4-6 hours of direct sun.

The women that suggested this bed for me suggested a 'Sweet Bay' magnolia or a river birch for this bed.

From the first photo, it looks like that oak's canopy covers most of the proposed rain bed. I'm assuming the photo angle is misleading -- but just how much of the bed is under the oak?

How far is the fence from the oak -- i.e., how much of the proposed bed isn't under the oak?

How large would the magnolia grow in your zone? How large would the river birch grow? How do you feel about having a tree that size in that location?

I find it odd that you want to build a rain garden because nutrients are running off from other areas. How do you expect that therain garden will affect that?

Are you concerned that you are putting excess nutrients in a nearby water body and that is why you want the rain garden?

The oak tree is so large that the canopy is pretty big but the lowest branches are really high. The entire area gets 7-8 or more hours of sun. The tree is 36' from the fence.

Laag - The whole idea of installing a rain garden is to be able to get something to grow there, as well as slow down the flow of water. Right now every time it rains, that area collects water or rushes off very fast. As a result, the soil is terrible and the grass won't grow well. We have ammended the soil several times and seeded but it never grows well. By installing a rain garden there, the use of certain plants with very long roots will help absorb and slow down the flow of water. Also, the plant selection is specific with plants that can tolerate standing water as well as dryness when it doesn't rain. I am taking a soil sample but I know I will have to ammend the soil before I plant. Hopefully with the right plants and the right soil ammendments, the area will thrive and control the water flow too.

I'm concerned about the fact that the tree is an oak tree. Mature oak trees tend to sicken and rapidly die if the ground within ten feet of them is regraded to be even as little as several inches different in height. By "installing a rain garden," do you mean digging out the area to make it permanently lower than it is right now, so as to collect more water? Because if so, this could kill the tree. Putting in a garden would be fine, but you would need to keep the soil at the same height it already is.

I definately plan to keep the soil the same level as it is now and I don't plant to plant anything near that tree. Right now we have a small amount of mulch around it and I'm going to just make that part of the rain garden bed.

I definately plan to keep the soil the same level as it is now

It appears from the photos that the neighbor's yard is below the level of your future rain garden. That may be one reason some earlier responses touched on the soil level. Are the photos misleading? Will you be adding a wall or berm to keep the rain from flowing out of the rain garden area onto the neighbor's property, and also at the gate to keep water from flowing into the backyard?

Or (I'm not an expert) is there a type of rain garden that is not a depression?