native growth/discouraging blackberries

my property backs against a "native growth area", that's filled with Himalayan blackberries (non native), native blackberries, raspberries, salmon berries, and vine maple. this small area of greenbelt seems to be where all the water drains from the neighboring farms, and this water then pools in the back part of my lot. when the house was build last year, the back area was cleared and planted with pasture grass, and turns into a mucky mess every fall/winter.
i was thinking i could plant it with some water loving, native plants. plants that will not become overrun by the blackberries that will surely spread there if we stop mowing them down. i'm not allowed to plant non-native species against the greenbelt, which is fine with me, i'm just concerned that whatever i do plant will be choked out by the blackberries.
i basically have three objectives for this project... 1) suck up some of that excess water. 2) provide some cover for local wildlife. 3) block some of the view of the neighboring farm.
i don't want the area to look "planted", i want it to look natural. i just have no idea where to start. the area is quite sunny, with rocky clay type soil.
how does one go about giving their land back to nature, without inviting the invasive plants next door? suggestions? recommendations?
thanks, Kelly Culp Snohomish, WA zone 8a
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culprit wrote:

starting with some Sword Ferns, perhaps? They tolerate darn near any conditions, and they're definitely natives. Even though you'd associate them with shaded areas, they do quite well in full sun if they get enough water. (There some varieties of rhodies that might like your setup, as well.)
There are a ton of good websites out there for ideas, here's a list of the one's I've personally gotten the most use out of:
http://www.gardengal.tv/annes_garden / http://home.earthlink.net/~rhodyman/rhody.html http://www.paghat.com/garden.html http://www.greatplantpicks.org / http://www.nwplants.com / http://www.rainyside.com/index.html
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