A weed-filled 8x35 ditch? You'll have a lot of work just getting the weeds out of the way before you plant. One thing to keep in mind: low-maintenance has nothing to do with height, so don't worry about keeping a groundcover to a few inches. What you want is a vigorous spreader, because you have so much space to fill (280 sq ft). It is so much space that you really ought to plant it in patches to see what thrives, what doesn't, and for the pleasure of variety.

So, vigorous spreaders for patches of a sunny ditch: evening primrose (Oenothera speciosa), cranesbill geranium, phlox (the species: subulata), catmint (Nepeta). And if you get tired of planting nice stuff and just want to fill one end of the ditch with a sort of grass on steroids: liriope.

Also, to avoid: english ivy and cotoneaster. Avoid them because they'd create a messy rat-nest heaven. You'd regret planting them.

One additional thing that slipped my mind, because I was thinking too much of garden plants: clover is an excellent groundcover. The best type apparently is white clover, which stays fairly short, out-competes weeds (after you first clear aside the established weeds of course), looks as nice as a lawn, blooms a lot, feeds the bees, adds nutrients to the soil. I have no place to plant clover so I had forgotten how useful it is. For your sunny ditch it might be best.

Zone 8 ... but where (city and state) are you? USDA zones are winter temps, and have little to do with summer survival.

How much water is 'average'.

Thanks Alangrr, very helpful info.

Lazygardens, I am in Seattle, WA. I say "average" water because the area doesn't serve as a runoff collection area as with most traditional rain gardens. Also, if you've never lived in Seattle, the whole "rains everyday" myth is not true.

All of western WA is very much focused on rain gardens. It is an attempt to limit storm run-off and retain and reabsorb any rainfall on the property. Seattle itself is rather pioneering in its move to create rain gardens in what it considers to be target run-off areas (potential for run-off to enter habitat streams and groundwater, eventually ending up in Puget Sound). Because of this, there are a lot of resources available, including many plant lists of rain garden suitable plantings.

Here are just a couple:
http://raingarden.wsu.edu/
http://www.stewardshippartners.org/programs/rain-gardens/

Or just Google 'western WA rain gardens' for many others.

Two groundcovers that are highly recommended on most lists are beach strawberries (Fragaria chiloensis) and creeping raspberry (Rubus calycinoides). Our native kinnikinnick (Arctostaphylus uva-ursi) is always an appropriate choice for a mostly sunny area as well.

Super helpful, thanks so much!!!

Creeping Jenny, which comes in green and yellowish green, is a foliage plant which does great in wet to moist situations. It is a short plant. I have some in a moist area under a Nuttall's oak in my zone 6 yard. It does best with a shady area.

Variegated Goutweed is listed as not liking dry soil, but, I don't know how it likes constantly moist soil. I really think it is worth a try for you. I have it growing under an oak tree. It is green and creme on the leaves. If you don't get the variagated form it is less "behaved":, and will spread alot. The variegated Goutweed, is much tamer. It is a nice plant, I want to divide my plants and put them under more of my trees. It shades the soil and helps keep weeds to a minimum. Okay Bye Poaky1

PNW rain garden plants are pretty specific in their requirements in that they must be able to tolerate seasonal moisture - sometimes even standing water for a period of time - as well as very dry summer periods. That's why PNW native plants are usually recommended - these are their normal growing conditions. Just any groundcover suggestion - moisture loving or not - may not work.

Folks not living in this area may have difficulty in imagining our climate and how rain gardens work. The soil in a PNW rain garden is moist/wet for only a limited period of time, typically for just a few months during winter. The rest of the time it can be very dry. Plants adapted to that type of seasonal wet/dry cycles are limited.

Sorry Gardengal, I should have considered that when it came to the Goutweed, but the Creeping Jenny is great for moist soil to a wet standing water situation. They can take all but a dry and sunny area. My friend had it in a pond area, it was floating in water, and was rooted in a very moist area. I can't think of anything that it couldn't handle in the PNW. It doesn't need a lot of warmth to grow. If I overlooked anything, and it is not what you want, or need that is fine, of course. I just know it is pretty versatile, and the more moisture the better it does here in zone 6 Pa. And I have an image of the PNW as moist and kinda cool. Even if it is hot in summer, as long as it is moist the Creeping Jenny will thrive even in full sun. If it isn't what you want, then whatever, ya know? Poaky1