It's just weeds now & have kept it mowed for several years but am now getting too old to mow it. I tried a pink colored sedum but it didn't spread fast enough. The ground is pretty much sand & in full sun all day. Would like something fast spreading to choke out weeds. Someone suggested either Sweet Woodruff or Lily of the Valley. I'll check those out while waiting for a reply. As far as I can tell, I'm in zone 4a.
Unfortunately most of the things you find that will spread faster than the weeds will be...well...other weeds! LOL
Lily of the valley does better in part shade, not sure if it would handle full sun in your area. Also it doesn't behave as badly out west as it does in other parts of the country, plus you're at the lower end of its hardiness range, so I think if you put all those factors together you will be disappointed in its ability to quickly cover the area. Galium odoratum may be a similar case--looking at its light preferences in Plant Files I don't know that western full sun will be ideal for it, and it needs consistenly moist soil, which in full sun on a slope with sandy soil in a dry climate is likely going to present some challenges (if it manages to survive, it won't spread nearly as quickly as you're hoping).
The key thing to keep in mind is that whatever perennial groundcover you plant is going to likely take a little time to really get going. There's a saying "first year they sleep, second year they creep, third year they leap" which often holds true for perennials. There are of course things that get around to "leaping" more quickly than 3 yrs, but even a faster growing perennial is going to be a little slower going than your weeds which are either annuals which by their nature grow faster, or perennial weeds that are already well established.
So no matter what you plant, you're going to need to put some effort into weed control for the first few years. Assuming you buy plants for the groundcover (vs starting them from seeds) I'd suggest using a pre-emergent herbicide (make sure to reapply according to package directions--it doesn't last forever). If your weeds are mostly annual weeds, then liberal use of pre-emergent will go a long way to controlling them. If you mostly have perennial weeds, I'd still use the pre-emergent to prevent new ones from coming up, but you'll also have best results if you kill the existing weeds before you start planting your groundcover (kill, not just mow down). Depending on your preferences, you could mow everything down then put black plastic over the whole area for a while to kill what's left, or you could get a big bottle of Roundup and spray everything, or you could hand pull as many weeds as you can.
When you do plant your groundcover, the best way to get it to fill in the area faster would be to buy more plants than you think you need and plant them closer together than the recommended spacing. That way it will cover the area a bit better.
I agree with ecrane3, I'm battling weeds to plant in my backyard, uhg!!!! As far as ground covers I got some ideas but you will have to look up for hardiness in your zone.
Saponaria ocymoides (soapwort), 4-8", it grow rapidily to trail up to 36" and reseeds, flowers are pink and bloom in spring, they can fill in rather rapidly with the resseding.
Gypsophila repens, 4", blooms in late spring and sporadically thru the summer, light pink, reseeding will give you a mat effect and they can fill in quite nicely.
Geum Mrs Bradshaw, 6-14", pink/org. (look for pix, not as bad as it sounds), blooms from late spring to frost, deadheading helps it a bit, besides you want a bit of reseeding.
If you'r interested in Lemon Grass (herb), it is about 12" and spreads like crazy, I have tons that needs to come out of the border, I'll ship for postage...lol.
I also have a mat forming Potentilla with yellow flowers that in 3 years has spread to 3-4, maybe 5' across. flower in summer at 2".
Dianthus deltoides Zing, when in bloom is 6", out of bloom is 2-3", it is allowed to reseed here and fills in nicely.
Phlox, creeping phlox can fill in fairly quick, also is rather drought tolerant. You might be able to find some locally from a neighbor or ????, that would be alot cheaper, colors are many, lavender, white, pink.
A fairly new one to me is Caliroe, 'Wine Cups", it is only a few inches high but can mound to 6-8", it's stems creep along the ground to a length of 2-3 feet, and bloom all summer.
A taller plant that will come back year after year is Cosmos, they are an annual but fill in nicely with flowers that can be cut for bouquets and are fragrant in mass. Each year they reseed themselves making sure I have plenty. I have the ones that get to 48", but there are newer shorter varieties available (18-24"). These could be mixed with a low ground cover that bloom earlier in the season. My patch of Cosmos has grown to 15feetX20feet easily, maybe more. I'm constantly pulling them out when then get in front of things that are shorter (in the border), they are easily pulled when young. You can easily pull them out after frost if the ground is moist or cut at the base, by spring most of the roots have decomposed from the previous year If you are ever interested I have bags of seed and would ship for postage...
Another great one for reseeding is the annual Nicotiana sandersae, if you try these do yourself a favor and get the fragrant varieties, in the evening they are just absolutely yummy. They can be started the first year either inside (under lights) or out, tho will begin bloom a bit later. Once you have a patch they will spread from year to year.
I have also allowed the wild sunflowers to seed into my border and do editing as needed. I didn't save any seed but could find you some...lol...it's out by the country road I live on...
Another idea is Linum perenne, flax...it blooms late spring at 18-24", reseeds and is blue...Home Depot has or had the seed fairly cheap, $3-4 for thousands of seeds. Just throw on the ground this fall and let them come up on their own. Then to clean them up in the fall you could use a weed eater to clip them about 3 or 4" above the ground.
Ok...those are a few ideas, but all require the ground to be clean first...I know cuz I'm retaking the ground from the weeds (feild grasses), where I live in the country. Don't let anyone talk you into weed barrier cuz it's horrid...may look good the first season but...after a few season the bark or rocks or whatever is put over the barrier,
gets weed seed and then you got a mess and still have to pull weeds.
I'm sure there are other things available but these are things I personally have growing in my garden and have obsered them over many years...
Also go to HighCountryGardens.com, thye are out of New Mexico and have ground cover they sell by the flat, prices are decent. Look at the whole online catalog for other suggestions and pix.
Try a search (google) for Groundcovers (stepables), you might find some goodies that way also, ie creeping Thyme and more...Kathy
pix2: shows the sunflowers at the back that seeded themselves in
pix3: geum Mrs. Bradshaw
pix4: gypsophila repens and jjs
pix5: Veronica Georgian Blue at 2-3", and blooms all season, forms a mat, think mine is about 3feetX3feet.