Answer: In most cases creeping phlox will outcompete grasses with a little help; some varieties of sedum can also be an effective groundcover once established. However, it is best to start with a clean slate, meaning getting rid of all perennial weeds and grasses first. (Unfortunately some weed grasses are nearly impossible to control by pulling alone, especially once they have infiltrated a bed.)
Then plant the groundcover and mulch well around it in order to keep down new weeds. If the area is next to lawn, it's a very good idea to use some sort of barrier between the two. A mowing strip and/or lawn edging are often used, in other cases gardeners will edge a bed by digging out a portion of turf and leaving a sharp drop and a bare stripof earth between the two.
If the weeds are coming up in between the started groundcover plants, you could try smothering them over time with cardboard or several layers of newspaper topped with mulch. (The groundcover will root down through the mulching materials.) If the weeds reappear, add another layer on top of them.
Another option is to carefully apply glyphosate to the weeds, but this can be difficult as it must not contact the groundcover. Some gardeners find the wipe application method easier to control than the spray application. In either case, do follow the label instructions carefully.
Finally, pulling is yet another option and do take heart -- as the groundcovers take hold the weed population does decrease. To speed things up, do try to provide the groundcover with adequate water and fertility until it has filled in; healthy plants will grow faster and have a better chance of outrunning any weeds that might find their way in.
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