Answer: Here are several considerations to keep in mind. Any healthy groundcover will naturally spread so you need to be prepared to contain it with either a physical barrier type of edging strip or an aggressive hand edging routine -- if it didn't spread well it wouldn't be an effective groundcover. Groundcovers need to have a well prepared planting area and regular watering and fertilizing until they are well established; this can take several years depending on the growing conditions and the plants involved. Your selection is going to depend somewhat on the soil type and microclimate as well. Finally, surface rooted trees such as maples and birches can be very difficult to plant around because not only do the trees rob the soil of nutrients and water but they also compete for sun and rain. You will also find that as the shrubs grow larger they outcompete nearly everything beneath them and that a layer of mulch which is raked and renewed occasionally may be sufficient until then.
Having said that, there are the old standby plants: ivy (Hedera helix) for almost anywhere, creeping myrtle or vinca minor for part shade to shade, and pachysandra for part shade to shade. These are all evergreen but only the vinca blooms in a noticeable way. For a sunny location creeping junipers (nonblooming) are very popular, as are the flowering creeping phlox and creeping thyme. You can expand the possibilities by considering other types of flowering perennials such as sedums, daylilies, Black Eyed Susans, and for shade, hostas in various sizes and shapes and colors.
Nearly any plant can be used as a ground cover as long as it is well suited to its location but unfortunately, I can't think of a groundcover that would flower from spring to fall; most perennial plants have a relatively short bloom period. For additional suggestions you might wish to take a look at a few basic gardening books. Two I particularly like are "Perennials for Dummies" by Marcia Tatroe (ISBN 0-7645-5030-6) and "Gardening for Dummies" by Michael MacCaskey (ISBN 1-566884-644-4). Both of these offer many planning suggestions and include sections on site analysis, soil preparation and plant selection. Good luck with your project!
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