Answer: I understand your concern! There are numerous types of junipers, and they come in a variety of heights, forms, colors (golds, blues and greens) and textures. But note that not all junipers are suitable for groundcovers. Some junipers are trees, while others fit the more usual image of "shrubs," i.e., plants that stand anywhere from knee-high to chest-high. Such plants are suitable for privacy screens and hedges.
Given the space you have to fill, I'd recommend Blue Rug Juniper Plants. The foliage is a silvery-blue, thus the first half of its common name. The second half comes from its growth habit, as it forms a dense, low mat (rug) -- making it an ideal groundcover. Height 4"-6", spread 5'-6'. Foliage turns a purplish-bronze in winter. Blue Rug spreads rapidly and is relatively resistant to some of the diseases that plague juniper shrubs. Incidentally, what look to be blue "berries" on junipers are technically cones.
Many varieties besides Blue Rug juniper plants are suitable for groundcovers. A green cultivar of Juniperus horizontalis, namely, 'Prince of Wales,' purportedly grows even more quickly, while another cultivar, 'Mother Lode,' bears greenish-gold foliage. Meanwhile, the 'Pancake' cultivar stays smaller than these, both in terms of height (an amazing 2"-3") and spread (2').
Other species of juniper groundcover are Juniperus procumbens and Juniperus squamata. The 'Blue Star' cultivar of the latter provides another option for those who seek that cool blue foliage. But Blue Star will get taller over time (up to 3') than Blue Rug and doesn't spread as much, proportionately (4').
No plant is "no-maintenance," but horizontal junipers are certainly "low-maintenance." Groundcovers that flower require much more care. Space 4'-6' apart to form a dense enough mat to crowd out weeds. To aid their weed-control efforts prior to maturity, make sure they're well mulched (but don't pile up mulch over the plants' crowns). Don't allow fallen leaves and branches to smother young plants. Thin out mature plants for better air circulation, which well help avoid disease; but don't prune severely.
Horizontal juniper plants thrive in full sun and prefer well-drained soil with an acidic pH. Before planting, prepare the soil with amendments.
Hope this information helps you make the right choice for your landscape!
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