Answer: A rule of thumb for annuals is to plant them as far apart as they ultimately grow wide. In other words, if the plant grows 8 inches tall and 12 inches wide, you would plant them about 12 inches apart. However, since you want a very full look I would suggest planting them slighly closer together than that, say 2/3 their width. This way they will grow into a solid covering sooner. You will also get a better overall coverage by planting them in staggered rows.
There are a number of varieties of impatiens, some much larger than others, so unfortunately I can't tell you exactly how far apart to plant yours. The label information should include the mature size for you.
Impatiens prefer a rich soil with plenty of moisture. Peat moss or any type of organic material will help the soil retain moisture and provide a better soil structure in general.
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