Answer: The easiest way to determine just how many plants you'll need for a specific bed is to find the average mature size and then add from 6" to 18" all around each plant. The spacing will allow you access to weed, walk, cut flowers for bouquets, etc. For instance, lantana can be either a shrub or a groundcover, depending upon which variety you purchase. The shrub can reach 6' high and wide unless pruned, the vining type will creep out about 6' in all directions. Milkweed grows about 36" high and wide; pansies remain only 8-12" tall and wide; dill generally grows about 2' tall and wide; marjoram about 8"; violets have a similar growth habit to pansies but can spread by seeds and underground roots. Butterfly bush can get quite large - usually about 6'.
Since you haven't decided on the shrubs, I'd make a note of the mature sizes (as listed on the plant labels) and then draw out your design, or purchase the shrubs first and set them in the garden, then fill in with the smaller plants. I would err on the sparse side - plants tend to grow larger and fill in spaces if they are happy.
I hope this information helps you determine just how many plants you'll need for your new butterfly garden. Enjoy!
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