Answer: Knot gardens are most effective on a flat site in full sun. Prepare your plan to scale on paper first using crayons or markers, then carefully transfer it to the planting area. (You can use flour or a bit of horticultural lime to mark out your planting locations.)
The plants you use will be dictated by the site (is it sunny or shady, hot and dry or damp and soggy, rich soil or poor) but some effective combinations might be: uniform annuals such as two (or three) colors of begonia; small shrubs such as dwarf red barberry and gold barberry or gray-leafed santolina; or clipped germander or boxwood infilled with colored gravel or low growing groundcover such as a small ajuga underplanted with bulbs.
Planting distances will be determined by the actual plant selection. Plant them on center at roughly two thirds the distance of the their mature width so that they will meld together nicely. Be sure to allow time in your schedule for frequent clipping if using shrubs--nothing spoils the appearance of a knot garden faster than stray tips on what should be a clipped line of plants.
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