From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156718-501794
Question: How do I plan for and plant a knot garden? What are the best
types of plants to use for a two color pattern? How far apart should
the plants be?
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.