There is no hard and fast rule regarding the spacing of groundcovers at the time of planting. The closer the spacing, the faster the bed will fill in. Generally, the spacing scheme is based upon the size of the plants, preference, budget, and the location of the bed. Keep in mind that the longer the bed takes to fill in, the more maintenance in the form of weeding and mulching will be required.
Liberal amounts of weed-free mulch are an essential part of a successful groundcover planting. The mulch helps to deter weeds, retain moisture and prevent heaving over winter. Shredded bark at 2-3" (5-7.5cm) covering is quite effective in preventing weeds. One cubic yard (0.75 cubic metres) of mulch will cover 100 square feet (10 square meters) to a depth of 3-4" ( 7.5-10cm).
Groundcover Fact Sheet
The Right Plant for the Right Place