4″-6″ voids seem to be the standard…
Laurentia fluviatilis
Blue Star Creeper – Sun to part shade. Evergreen in warmer climates, semi-evergreen in colder areas. Tiny-green foliage makes dense, low mat. Tolerates heavy foot traffic, 3 or more times daily. Prefers moist soil. Blooms blue Spring-Fall.
Mature height 3″.
Thymus serpyllum ~~
Creeping thyme seed produces a perennial evergreen groundcover that emits a fragrant scent when walked upon and blooms during the spring and summer months. Considered to be an Eco-Lawn, creeping thyme is one of the best low growing groundcovers due to it’s low maintenance needs and ability to handle the foot traffic of a grass lawn. This variety of creeping thyme (Thymus serpyllum) is not to be confused with other taller varieties (T. vulgaris) which do not tolerate foot traffic.
Ajuga
8SER30 Herniaria glabra
Mossy groundcover, yellow green foliage, tiny white flowers, can take foot traffic, grows only 3″ tall, hardy to zone 5.
Arenaria sp. ‘Wallowa Mountains’
…as a small scale groundcover, as a crack filler between flagstone or paving stones. Because the plant never blooms, it stays looking neat and tidy. Zones 4-8
Veronica liwanensis
…ground cover “lawn”. With the first hint of warmer weather to come, the small but numerous spikes of flowers paint the garden blue like a reflection of the sky. With a little extra water, Veronica liwanensis reblooms lightly later in summer. Needs afternoon shade in hot summer climates. Zones 4…