Mulch was not invented by humans. Most plants make some sort of mulch naturally. Even desert plants that live on bare ground shed foliage that decomposes to be recycled back into the soil,and provide nutrients for the roots below. Redwood, most pines and most eucalyptus are extreme mulchers that generate thick layers of foliar debris that benefit their own roots, but inhibit the growth of competing trees. Knotweed, Hottentot fig (freeway ice plant), ivy (both Algerian and English) and other dense ground covers are their own mulch.
There are a few advantages to mulch. Although ground cover mulches consume some degree of moisture, mulches benefit plants by retaining moisture at the surface of the soil. Mulches also insulate the soil, so that it is more comfortable for roots that want to be near the surface. Most weed seeds that get covered by thick mulch cannot germinate and emerge through it. Those that try to germinate on top probably cannot get their roots through to the soil below. Besides, mulch simply looks better than bare soil.
Mulch is generally spread in early spring, before weed seeds are completely germinated, and while the soil is still damp. However, moisture retention is still a concern through the warm and dry weather of summer. A thin layer of finely textured mulch added over thinning ground covers (without completely burying the foliage) can rejuvenate tired old stems by giving them something more to root into. This works well for knotweed, English ivy and trailing gazanias.
Mulches should be well composted so that they do not take too many nutrients out of the soil for their own decomposition. However, for the same reason, uncomposted coarse wood chips, like those often recycled from tree services, are even more effective at controlling weeds while fresh.
Small volumes of mulch can be purchased in bales at nurseries and garden centers. Composted redwood soil conditioner is a popular soil amendment that can alternatively be a nice finely textured mulch to spread thinly over small areas or in planters. Larger volumes of more coarsely textured and less expensive mulching materials can be obtained by the yard from garden supply stores.
There are no fancy varieties, but many different fancy names for knotweed. It used to be known as Polygonum capitatum, but is now Persicaria capitata. The many common names include pink knotweed, pink clover, pink fleece flower, pinkhead, pink bubbleweed and smartweed. Obviously, the tiny and spherical blooms are pink like bubble gum. Each small leaf has a distinctive brown chevron, which makes the collective foliage rather bronzy. The wiry stems can not stand much more than three inches high, but creep indefinitely, rooting as they go. Knotweed is an excellent but potentially invasive ground cover, and is also a nice component to mixed plantings in large urns or behind retaining walls, where it can cascade several inches over the edges. A bit of partial shade is no problem. Bloom continues through summer, and resumes at the end of winter.
Captions for photos:
40806+: Knotweed by any other name, (and it has quite an accumulation to choose from!) is a practical and resilient ground cover.
40806A: Ground cover might look better than mulch, but would also require irrigation when there is not enough water to go around.