Groundcovers, do they need to be green all the time?

As I think about the landscapes that I have idealized from an early age, they are green and full of life all year long.  But I grew up in an area that did not have four seasons and at a time when droughts and water management were not something that we worried about.  So the expectation of having some sort of green ground cover 365 days a year was not difficult to achieve.  Now after living in the Northern half of the US for 40 some years I have come to realize that having four seasons means that there is a good chunk of time when the plants in the landscape take a break and rest. 

Vinca minor is a widely used and invasive groundcover, originally from
central and southern Europe, that is often used where an evergreen carpet is desired.   

English ivy forms a dense evergreen cover that spreads by both
 runners and seeds.  It is particularly damaging when it is allowed to climb trees.

This brings up a question, does all the ground need to be covered all the time?  Yes, for protection from erosion, extreme temperature fluctuations and evaporation coverage is beneficial.  Groundcovers, both living and dead, also provide wildlife habitat.  There has been increasing awareness of the importance of  providing habitat for overwintering insects as they are critical sources of food for birds and other predators.

Alleghany spurge, Pachysandra procumbens, native to the mountains
of the Southeast, acts as a more naturalistic groundcover.

Does it need to be green all the time? No, not really.  But there is a desire among many for a lush green ground covering year around.  This desire is' might I say, for primarily aesthetic reasons.  To get this year-round green in temperate climates many folks are drawn to non-native species like Japanese pachysandra, English ivy, Vinca minor , yellow archangel (Lamiastrum galeobdolon), wintercreeper euonymus, mondo grass and lilyturf.  Most of these species are invasive in many parts of North America.

Here's my listing of the importance of having a ground cover, roughly in order of importance:

Minimize Erosion by holding soil against the forces of rain and wind;
Water management, by improving water infiltration and reducing evaporation;
Soil Health, by avoiding compaction, maintaining good aeration, moderation temperature and moisture extremes.  All these help maintain a healthy soil microbiota;
Maintain ecology by providing places for native insects to overwinter, less disturbance provides better survival rates for overwintering insects and nesting sites;
Weed management, its harder for seeds to geminate on shaded soil, in competition with established plants or where roots cannot take hold; and,
Aesthetics, create a visually pleasing composition.

As I alluded to above, the ground cover does not need to be green and growing year round.  In fact, depending on the climate and the environment the most appropriate ground cover may not be green at all.

In the majority of cases the material covering the ground will be some sort of plant material.  Having a actively growing root network under the soil to stabilize it and keep it biologically active is the ultimate goal. Where climate and conditions allow having a living mulch is an ideal solution for protecting the ground. The plants making up this living mulch need not be green though all four seasons.  They just need to continue to to hold the soil in place.  Green and actively growing on top is nice but not a year-round necessity.

The following are descriptions of how natural groundcovers function in three habitats that are similar to what you might encounter in a residential setting: Woodlands, Grasslands, and Deserts.

Mayapples, Podophyllum peltatum, provide a dense cover throughout
the spring as the forest canopy leafs out.  They hold the soil
with a wide spreading network of rhizomes.

Spring beauties are another spring ephemeral that are found on forest edges.

Woodlands: In Eastern deciduous forests the ground is covered year around by a layer of decomposing fallen leaves, natural compost.  Similarly, in coniferous forest there is a slowly decomposing layer of fallen needles.   Depending on the amount of light there is an understory of perennials and shrubs adapted to those conditions.  Where there are gaps in the canopy and accessible soil, new plants can get a start.  Depending on the composition of the seed bank these may be native or non-native species.  In a well balanced system the amount of undergrowth will be determined by the available resources.  Because of the way the available light changes through the year the nature of the ground cover plants also changes through the seasons.  In springtime the ground may be covered with ephemeral species like mayapples and Dutchman's breeches.  By summer these disappear and the ground plane is less densely covered with shade tolerant species like ferns and sedges and understory trees and shrubs.  Fall and winter feature the return of fall leaves which slowly decompose to replenish the soil with nutrients as well as serve as habitat for overwintering creatures.

Grasslands: In grasslands the winter ground cover consists of dead leaves and stalks from the past growing season and a network of crowns of the overwintering plants.  A healthy layer of this plant debris serves to protect the soil and fill the necessary roles of a ground cover.  The root networks of perennial grasses and forbs help to hold the soil.  Standing stalks of tall grasses and perennials form important habitat for some overwintering birds and insects.  Where the ground has been cleared by natural (fire) or unnatural (cutting and raking) processes, space is opened up for new plants to grow, both desirable and unwanted, depending on the seeds that are present.

Little bluestem, Schizachyrium scoparium, is a common
component of many eastern grasslands and dry meadows. 
Even after it dies back in winter it maintains its visual appeal.

Arid lands: In desert environments growth is limited by the availability of resources, particularly water.  Here you typically see plants growing much further apart because they are limited by the amount of available moisture. Root systems of perennial species tend to be much larger in order to access sufficient moisture for growth and survival.  Particularly in hot deserts most of the organic material that falls to the ground gets burned up and does not contribute to build organic material in the soil for the long term.  In these environments a natural ground cover would be inorganic in nature, rocks, stone and sand. While plants may not stabilize the soil on the surface, many have very wide spreading shallow roots systems adapted to harvest rainfall before it evaporates.   These wide systems can help to stabilize the soil, leading to less erosion.  Trying to maintain a green ground cover in a desert environment can be very resource demanding and not at all sustainable.

The ocotillo and purple-tinged prickly pear cactus each have shallow
wide-spreading roots to scavenge water thus limiting what else can grow nearby.

If green in winter is really what you need there are several native species here in the Mid-Atlantic that fit the bill.  Native (near)evergreens include, wintergreen (Gaultheria procumbens), green and gold (Chrysogonum virginianum), heuchera, tiarella, Alleghany spurge (Pachysandra procumbens), golden groundsel (Packera aurea), evergreen ferns (Christmas and intermediate wood ferns) and some sedges, like creeping sedge (Carex laxiculmis) are good for partly shaded locations.  For sunnier sites moss phlox (Phlox subulata), wild strawberry (Fragaria virginiana), common and creeping junipers, barren strawberry (Geum ternatum), as well as some grass-like plants like prickly bog sedge (Carex howei).  On most sites these plants will still be green through most of the winter, but they generally will not be lush with fresh foliage

Golden groundsel in winter.  The interconnected leaf rosettes
are held close to the soil.

In spring golden groundsel puts on fresh growth and sends up flowers.