Ground-cover plants are distinctly valuable for a use covered by the literal interpretation of the word. The conditions which they overcome may not be unsightly or entirely barren. Their general use is to provide on the ground an interesting carpet, which may be close-growing, as in the instance of vines and the very low perennials, or taller-growing, as in the instance of yellowroot and flowering raspberry.
We may say that ground-cover plants are used to make a more interesting mat, which is an aesthetic use, or to form a carpet for the purpose of preventing excessive evaporation. Many of these plants are useful because of their fruiting characteristics, and also their ability to retain their foliage, both during the hot, dry summer months and during the winter months. In the selection of this material there are many different and distinct uses which can be definitely grouped under various headings, as shown in this chapter.
As a matter of fact, any plants, whether high or low, serve as a ground cover in some sense of the word. The tall-growing plants, which are often seen planted in close masses and included in the groups of tall-growing shrubs, are discussed under the headings of "mass plantings" and also "undergrowth plantations." This discussion does not attempt to treat those groups.
It has seemed best, rather than to include in one general list all of the plants used for ground cover, to make an effort, at least, to separate into a number of subheadings the specific uses for which ground-cover plants may be selected. The person familiar with plant adaptations knows that there are distinct groups of plants which are adapted to low, moist conditions, as there are distinct groups of plants adapted to dry and sunny conditions also. Those of us who have had considerable experience in the planting of perennials in different types of garden soil, as affected either by the type of soil or the kind of drainage, know that certain plants will thrive in a very moist soil, while other plants take a great dislike to moist conditions, and will thrive only when the soil is well drained or light and sandy. There are some plants, however, like the moss pink, the sea thrift, and the Japanese evergreen ivy, which seem to thrive almost equally well in moist situations or dry situations. The plants shown in Group A are those which have proved their value as being adapted to conditions which are continually moist, and should preferably be grown in the open sunlight and not subjected to any considerable degree of shade.
Plants which are adapted to dry situations, especially conditions of sandy soil or extreme drainage where grass will not thrive, include a small group which have proved themselves very hardy. The barberry, the Japanese spurge, the moss pink, and the stonecrops are fully representative of this group. The mat of foliage formed by the plants in their more mature development serves to shade the ground beneath and, to a certain extent, to retain much moisture in the soil which otherwise would be lost through evaporation. This group includes the close-growing types of plants which are selected mostly because of their ability to form a definite mat. Many of them such as the stonecrops, the tunica, and the moss pink, are extremely valuable because of their flowering habit, although effective during a short period only. The Japanese spurge, the partridge berry, and the bearberry are valuable distinctly on account of their foliage habits.
The plantsman is often called upon to select material which may be vines, perennials, or low-growing shrubs, to be used for ground-cover purposes under large trees, and in situations heavily shaded by buildings. Most of these plants should have, for their most successful growth, an excellent topsoil containing a small percentage of clay, or a rich woodland loam consisting mostly of well-rotted leaf mold and fibrous roots. These plants are valuable because of their ability to thrive under extreme shade. One often sees in large lawn areas and at the edges of woods, or on the shady side of buildings, spots which receive little or no sunlight where grass will not thrive and where most of our ordinary shrubs and perennials grow thin, leggy, and not vigorous. In such locations the only real solution lies in the selection of plants which will form a ground cover and thus preserve a mat of interesting green foliage. Many of these plants also, like the bunch-berry, the ground yew, and the partridge berry, have interesting fruit. The waterleaf, the moneywort, and the Japanese evergreen spurge are types valuable only because of their foliage, and the stonecrops, the periwinkle, and the wake robin are valuable also because of their interesting flowers. This list of plants does not endeavour to cover the group shown under "perennials valuable for ground cover in wild garden areas," but if one is seeking more complete information on this subject both this group and the discussion on "wild garden areas" should be consulted. (Chapter XXXI (Perennials For Different Purposes)-C.)
The question of how to make interesting those embankments and slopes that are otherwise unattractive is one which often confronts us. The plants which are used for this purpose are included under the groups of shrubs, perennials, and vines, and the kind of materials selected, whether shrubs, perennials, or vines, depends upon the scale of the effect which is desired. That is, for coarse, rocky embankments, marked with large boulders, and which are to have a rocky appearance, the larger shrubs and vines are most valuable, while for a more refined effect in the intimate portions of the landscape setting the perennials and smaller-growing vines are more effective. Most embankments and rocky slopes are composed of sandy, well-drained, and generally dry soil. Here plants such as roses, matrimony vine, and buffalo berry are valuable for their fruit. The honeysuckles and the yellow-root are valuable for their foliage, and the flowering raspberry and prairie rose for their flowers. Most plantings in such situations will require considerable care and watering during the first two years after transplanting. But subsequent to this time these plants, if properly selected, will continue to thrive, having been thoroughly acclimated to the new location.
Another valuable group of ground-cover plants are those which are used to fill crevices between stepping-stones or between the flagging of paved terrace areas. This list is composed of small-flowering and foliage plants, both perennials and annuals, which are usually planted in small soil spaces between the stones. Their greatest value is to relieve the monotonous, bare effect of walks and terrace areas, to which so much interest would be added by a touch of colour, either in foliage or flowers, obtained through a proper planting of well-selected material included in this group. Many of these plants, if left to themselves, will after the first two or three years spread rapidly and will require much attention to keep them within the proper limits. Many paved walk and terrace areas are overplanted and consequently a careful selection of a few of these plants is much better than an over-supply. The rock cress, sea thrift, stonecrop, and creeping phlox are types which are adapted to the small, refined spaces, while for terraces on a large scale, the dwarf iris, evergreen candytuft, tunica, and speedwell are plants which should be used. It is quite probable, in many instances on paved areas which to be on a firm foundation have but a small layer of loam between the stone and the cinders, that these plants will be frequently winter-killed and require replacing. If it so happens that these paved areas can be successfully laid, because of local climatic conditions, upon a good depth of sandy loam, then these plants must seldom be replaced, but rather frequently thinned out.
There are many indigenous mosses which can readily be transplanted in tufts to fill the crevices between the stones on paved walks, thus presenting the appearance of age during the first years after construction. Most of the mosses require a considerable quantity of water to make them thrive. There are a few varieties, however, found in open, sunny locations, that will thrive with little moisture. Therefore, before using moss to fill the crevices between flags on paved areas, the natural habitat of the moss to be used should be known.
In one of the former groups of plants for dry locations and for embankments, the discussion was directed toward the effects of drying out. There are instances where the open exposure and the effect of the sun develop a situation requiring plants that will withstand extreme sun exposure. These plants can be adapted to light, sandy soil, and they form a small group, with the stonecrops, the Adam's needle, and the maiden pink as typical varieties, which may be planted under the most adverse conditions of exposure and sun.
Quite often one finds plantations of rhododendrons and azaleas which have fallen just short of being really interesting because of the lack of some ground-cover planting to give the added and desired touch of interest. This may be for the purpose of relieving the bare ground around the edges of the plantation, so often covered in a successful manner with Japanese spurge, periwinkles, ferns, and andromedas. It may be that one desires a touch of colour so often obtained by the introduction of the different types of lilies, which can be successfully grown in plantations of hybrid rhododendrons. Especially in plantings of large and native rhododendrons, many of these ground-cover plants can be introduced with a great degree of success to relieve the "leggy" appearance of these plants and to make an interesting mat over the ground which might otherwise be more or less bare. The ground among rhododendrons is subject to more or less heavy shade for two reasons. In the first place, the rhododendron foliage itself provides considerable shade at the base of the plant, and the nature of the rhododendron plant requires shade for its successful growth. Consequently, these ground-cover plants should be such as are adapted to the general conditions of woodland shade. It is inadvisable to cultivate the soil around the base of rhododendrons and azaleas. These plants ought to be such that when once planted they will require no further cultivation other than the addition of a small amount of leaf mold from year to year, to provide the necessary food supply.
The last and one of the interesting types of ground cover is that used in the development of rose gardens, to provide a mat of foliage or flowers between the rose bushes. Plants used for this purpose, such as the tufted pansy, the common verbena, and rose moss, ought to be shallow-rooted types, with low, spreading characteristics. The reason why a ground cover is desired in a rose garden is that during a portion of the summer months the ground is often bare. There is an argument, however, against the use of any ground cover throughout the rose garden in that the constant cultivation which is the best aid to the good development of roses cannot be done. Many of the successful English rose gardens are filled with these ground-cover plants. Where plants of this kind are used the roses should be well cultivated in the early spring and should be well cultivated again in the early or late fall, and they should be well fertilized also to insure sufficient food in the soil to provide for both the growth of the ground-cover plants and the roses. These plants, all of which are interesting for their flowering habits, provide an interesting group of colour at a season of the year when most of the roses have passed the height of their bloom.