A next logical step along the continuum of using native plants or native plant communities in the designed landscape is to depart from single-species ground-cover plantings by planting a diversity of species in mixes which are matched with the soil/light/moisture characteristics in different zones. Using the upland Piedmont landscape example again, a semi-shaded area might be planted with a mix of bracken fern (Pteridium aquifolium), fire pink (Silene virginica), bluestar (Amsonia tabernaemontana) and blue-eyed grass (Sisyrinchium atlanticum). In deeper shade, a ground layer mix might combine Christmas fern (Polystichum acrostichoides) with woods phlox (Phlox divaricata), wild geranium (Geranium maculatuni) and hepatica (Hepatica nobilis). With mixed plantings such as these, the likelihood of a climatic extreme or disease or insect pest eliminating all the plants in the ground-cover bed is greatly reduced. Additionally, a wider array of plant species simply provides greater potential for aesthetic richness with a range of textures, colours, and flowering times, than does a single-species planting.
The practice of ground-layer diversification can range from combining two or three species in a few square feet to meadow-like plantings of 20 or more species replacing large lawn-areas. As in any planting design, the two components of this process are: the selection of species and designating their placement at the time of planting.
Species selection has both an ecological and an aesthetic basis. Ecologically, of course, it is essential that each species be placed in the appropriate soil/light/moisture environment. Aesthetically, colour and texture relationships, in particular, can be planned to create specific visual effects. Selecting from ground-layer species that occur together naturally in a native community, one may select analogous, or closely related, colours, as in the woods phlox-wild geranium-hepatica example mentioned earlier, with its pale blue-violet-pink flower colour combination. Or one could select complementary, contrasting colours, as with the yellow-flowered green-and-gold in combination with the blue-flowering crested iris (Iris cristata). In selecting plant textures, fine, fern and grasslike textures provide refinement and a soft appearance, creating a good backdrop for more dramatically textured species. An example of this might be the fine New York fern (Thelypteris noveboracensis) in a mass, with the bold, broad foliage of Mayapple (Podophyllum peltatum) as a contrasting texture.
In determining ground-layer plant placement patterns, the method of planting comes into consideration, i. e. whether seedlings will be individually placed or seed will be sown. The planting of propagated plants lends itself to shaded woodland plantings or to sunny borders. Seeding is best adapted to larger expanses, such as meadowlike plantings. The advantages of planting seedlings include the control one has over the placement of individual plants, and the competitive advantage such plants may have over weed species whose seed is in the soil. The disadvantages of using seedlings include higher costs and, in some cases, the limited availability of plants in the quantities required. Seeding has the advantage of being relatively inexpensive and often leads to a more natural distribution of plants, but has the disadvantage of being slow to establish and is therefore more vulnerable to weed invasion.
When starting with propagated plants, a useful model is the mingled ‘drift’ pattern which occurs so abundantly in naturally evolving landscapes. Using the ‘drift’ approach, an individual species is planted in a higher density in the centre of any particular group, but with more widely spaced individuals trailing away from the group. As one species diminishes in density, a second or third species increases. Thus, dynamic interactions between species are created. Because of the potential for the reproduction of the various species, open spaces may be left within mixed ground-layer plantings, just as there are open spaces on the natural forest floor. Up to a third or half of areas designated for woodland ground layer may be permitted to remain open initially, thereby providing an opportunity for the originally installed plants to spread into those spaces.
The alternative of planting by seeding, as noted above, lends itself particularly well to the planting of sunlit meadow-like communities. Using this technique, pattern is achieved through varying the composition of seed mixes in adjacent zones, leading to contrasts in colour and texture at a broad scale.
In such plantings, as in the natural prairies after which they are modelled, grasses play an important role, functionally and visually. They are effective at erosion control, with a network of deep roots beneath the soil and persistent leaves above the ground that intercept rainfall. Visually, they provide a linear filter of the broad-leafed and showy forbs or wildflowers with which they are planted. Further, their foliage tends to remain standing through the winter, providing structure in the winter landscape, often with the added benefit of rich, warm colour, as in the copper colour of various Andropogon species, and the gold and tan of switchgrass (Panicum virgatum) and Indiangrass (Sorghastrum nutans). Grasses might well constitute 80% of the seed mix in a meadow planting, with the remaining 20% comprised of a variety of forbs or broad-leafed wildflowers matched with the micro-environment.
In determining seed distribution patterns, zones that tend to ‘flow’ most closely emulate the patterns in natural landscapes, and relate well to undulating topography. In adjacent zones, there will be some variation of species, both grasses and forbs, but there should usually be some continuity of species between zones in order to present a soft ‘continuum’ effect as opposed to clearly defined lines between zones.
There are no hard-and-fast rules determining how many different species should be incorporated in a particular area of meadow/grassland planting, but some guidance can be drawn from earlier grassland studies. Most of the research on this community type has been done in the US Midwest, where a typical species density of 40-70 species per acre has been observed. By far the greatest proportion of the vegetation is made up of a small number of grass species, with the rest comprised of a large number of forb species in relatively small quantities. Translated into seed-mix terms for a zone within a meadow/grassland planting, this could suggest three or four grass species making up 80% of the mix, and 10 to 15 forb species constituting 20% of the mix. An adjacent zone might have a similar ratio of grasses to forbs, with, say, two of the same grass species carrying over into it and five to eight of the same forb species, whilst there may be a greatly reduced grass component in zones where a high-intensity floristic display is required. There would logically be less continuity between zones if there were sharp environmental differences between the zones, for example a low, poorly drained area bordered by an upland slope.
In implementing mixed grass-and-forb plantings, the seed of all grasses and forbs for a particular zone may be mixed and planted, either through broadcasting or drilling. Or the grasses may be drilled mechanically, and the forbs broadcast in ‘drifts’ within the zone. The second option provides a greater opportunity for controlling the distribution of forbs, and perhaps more closely emulating natural distribution patterns