Prairie vegetation is naturally restricted to North America, where it stretches from the east of the Rocky mountains to the Appalachians (Figure 6.4). Its most northerly occurance is in Saskatchewan, Canada (45°N), and its most southerly, Texas. Prairie is a recent semi-natural vegetation, having developed within the past 10,000 years. Humans have played a major role in its evolution through the annual aboriginal burning of woodlands and savannah in combination with the grazing of wild ungulates. Without regular burning it is invaded by scrub, trees, weedy forbs and grasses, and slowly declines.
Whilst prairie is very rich in perennial forbs, it is typically dominated by warm season grasses. These are also referred to as C4 grasses, and are more drought-tolerant than the C3 or cool season grasses that dominate European and many montane grasslands in warmer parts of the world. As the name suggests, warm season grasses grow during the summer months and are fully dormant during the winter. The forbs in prairies are largely species that grow vigorously at lower temperatures. As a result of these two different growth strategies, there is, in essence, a window of low grass competition in spring that is exploited by the forbs. When cool season grasses invade prairie vegetation, whether semi-natural or created in urban landscapes, this has a detrimental effect on the vigour and survival of many forbs.
As can be seen from Table 6.5, soil moisture is again a key factor in determining the species composition of prairie, although some species occur across the full range of moisture conditions, for example Aster laevis and Monarda fistulosa. It is, however, unlikely that the genotypes of either of these species in a wet prairie are the same as those in a dry prairie. Species also have specific distributions within the prairie region, in east – west and north-south directions, for example Aster turbinellus is largely restricted to the region from central Illinois to Kansas (Gleason 1963). Most prairie species flower from July onwards. As a result of this and the presence of late developing warm season grasses, unlike most meadow vegetation, prairie is most attractive and ‘tidy’ in late summer and autumn. Prairie grasses remain structurally intact and attractive until the first frosts of autumn, when the foliage of many species turn yellow, orange or red.
As with all grassland vegetation, the height and openness of the community is determined by the combination of soil fertility and moisture availability. Dry prairie is more open than moist, and wet. Species of dry prairie are also typically shorter, for example Asclepias tuberosa, Euphorbia corollata, Liatris aspera, Petalosporum purpureum and Schizachyrium scoparium. They are also more intolerant of shading (particularly when grown in cooler climates) and, when used in designed landscapes on fertile moist soils, are likely to be eliminated by the growth of
Table 6.5. Commonly cultivated forbs and grasses characteristic of prairie vegetation in the northern US—grasses are in bold (derived from Curtis 1959; Gleason 1963; Ladd 1995; plus the observations of the author)
weedy species or taller growing prairie plant neighbours. As with meadows, dry prairie communities are most manageable when established on dry, infertile soils. They have the potential for use on low-volume roof gardens, as discussed by Dunnett (2002), however, they are generally less tolerant of extreme infertility and drought than European dry meadow species (Hitchmough et al. 2003).
Some prairie species are highly palatable to slugs and snails when cultivated in Britain, and this is a key factor in determining species choice. This is discussed in greater detail in the section ‘Creation of naturalistic herbaceous plant communities in practice’. Many of the most robust of the prairie species that are traditionally cultivated in Britain (for example Filipendula rubra and Veronicastrum virginicum) are naturally associated with wet prairies.
Whilst prairie vegetation is generally thought of as a mixture of grasses and forbs, in some situations grass-only prairie communities can be developed. Prairie grasses require warm soil conditions for germination and establishment, but are not subject to mollusc damage and have highly attractive textures, subtle leaf colour and potentially stunning autumn leaf colour. Prairie grass establishment from seed is discussed further in the section ‘Creation of naturalistic herbaceous plant communities in practice’. When planting is employed to create prairie-like communities, other C4 grasses, such as Miscanthus, can also be used.