Herbaceous plants regenerate by either vegetative means via stolons/rhizomes or from seed, and in some cases by both. Species that can do this successfully in naturalistic vegetation are useful (providing they are not too aggressive), in that they are more likely to be able to fill in gaps in planting and to compete with invading species irrespective of initial sowing or planting density. This process potentially makes such communities sustainable in the long term, by providing a buffer against the loss of key species to aging, competition or herbivory.
All herbaceous plants spread outwards as they add another ‘layer’ of growth points to the previous year’s growth. In many species this process slows down with aging, restricting the area any one plant can cover. This area may then diminish as individual plants age and decline, as, for example, in Achillea species and Echinacea purpurea. Other herbaceous species are more permanent and they slowly grow larger, as in the case of many Aster and Geranium species. The next group of species are those that spread aggressively by vegetative means, for example Filipendula ulmaria, Inula hookeri and Euphorbia griffithii, forming what are known as ‘clonal’ patches as a result of competitively displacing sown, planted or spontaneously occurring neighbours. With these types of forbs, a monoculture is likely to result in the longer term, unless the forbs chosen all share the vigour and growth habit of the most aggressive species present. The spread and persistence of clone-forming species is often relatively independent of management. Species with these growth habits are, in any case, essentially immortal and, therefore, the need to employ management to aid persistence is greatly reduced.
Species that have the first growth habit rely on regeneration from seed for their longterm persistence and are much more dependent on management. This is clearly most marked in annuals and biennials. The successful establishment of seedlings in naturalistic herbaceous vegetation is often an occasional event for the following reasons:
– low seed production or heavy seed predation by weevils or birds
– insufficient germination microsites free of competing plants
– a layer of moss or organic debris restricting seedling access to mineral soil
– intense slug predation on seedlings
– the elimination of seedlings through shading by surrounding adult plants.
As the potential productivity of a landscape planting increases, the likelihood of species establishing successfully from seed in established vegetation decreases. Germination and establishment requirements often differ considerably even between species in a genus, for example in Solidago, see Goldberg and Werner (1983). In general, however, breaking up layers of organic surface debris to improve seed access to mineral soil, and increasing light availability at the soil surface, are important. In grass-dominated native meadows, seedlings of many species often germinate in late summer to autumn, as well as in spring. Cutting these meadows close to the ground in late summer to autumn, followed by heavy scarification of the surface, appears to be effective. In plant communities not subject to defoliation during the growing season, for example North American prairie vegetation, seedling germination and emergence rarely takes place before March or April. Seedling establishment can often be promoted by avoiding burning or similar forms of management in that year. This assumes, of course, that the community is relatively weed – free. Seedling survival will be further improved by high mowing to maintain higher light – levels for seedlings in early summer. Even with these inputs, the author’s experience suggests that relatively low levels of seedling establishment should be anticipated.
Within communities of annual and biennial species, management to promote germination from the previous year’s seed is essential if the community is to continue to exist. This often takes the form of shallow cultivation to disturb surface debris and weed colonisation, and to stimulate buried seed to germinate. On sites that have become heavily colonised by perennial grasses, the use of a nonresidual herbicide, such as glyphosate, prior to the germination of the desirable annual species may be necessary, as few annual species can germinate and establish satisfactorily in the presence of adult plants.
Naturalistic herbaceous vegetation is a potentially valuable addition to the repertoire of planting styles for public and institutional landscapes. In aesthetic terms, it is likely to challenge the preconceptions of many lay observers, however there are reasons to believe that this will gradually change as this type of planting becomes more common. At present, this is hampered by a lack of understanding at both a scientific and practical level of these types of vegetation, however this is gradually changing. Successful creation and management does, however, require that designers, clients and managers are prepared to adopt a more ecological approach than is normal in conventional herbaceous vegetation. It remains to be seen whether this will materialise in practice.