Establishment and Management of Prairie Grasses
Royal Horticultural Society Research, UK, 2007
Establishment of North American prairie grasses by field sowing was investigated at the Royal Horticultural Society Garden at Wisley. This experiment is part of a larger programme of work to investigate the use of North American prairie wildflowers and grasses as a style of planting in gardens and parks in Britain, which is a modern, informal and low maintenance. It is particularly appropriate for amenity planting.
Grass species
Schizachyrium scoparium Prairie grasses with high horticultural potential include little bluestem, (Schizachyrium scoparium); indian grass, (Sorghastrum nutans) and prairie dropseed, (Sporobolus heterolepis). All three species are widely used in contemporary garden design in Europe and North America. They also have a considerable tolerance of drought, which can be an important characteristic on sandy soils in southern England.
The Schizachyrium population used in the study has blue grey foliage in summer, which turns red in autumn, with this colour being retained throughout the winter. Sorghastrum is a taller grass with attractive bronze flowers in September, and apricot autumn leaf colour. Sporobolus is the slowest growing of the three but has brilliant green, narrow leaves and forms a fountain-like tussock. The leaves turn orange in autumn.
Germination of seed
Our goal was to see whether it was possible to establish these grasses by sowing seed in situ rather than transplanting. This is potentially an economically attractive technique when large areas of grasses are required as part of a naturalistic planting scheme.
All three of the species used in this study are categorised as warm season (C4) grasses; they typically require relatively high soil temperatures (above 20°C) and moist soils for successful germination and establishment. This poses problems for establishment by field sowing in Britain as a result of typically low soil temperatures.
Grass sowing treatmentAn experiment was designed using mulches to increase soil temperature to promote seed germination. The experiment started in the spring of 1997. Untreated seed was sown in May at a density of 550 seeds per sq m onto topsoil plots (control). This was a baseline for comparison with the following mulching treatments. These were topsoil with charcoal mulch, and topsoil covered with transparent, perforated crop polythene post sowing.
Site assessments
Seedling establishment was recorded on all treatments. The greatest number of seedlings was recorded in the clear polythene mulch followed by the charcoal mulch and the control with no mulch.
Weed control by clippingCompetition with weeds is a major factor limiting the establishment of new seedlings. Weed growth that developed on the plots was managed by cutting with shears to 75mm above the soil surface. Hand weeding was not attempted; we wanted to see if much cruder weed control would be sufficiently effective.
In late October the growth of all of the prairie grasses during the summer was assessed. The plants were cut off at ground level with shears, placed in bags, dried and weighed to establish the mean weight of each species in each of the three treatments. The smallest plants were those in the control, the largest in the charcoal and polythene mulch plots. There was no statistical difference between dry weights in the charcoal plots and the polythene plots. Some plants of Schizachyrium and Sorghastrum were large enough to flower in September. Dry weight of all grass species in October was strongly negatively correlated with percentage weed cover in June indicating that weed growth had an impact on seedling growth. The weight and therefore size, of all the grasses was reduced by the growth of weeds.
Implications
The study shows that prairie grass establishment by direct sowing is possible in the south of England, especially when soil temperature is elevated by either charcoal or clear polythene mulching. Overall the charcoal treatment was most successful as this mulch also reduced weed cover early in the first half of the summer. Management from 1998 onwards has been restricted to burning the dead grass leaves in spring each year with a propane gas flame gun a technique which is less laborious than hand weeding, and could be used in larger scale plantings without the use of herbicide. Five years after establishment the plots that supported a high density of prairie grasses in October 1997 (mostly charcoal and clear polythene mulch) are still the most successful, and are largely weed free. This shows that prairie grass planting can be achieved cost effectively using seed sown in open ground, in southern UK. The management regime for this planting could be adapted for the amateur horticulturist.
This work was carried out by the University of Sheffield at RHS Garden Wisley. The summary was written by Professor James Hitchmough, who carried out the work.