Sow seeds at a higher density

Supporting evidence from individual studies

1. A replicated, randomized, controlled study in 1997–1999 in a grazed wet grassland in London, UK (Gilbert et al. 2003) found that sowing grass seeds at a higher density did not increase similarity of the vegetation community to that of the target community. The similarity of the vegetation community to that of the target community did not differ significantly between areas sown with seeds at rates of 10, 25 or 40 kg/ha (data presented as similarity coefficients). In 1997, seeds of eight grass species were sown in varying proportions onto bare soil in each of three 15 x 15 m plots at rates of 10, 25 or 40 kg/ha. Cover of all species in each of the nine plots was estimated within 10 randomly placed 1-m² quadrats in June 1997–1999. Similarity of the plant communities to the target community, a mesotrophic grassland, was assessed using the UK National Vegetation Classification.

   Study and other actions tested

   Referenced paper

   Gilbert J.C., Gowing D.J.G. & Bullock R.J. (2003) Influence of seed mixture and hydrological regime on the establishment of a diverse grassland sward at a site with high phosphorus availability. Restoration Ecology, 11, 424-435.

2. A replicated, randomized, paired, controlled study in 2001–2004 in a former agricultural field in Kansas, USA (Dickson & Busby 2009) found that sowing seeds at higher densities increased grass and forb cover, and forb species richness. The results of this study did not allow for statistical significance to be assessed. After 1–3 years, average grass cover was higher in plots where grass seeds were sown at high density (35–78%) than in plots where grass seeds were sown at a low density (23–64%). There was a similar pattern for average forb cover (high density: 38–57%; low density: 24–34%) and forb species richness (high density: 7–8 species/plot; low density: 5–6 species/plot) when forb seeds were sown at high and low densities. The field surrounding the experimental area was sown with native species in May 2001. A few days later, the experimental area was sown with forb and grass seeds at three combinations of densities: low grass/low forbs, low grass/high forbs, high grass/low forbs. Each seeding density was sown in six 2 x 2 m plots. Seeds were hand sown and raked into the bare soil. The entire field was mown in June–July 2001 and 2002, and burned in March 2003. Plant species cover was estimated using a 1-m² quadrat in the centre of each plot in May/June and August/September 2001–2004.

   Study and other actions tested

   Referenced paper

   Dickson T.L. & Busby W.H. (2009) Forb species establishment increases with decreased grass seeding density and with increased forb seeding density in a northeast Kansas, U.S.A., experimental prairie restoration. Restoration Ecology, 17, 597-605.

3. A replicated, randomized, paired, controlled study in 2005 in a grassland site in California, USA (Orrock et al. 2009) found that plots sown with a high density of purple needlegrass Nasella pulchra seeds had more seedlings and plants than plots sown at a low seeding density. Eight weeks after seeding, there were more seedlings in plots sown at high density (344–517/500 cm²) than plots sown at low density (168–327/500 cm²). Twenty-one weeks after seeding, the number of plants remained higher in plots sown at high density (134–418 plants/500 cm²) than plots sown at low density (79–218 plants/500 cm²). In January 2005, sixty-four 1 x 1 m plots were set up in grassland, in eight blocks of eight plots. In February 2005, locally-sourced purple needlegrass seeds were sown onto the surface of tilled soil. Twenty plots (four random plots in each of five blocks) were seeded at high density (1,000 seeds/m²) and 32 plots (four random plots in each of the eight blocks) were seeded at low densities (500 seeds/m²). Parts of the plots were also subject to different grazing treatments. The number of plants in four 500-cm² areas within each plot was counted in March and June 2005.

   Study and other actions tested

   Referenced paper

   Orrock J.L., Witter M.S. & Reichman O.J. (2009) Native consumers and seed limitation constrain the restoration of a native perennial grass in exotic habitats. Restoration Ecology, 17, 148-157.

4. A replicated, controlled study in 2006–2009 in a former arable field in Nebraska, USA (Nemec et al. 2013) found that sowing seeds at a higher density did not alter the cover of seeded species, unseeded species or invasive plant species compared to areas that were sown at low density. Cover of seeded species did not differ significantly between areas sown at high density (3–34 cm) and areas sown at low density (3–28 cm). Cover of unseeded plant species (3–35 vs 3–32 cm) and invasive plant species (0–1.4 cm vs 0–1.1 cm) also showed no significant difference between areas seeded at high and low densities. In March–April 2006, twelve 55 x 55 m plots were sown with grass and forb seeds at a high density (328 seeds/m²) and twelve plots were sown at a low density (164 seeds/m²). All plots were burned in March 2008. In July 2008, invasive plants were sprayed with glyphosate herbicide. Vegetation cover was recorded using five 55-m transects in each plot in June 2007–2009. Cover was measured at six points along each transect.

   Study and other actions tested

   Referenced paper

   Nemec K.T., Allen C.R., Helzer C.J. & Wedin D.A. (2013) Influence of richness and seeding density on invasion resistance in experimental tallgrass prairie restorations. Ecological Restoration, 31, 168-185.

5. A replicated, controlled study in 2005–2009 in an abandoned field in Saskatchewan, Canada (Wilson 2015) found that sowing thickspike wheatgrass Elymus lanceolatus seeds at higher densities resulted in higher thickspike wheatgrass cover and lower cover of non-native species. In two of three comparisons sowing seeds at a higher density increased cover of thickspike wheatgrass (highest seeding rate: 86–98%, lowest seeding rate: 2–25%), however in one comparison there was no change in cover as seeding rate increased (highest seeding rate: 5%, lowest seeding rate: 1%). In two of three comparisons sowing seeds at a higher density reduced cover of non-native cover (highest seeding rate: 6–13%, lowest seeding rate: 60–94%), but in one comparison there was no change in non-native cover (highest seeding rate: 62%, lowest seeding rate: 59%). In June 2005–2007, thickspike wheatgrass seeds were sown at a rate of 30, 300, 600, 1800, and 3000 seeds/m².

   Study and other actions tested

   Referenced paper

   Wilson S.D. (2015) Managing contingency in semiarid grassland restoration through repeated planting. Restoration Ecology, 23, 385-392.

6. A site comparison study in 1985–2012 at nine former coal mine sites in Montana and Wyoming, USA (Rinella et al. 2016) found that sowing grass seeds at a higher density reduced the cover of native grassland shrubs but did not alter the cover of unseeded non-native grasses and forbs. After 8–20 years, the cover of native grassland shrubs was on average lower in fields that were sown with higher densities of grass seeds (data reported as statistical model results). The average cover of unseeded non-native grasses and forbs did not differ significantly between fields that were sown with low, moderate or high grass seed densities (data reported as statistical model results). In 1985–2005, seed mixes of grasses, forbs and shrubs (average 15 species) were sown in 327 fields at nine former coal mine sites. Grass seeds were sown at low (0–4 kg/ha), moderate (>4–8 kg/ha), or high (>8 kg/ha) densities (number of sites for each not reported). Crushed rock and topsoil were added prior to sowing. In 2011 and 2012, vegetation was sampled within 20 frames (20 x 50 cm, seven sites) or 1–3 areas (4.6 x 45.7 m, two sites) evenly spaced along a transect in each field.

   Study and other actions tested

   Referenced paper

   Rinella M.J., Espeland E.K. & Moffatt B.J. (2016) Studying long-term, large-scale grassland restoration outcomes to improve seeding methods and reveal knowledge gaps. Journal of Applied Ecology, 53, 1565-1574.