Sowing Depth

For turfgrass seed to germinate, emerge as a seedling and establish requires a range of conditions for a successful outcome. The depth at which the grass seed is sown is one factor which can influence the outcome. Too shallow a depth and the seed can become more susceptible to desiccation, inadequate soil moisture to explore once germination has occurred at the surface, as well as being more readily consumed by birds as seed is more likely to be exposed at shallow depths, especially if windy conditions arise following seeding, potentially uncovering the top-dressing protecting the seed. Temperature variation will be more variable the nearer the surface and might influence seed growth, although this will be most likely be a minor consideration, or insignificant. The size of the seed, typically related to the species and also the cultivar, will also influence the depth at which a seed can be sown, with smaller seeds typically being needed to be sown at a shallower depth than larger seeds.

If a seed is sown too deep then whilst germination may occur, the seed might have insufficient energy resources to be able to adequately push up to the surface and expose its primary leaf to sunlight. Where the seedling does establish this can initially be slower than for seed at more suitably sown depths therefore resulting in slower establishment, although generally catching up over time, which can be a particular problem if the time from renovation to start of play is relatively short, which is the case for many sports surfaces. Too deep a sowing and the seed may also have too much moisture retained within its locality, resulting in seed rot.

As a guide the optimum sowing depth, for all three conditions of germination, seedling emergence and initial establishment for a range of different species and cultivars can vary slightly. As a general guide a sowing depth of 10mm could be planned, although minor adjustments to decrease or increase sowing depth could be made to improve the overall outcome slightly.

Where several evenly spaced observations are carried out on a daily basis and responses to findings are undertaken, for example. additional watering where drying out has occurred, or additional light top-dressing where seed has become exposed, then slightly shallower depths could be considered, if appropriate.
• Perennial ryegrass: 10-15mm where adequate moisture maintained; 10-20mm (or to 25mm) where drier conditions are expected.
• Smooth stalked meadow grass: 15mm-20mm, with this grass being slower to germinate and establish, having too shallower a sowing depth increases the likelihood of drying out, without realising it because expectations are that the slowness to emerge is due to the nature of the species rather than environmental conditions.
• Browntop bent: 5mm where moist up to 10mm where drier.
• Red fescue: 10mm in moist conditions and 15mm where it is drier.

Where oversowing takes place on established fine turf surfaces, especially golf or bowling greens, then the top-dressing application rate might only be a few miliimetres, giving minimal protection to the seed. Where seed has not settled into pre-seeding preparation work such as scarification grooves, dimple seeder holes, or aeration tine holes, then there is greater potential for seedling loss, even though initial germination might be very high, so post germination monitoring and nurturing will be greater than for where seed has been sown slightly deeper, for example 4-8mm, although this can be quite nuanced in determining optimum success.

References:
• Poa pratensis, Barenbrug.
• Beard, J. B. (1973) ‘Turfgrass: Science and culture’, pp512-513
• Shildrick, J.P. ‘Lolium perenne for renovation, with and without other species’, in The Journal of the Sports Turf Research Institute, No. 55, 1979, pp82-88.
• Shildrick, J. (undated) ‘Turfgrass Manual’, p3.