Depth of sowing

In general, seeds of many species of native grasses are equipped with ancillary structures that aid in their germination and establishment if they are surface sown. Available evidence suggests that, at least with some species, (e.g. T. triandra) best results are obtained by surface sowing untreated dispersal units. If post-harvest seed treatment removes these structures or renders them inoperative, then the seeds must be sown at an appropriate depth for the species and conditions and close contact between the seed and the soil achieved. If the seeds do not have these structures (such as seeds of Panicum or Eragrostis spp.) then it is essential that they be sown at an appropriate depth or adequately covered if sown on the surface. If whole dispersal units are to be surface sown onto bare soil prone to surface crusting, the soil should be cultivated first and the seed sown as soon as possible to enable sufficient burial of dispersal units prior to rain.

There is a general association between large seeds and the ability to emerge from deeper sowings when seeds without ancillary structures are used. This attribute could be an advantage in certain environments as seeds which are deeper in the soil have more time to germinate following rain and a greater chance that their radicles will keep ahead of the drying front than those near the surface. However, there are likely to be interactions among species, sowing depth, soil type and water availability.

Most of the depth of planting studies using Australian grasses have only involved one or at most two species and, not surprisingly, the results have depended on the species involved and the experimental conditions. Most such studies have been under artificial conditions because of the difficulties involved in controlling the depth of planting of small seeds in the field. As a consequence, the results are often conflicting because the successful establishment of a seedling depends very much on the micro-environment in which each individual seed and developing seedling is situated.