Effects of Burial in Sand and Water Supply Regime on Seedling Emergence of Six Species

Probing the depths

The sowing instructions on seed packets that I buy from my local garden centre are very clear about the depths at which seeds of particular species should be sown. But does it really make any difference? In the wild, seeds have no human agency to regulate the depth at which they are buried, if indeed they are buried. However, depth of burial has been shown to affect germination in a number of species and this has been confirmed by the work of Zheng et al. (pp. 1237�1245). Their motivation was the need to rehabilitate areas of affected by desertification. To do this it has been customary to scatter seeds from the air, but the success rate is very low. Suspecting that this seeding method did not expose the seeds to ideal germination conditions, the authors have investigated the effects of depth of burial in sand and of water supply on the germination of seeds of six species typically used in re-seeding operations. The results are very clear. Firstly, seeds that remain on the surface germinate very poorly, whatever the watering regime. In different species this failure may be ascribed to desiccation or to the inhibitory effects of light. Secondly, nearly all species are affected by the amount of water they receive, with more copious water supply generally leading to greater germination success. Thirdly, depth does matter but how much it matters varies between species. At one end of the scale are species such as Artemesia sphaerocephala, which show a very �tight� optimum depth of 5 mm; at the other end is Medicago sativa, in which germination success is similar at all depths from 5 to 40 mm but then declines sharply so that there is no germination at 60 mm. Other methods clearly need to be developed for re-seeding the desert (and perhaps I should pay more attention to instructions on seed packets!).