Bracken litter as mulch: glasshouse evaluation of phytotoxicity

Abstract

Summary. Austral bracken (Pteridium esculentum) has a major impact on forestry and the pastoral industries in Australia and New Zealand, as does P. aquilinum in parts of the northern hemisphere. There is a large body of, often conflicting, evidence on the toxicity of P. aquilinum to growth of other plants but there is little evidence for P. esculentum. Control of bracken is currently expensive and difficult to maintain as there is neither a marketable product based on bracken, nor a significant use for it on site. Current interest in commercial uses for bracken as mulch or compost requires analysis of the effects of bracken fronds on germination and growth of a representative range of plants.

We report the effects of a mulch of chopped recently-dead (standing) bracken fronds on germination and early growth (to 9 or 12 weeks) of white mustard (Sinapis alba), of the improved pasture species, white clover (Trifolium repens) and perennial ryegrass (Lolium perenne), and of 2 native woodland species brown barrel (Eucalyptus fastigata) and yellow teatree (Leptospermum polygalifolium). A control mulch was prepared from chopped recently-dead bracken fronds by boiling and washing to extract possible allelochemicals and destroy associated microbiota.

Bracken mulch stimulated seedling growth of white mustard, white clover, perennial ryegrass and brown barrel to a statistically significant, although moderate, extent (8–20%). Shoot length of perennial ryegrass and brown barrel, and leaf number and leaf size of brown barrel were more markedly enhanced (≥30%) by the application of bracken mulch. In contrast, bracken mulch reduced germination of yellow teatree to 25%, and leaf number, during early growth, to about one-third of that in the controls. The effects of bracken mulch may be due to release of active phytochemicals and mineral nutrients, with or without mediation by associated microflora including mycorrhizae.

These glasshouse experiments suggest that recently-dead fronds of P. esculentum may prove a generally useful mulch, although monitoring for adverse effects on establishment and growth of particular species will be required.