Development of an alternative coating (pelleting) technique for eucalypt seed.  B. Roberts.  August 2000.  22pp. (unpubl.)

SUMMARY

Increasing the particle size of eucalypt seed, by coating (or pelleting), assists in spreading the seed in ground-based forest regeneration operations. Until recently, mucilage and kaolin powder have been used to coat the seed. Despite difficulties being identified with this current pelleting technique, it has been in use for many years. Worsening problems with the supply of mucilage and the low stocks on hand have necessitated the development of an alternative operational seed coating technique.

A literature review showed that there is little information available with respect to the coating of eucalypt seeds, or tree seeds in general.

In developing an alternative coating technique it became apparent that there were competing demands and that compromises were required. Coated seeds must meet the needs of operational use by having suitable toughness for bulk handling during storage and sowing operations, while the coating medium also needs to disintegrate readily in the field to enable effective germination.

Investigations were undertaken to derive an appropriate coating formula. The combinations previously documented, using methyl cellulose/lime or Elvanol^®/silica flour, proved to be ineffective. Therefore a more exhaustive process, using trial and error was used to find a suitable blend of components. A suitable sticker solution that met all objectives was difficult to achieve. Eventually a mixture of egg white powder, methyl cellulose, water and glycerine was found to be successful as a sticking agent to bind kaolin powder to the surface of individual seed particles. This technique when applied at an operational scale successfully produced coated seeds to meet field regeneration requirements. The other coating powders that were trialled were not as fine as kaolin. As a consequence they failed to produce pellets of the same quality.

Both laboratory and field trials indicated that Eucalyptus regnans seeds coated in this way did not show any inhibitive effects on germination. In fact, the coating material appeared to have a beneficial effect on the germination rate, resulting in increased percentage germination at the conclusion of laboratory seed tests compared to uncoated seeds after the same period. Similarly, a field trial showed that coated seeds germinated more rapidly and subsequently produced a higher number of seedlings (seedling percent 4.3%) compared with uncoated seeds (2.5%) after 12 months.