Eucalyptus Seed Collection: The Importance of Good Timing
Recolección de semilla de eucalipto: importancia de la época de recolección
Eucalyptus seed is the easiest and most common type of germplasm for propagation of species in this genus. Depending on the species typical flowering habit (profuse or shy, from early ages or taking several years, in just one season of the year or during several, etc) and the particular conditions of a specimen tree in its environment (soil and climate matching native habitat for the case of eucalypts grown outside of Australia, microclimates in the case of native habitat) flowering age can be quite variable. For some species the timeframe from planting to flowering can be as short as 1 year, but for others it can easily take 10 years or more. Due to this variability there is an important and ongoing task assigned to every grower of eucalypts: observing the phenology of every cultivated species and keeping comprehensive records of this for each local planting area in the garden record books.
After successful flowering and pollination, development of fruits begins. The timeframe between flowering and seed maturation is normally long enough to allow the grower a regular observation and preparation for seed collection at the right time. This maturation process has, again, a variable duration. It can be as short as 1 month for some species, but normally falls in the range of 8 to 16 months after flowering. Which means that mature capsules containing viable seed can be found in an individual tree at the same time immature capsules and newer buds or blooming flowers can be found. In addition, for many species mature capsules remain on the tree for up to 2 years after capsule valves are partially open, and they retain some viable seed, especially at the bottom part of the capsule, the last part releasing seed. So it is perfectly possible finding an eucalypt tree having at once flower buds in formation, blooming flowers, pollinated but yet immature capsules, fruits reaching maturity within the same year of collection, and older senescent capsules of at least the previous two seasons.
Eucalyptus Seed Collection / Recoleccion Semilla Eucalipto
Fig. 1: Maturation stages for fruits collected on Corymbia calophylla trees in a single visit, showing some of the internal organs within Eucalyptus infrutescences in each phase.
1 - Pollinated flower: It has lost its external reproductive organs (stamens and style), starts forming seeds within the hypanthium.
2 - Immature capsule: Size increases as capsule walls grow thicker and seed continues its development within the ovary chambers (loculi)
3 - Semimature capsule: Size increases and external colour and texture starts change to brown and coarse. Fruit walls start becoming woody. Some seed might be viable already.
4 - Mature capsule: Capsule walls become totally woody, and external colour is totally brown. Seed valves star to open. Majority of viable seed is now in the right stage for collection.
5 - Senescent capsule: Reached maturity 1 to 2 years before visit. Seed release can be gradual, so they can be totally empty or retain part of the viable seed crop.
Eucalyptus seed collection from specimen trees is best done in a way that does not compromise further seed production. Harvesting flower buds, blooming flowers or immature capsules has no benefit from the point of view of obtaining viable seed. Fruit harvest should focus on mature (4) and senescent (5) capsules thus allowing the trees to provide us with a further crop the next year. If selective harvest is not possible, full branch cutting from specimen trees should be restricted to just a small total of tree crown thus allowing easy recovery, low stress and good chances for a heavy flowering in future. Seed orchards and plantation plus tree seed harvest are another story.
© 2007 Gustavo Iglesias Trabado / GIT Forestry Consulting - Consultoría y Servicios de Ingeniería Agroforestal