Persoonia pinifolia seed propagation... I hope!

Persoonia pinifolia is a notoriously difficult species to propagate by seed. So I've set myself the challenge!

Image: Australian National Botanic Gardens

Also known as the Pine-leaved Geebung, this species of shrub is indigenous to a small region of Australia, between Broken Bay to the Royal National Park and the lower Blue Mountains. The difficulty in germinating appears to be caused by the hard, dense endocarp surrounding the seed embryo.

On the left, is an unripe fruit (a drupe), as I collected it from the shrub. In the centre, a ripened drupe. I facilitated the ripening by keeping it in a paper bag along with a ripe banana for a few days. Bananas when mature release the gas ethylene, which is an important plant growth hormone. (If you have a bunch of bananas that aren't ripe, and you put them in a bag with a ripe banana, all will ripen quickly. Handy, huh?)

On the right is a seed with the flesh of the drupe--the seed's exocarp--removed. What you can see is the endocarp, and inside that is the embryo.

Many species of seed need to have their endocarp breached in some way to allow the influx of water and oxygen so the embryo can germinate. There are many ways of doing this, depending on species and their evolution. Sometimes, the seed is placed in boiling water, as with Acacia seeds. Another methods include abrading or piercing the endocarp, or burning the seed. Seed dormancy can also be overcome by stratification (chilling) in the case of seeds from alpine and semi-alpine habitats, or by the application of water through which smoke has been bubbled.

Just about everything has been attempted on Persoonia seeds by various horticulturists and plant scientists, without much success. One paper, "Propagation of Persoonia virgarta for the development of a new floricultural export crop," by Lynda Maree Bauer (ne Ketelhohn) and Margaret Johnston of the University of Queensland, suggests that one of the major factors inhibiting germination is the sheer hardness of the endocarp. Merely abrading or piecing it is insufficient, suggesting that there is in part a simple mechanical barrier to seed growth.

So taking their research as my guide, I am going to try to break down the endocarps. Having removed the exocarps, I'm now soaking the seeds in water in an attempt to soften the endocarps by fermentation, and then will use pliers to remove them. The risk here, of course, is that I damage the embryos, and there is also a real possibility of contamination by bacteria or fungi.

Thus it might all end up in a heap of failure, but it's worth a try. If I don't succeed, the next step will be to try propagating Persoonia pinifolia by cuttings. Apparently that's not always successful, but more efficacious than by seed. Watch this space!