Panning for seeds

Sorbus pseudohupehensis in October: one of the many species of Sorbus that produce genetically identical seedlings through apomixis. 

My post 'Celebrating Sorbus' (8 December 2013) prompted a colleague at an American botanical garden to request some seed, so before Christmas I gathered bunches of fruits of different species to extract seed from. Gathering seed from any collection of garden plants is fraught with the risk of unwelcome hybridisation and in general such seed is not useful where accurately named material for research is needed. But many species of Sorbus, as with other members of the Rosaceae, produce seed by apomixis, which  means that there is no fertilisation and no recombination of genes and that in consequence the seedlings are genetically identical to the parent plant - and thus safe to distribute.

The fruits of each species ripen at different times and become variably softened as they do so. Some of the pink-fruited ones, like S. pseudohupensis and its relatives, remain very hard for a long time into the winter (which is why they give such a long season of interest), while many orange- or white-fruited species have softened and gone long ago. To encourage the softening process, as extracting seed from hard fruits is a very tough job, I bagged each collection in a plastic bag and left them to sweat in a warmish place (nowhere in this house being aptly described as warm).

Sorbus and Cotoneaster fruits going squishy in sealed plastic bags.

 Today, having some spare time, I cleaned the seeds, using a very useful technique I learnt when cleaning tomato seeds long ago in Holland. The fruits are squashed firmly but gently, breaking open their skins and releasing the bulb and seeds. This is most easily done by squeezing the fruit through the bags. After this the squashy (and somewhat smelly) mess is tipped into a bowl of water and stirred round. Another rub of the fruits will wash the seeds out of the pulp. Being the densest objects in the 'soup' the seeds will sink to the bottom of the bowl and the light, floating sludge can be be gently poured off the top.

The first stage of cleaning: fruit pulp in a bowl of water.

This will leave a little heap of the heavier material at the bottom, with a certain amount of extraneous gubbins. Clean water is added and successive washings and stirrings, with the floating debris being carefully poured off each time, will leave almost pure clean seed in the bottom of the bowl. It's exactly the same principle as that employed in panning for gold - the heaviest particles are left at the bottom of the pan - but can be done in the comfort of the kitchen, and with the certainty of finding something useful at the end. The number of washings will vary but two or three will usually suffice.

After the final wash: almost pure seed left in a little water.

A final swirl of water is used to wash the seed into a sieve, from which it's tapped out onto a sheet of kitchen roll. There will be a few impurities left, but these are most easily removed once the seed is dry by the usual blowing and vibrating techniques used to purify a seed sample. I'm giving the seed I'm keeping  a couple of weeks in the fridge before I sow it: most Sorbus seed needs a long cool period for best germination, so this will at least help them along the way.

Almost pure Sorbus seed knocked out of a sieve onto kitchen roll.