Seed Cleaning, Post and photos by Forrest Edelman
I am Forrest Edelman, I work for the Center for Natural Lands Management within the Nursery Department and perform the functions of a seed processor.
After flowers have finished and seed pods have ripened, the plant material is harvested and laid out to dry in a drying shed. When the material is good and dry it is binned up and goes to the seed processing shop for the separation of seed from the rest of the material. The species being processing here is Primula pulchellum or the Few Flowered Shooting Star.
Most of the seed falls out of the open cup style pod during the drying processing, for those that are caught in the pods and between the material, they will be sent through a hammer mill to dislodge the remaining seed.
The hammer mill is a threshing type machine which uses a motor to spin and swing nylon rectangles which abrade the material in a closed metal compartment, thereby mechanically dislodging seed from inert material.
The material is fed down a metal chute through a metal gate into the threshing compartment. All of the seeds and plant material will be threshed and dropped in a single steam, to be screened down next. In short; when the machine is spinning, material is fed into the machine and it lightly chews it up and spits the mess into a bin to be moved on to the next step.
Removing most of the large chaff with a brass sieve is the next step. It consists of dropping handfuls of material into the sieve and shaking the smaller pieces of plant material and seeds through then removing empty pods, sticks, and other large chaff.
Afterward the material should be more even in size and drastically reduced in bulk, which makes it easier to load into the next machine, an office clipper.
An office clipper is an air screener separator machine, it allows seeds to drop through a larger top screen and get rid of more chaff.
The lower screen allows for fine chaff and dust to fall through but seeds and material of similar size roll off the lower screen.
This in between size material falls into a compartment which has air blowing up through it. Seeds are generally heavier than dried plant material fragments of the same size, so the air stream will winnow away the remaining chaff and drop good seed into a bin. After a couple of runs though this machine the final product is seeds.
This is just one method for one species. There are many ways to reach a final product for each of the 80 or so plant species processed every year.