Harvesting, Cleaning and Storing Seed

Your own seed needs several working steps after the harvest before it can go into the freezer. After threshing, the seeds are sieved and separated from empty seeds, then dried and sealed. Each step ends at a condition you can see or weigh. Keeping to the order gives you a clean batch that stays viable for years.

When are the seeds ripe for threshing?

Ripe seeds are hard and have turned from green to brown, and the harvest date follows this. No source gives a number of days from pollination, so the seed decides and not the calendar. After that, the whole plant is dried upside down or only the flowers without stems, and the second way is faster.

Threshing only starts once the flower material is completely dry. Threshing means freeing the seeds from the flowers. Freshly threshed seed, by contrast, still carries 22–30 % water and can heat up after just two to four hours.

  • Only thresh when the small stems snap as you bend them.
  • If you thresh fresh, spread the seed thinly or ventilate it within three to four hours.
  • Bring freshly threshed seed into drying within three to five days.

How do I sieve out the seeds and blow out the chaff?

Sieving takes two passes, and the clinging husk dries off in between. Rub with light pressure, without scratching or crushing the shell. This set of sieves has proven itself:

  • a coarse sieve of 4–5 mm, through which the flowers are rubbed
  • below it 3.6 mm, 3.0 mm, 2.5 mm and 1.8 mm for sorting by size

Which mesh sizes fit depends on the variety, since measured seeds are 4–5 mm long and 3–4 mm thick. Loose chaff then goes out with an airflow, and your own breath is enough for this.

  1. Rub the dry flowers over the coarse sieve until everything has passed through.
  2. Dry the sieved material in flat crates of 60 × 40 cm with a 7.5 cm rim until the husk is dry.
  3. Rub a second time over the finer sieves; the capsule remains pass through the finest mesh as broken material.
  4. Hold the crate at a slight slope and shake everything to the lower side.
  5. Blow the chaff up the slope from the lower end and remove the coarse chaff at the top by hand.
  6. Repeat with light shaking until the seeds lie clean at the bottom.

When do I dry the seeds?

After sieving, the seeds lie spread out thinly in room air until their weight stops changing. This gives the starting value for weighing and evens out the batch before density separation. Room air does not bring the seed down to the storage value of 5–7 %, though. How to dry further and measure the end is covered with the drying calculator in the article on drying seed.

How do I separate empty seeds?

Empty seeds are removed by density separation, which sorts by weight per volume in an aspirator. An aspirator is a tube in which air flows from bottom to top. With seeds of the same size, the airflow carries out the empty ones and lets the full ones fall. A sieve cannot do this, because a full and an empty seed of the same size end up in the same fraction.

A moist seed is heavier than the same seed when dry, and so dry seeds should separate more sharply. This has not been measured.

  • Sieve by size once more before the aspirator, so that only density varies in the airflow.

How do I store the batch?

The finished batch belongs in a vapour-tight container, otherwise it takes up water from the room air again. Which containers stay tight is covered in the article on seed storage containers.

A germination test shortly after harvest or after thawing easily underrates the batch. Freshly harvested hemp seed rests for about 35 to 42 days and germinates poorly during that time. After two years in the freezer, only 66 % germinated in one trial, yet around 90 % were still alive. How to shorten this dormancy is covered in the article on seed dormancy.

  • Only open containers from the fridge or freezer once they are at room temperature.
  • Before a germination test or sowing, let the seed take up water over moist air for 24 hours.

Sources