Horticulture in Winter: Seed Cleaning Techniques

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This time of year, our greenhouse staff, with help from volunteers and other members of the horticulture department, embarks on the task of cleaning the seed that was collected throughout the warmer seasons from the grounds and surrounding areas.  In 2011, we collected approximately 600 different species, so we definitely have a lot of work going into these colder months!

Seed is collected either ‘dry’ or ‘wet’.  The ‘wet’ seed (think of saving seeds from tomatoes, squash, or even some species of fruit- or pod-bearing trees) is processed and cleaned almost immediately.  However, the ‘dry’ seed is collected from a plant once the inflorescence, the showy petal part of the flower, has dried up and the ovary, containing the seed, has begun to harden.  In some species, such as the opium poppy (Papaver somniferum) featured below, it is very easy to locate the ovary containing the seed.  In other species, such as a yarrow (Achillea spp.) shown below, the ovary and seed are much smaller and it can be difficult to know which part should be saved.

Papaver somniferum

Once these parts of the plant have been collected, they are stored in paper bags to allow them to continue to dry out.  These collections might sit for up to six months before they are cleaned; as long as they are kept in a relatively dry, cool room, this will not affect the long-term viability of the seeds.  Because it is often easier and less time consuming to collect the seeds with large sections of stems, these bags will be filled with both seed and chaff.  Chaff is any part of the plant, including stem, leaves, and petals, that needs to be separated from the seed.

In order to separate out the seed from the chaff, we use a variety of methods, some time-tested and some a bit more home spun.  The basic techniques are threshing and winnowing.  Threshing is the process of breaking up seed pods to release seed.  Some examples of threshing are rubbing the seed pods between your hands, lightly grinding the pods with sandpaper or rubber blocks, and, in the case of some very hard seeds, striking the pod with a hammer.  Once the seeds and chaff have been threshed, winnowing can be used to sort the chaff from the seed using wind.  In our lab, we use a hand-held blow drier and a set of geologic sieves to accomplish this task.  Larger operations use equipment such as gravity separators and modular air screen machines that are able to separate seed from chaff by programming the weight of the seed into a machine.

Achillea sp.

In some cases, winnowing and threshing are not enough to finish the job.  Often times, the final step in cleaning a sample of seed is removing the chaff by hand.  This can be time consuming, but storing your seed in the cleanest state possible will help prevent decomposition of the seed over the term of its storage.  Even after the seed has been cleaned, we store it in paper envelopes (rather than plastic) to lessen the chance of molding caused by seeds that contain too much moisture.

In general, we keep our seeds in storage for about five years.  During this time, the seed will most likely be grown for use in one of our gardens or for sale at the Spring or Fall Plant Sale.  There is also a chance that it will be requested for use by a different botanic garden through our involvement with the Index Seminum program that allows for seed exchange among public gardens in the international community.

As our seed collection efforts have increased over the past five years, we have moved into the next phase:  creating a seed herbarium containing all the wild and garden collected seeds in our collection.  This catalogue currently only exists in physical form, but we hope to someday soon move it to a digital format.

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At the Gardens

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