Cleaning Seeds of Dry Crops

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All plant structures that contain seeds are technically called fruits. So a pea pod is a fruit and a lettuce seed is actually a fruit.

When should dry fruits be harvested? Seeds of dry-fruited plants must be mature before they are cut off the plant. For most types of plants with dry seeds, the seed heads or pods are ready to harvest when they are no longer green. Another indicator of maturity is when seeds start to disperse – the pods or heads break open (shatter), produce fluff for wind dispersal, or fall off the plant. Keep an eye on your crop and harvest the seeds as soon as they are dry. 

What if the seed heads or pods aren’t all mature? Many plants produce seeds over a few weeks, so you will have both green and mature seeds to deal with. You can cut branches with mostly mature seeds and leave other branches to continue to mature. Types with seed pods, like peas, can be harvested in small batches of mature fruit. Seed heads of carrots and broccoli can be cut off as they mature. 

If you have a large crop of seeds, harvest the whole crop when two-thirds are mature. This can be done by pulling up the plants or cutting off branches. Remember to do this by bending the plants over a container or tarp to minimize seed loss. 

Can the seeds be extracted from the fruit immediately? All types of dry seed fruits need to be dried further after they are harvested. Put them someplace out of the rain or dew. Spread them out so they will dry before they mold (don't put them in a plastic bag until they are totally dry). Let the cut plants, branches, or seed heads dry for at least a week.

How are seeds extracted from the fruit? Threshing is the process of applying force to the plant material to break up the fruit and release seeds. Depending on the type of plant, this can be done by shelling, stripping, flailing, stomping on, or using threshing machines.

Some type of seeds, like peas and onions, are delicate and can be damaged by threshing. Other types, like beets, radish, and kale, can withstand much rougher methods, including being driven over by a vehicle. 

What about all the plant material mixed with the seeds? After threshing, there is often more plant material, called chaff, than seeds. Although it isn’t strictly necessary to remove the chaff, doing so will help prevent plant diseases from being carried into next year’s crop, and increase the longevity of the seeds. Chaff is removed by putting the seeds and chaff through appropriate sized screens. Then the remaining chaff and seeds are winnowed in front of a fan or outdoors in the wind.