On this page, you'll find information on methods of harvesting and cleaning seeds.
There are several sections on this page:
Winnowing
Winnowing is a process used to separate dry seeds from the chaff and other debris left from harvesting. The simplest method of winnowing involves tossing the mixture into the air and letting the wind blow away the lighter particles, leaving the heavier seeds behind. This method was developed by ancient cultures and is still practiced today. Modern winnowing techniques include the use of a fan instead of the wind, as the wind can be too variable and seeds can be lost (Ashworth, 2002).
Another method of winnowing called “screening” involves using a screen with holes that are just large enough for seeds to pass through, leaving the larger chaff behind. Often with this method a second screen with holes just smaller than the seeds is then used to separate the remaining small debris.
A method that works well with more rounded, spherical seeds is done by placing handfuls of seeds on one end of a flat board (such as a cutting board) and carefully tilting the board upwards until the seeds roll down the board into a container, leaving the chaff behind (Ashworth, 2002). This can also be done using a gentle fan at the bottom of the board which will simultaneously blow the chaff and lighter debris upward off the board as the seeds roll down.
Washing
After scraping out or mashing wet seeds, the seeds and pulp should be placed in a large bowl or bucket and covered with water (at least twice the volume of the seeds and pulp) (Ashworth, 2002). This mixture is then stirred vigorously. As they settle, viable seeds (which tend to be denser) will sink to the bottom of the container and the less viable seeds will float. They can then be poured or scooped off with any other floating debris. This process is repeated (adding more water as needed) until only clean, viable seeds remain. The seeds are then strained, rinsed under running water, and dried.
Drying Seeds
Drying seeds is most important for wet varieties that have been harvested and cleaned, or for seeds that have undergone a hot water treatment. While seeds are still in the strainer after they have been washed, a clean towel should be used to soak up as much moisture from the bottom of the strainer as possible. Then the seeds are dumped onto a non-stick surface (glass, ceramic, wood - anything but paper or cloth, as they will be difficult to remove once they are dry).
Warm, wet seeds are likely to mold (or germinate prematurely), so drying them quickly and thoroughly is important. The easiest way to do this is to spread them out in a single layer and put them in a warm, dry place. Never expose them to temperatures exceeding 95° F (Ashworth, 2002). It is not advised to attempt to dry them in the oven or in direct sunlight. Using a fan to speed the process is possible, as long as it is placed far enough away that it does not blow the seeds around. A good non-stick surface to use for drying is a window screen, as it allows excellent air circulation. As they dry, stir seeds around, several times during the day if possible. The drying process should take a few days. Dry seeds should break, not bend (Ray, 2012). If you are unsure about the dryness, let them dry for another ay to be safe.
Silica gel is sometimes used to absorb excess moisture from seeds. It can be purchased in bulk online or in most craft stores. Color-indicating silica gel makes it easy. The small beads are a deep blue when completely dry and as they absorb moisture they gradually turn pink. These beads can even be reused by drying them out again in an oven or microwave (Ashworth, 2002). When not in use, make sure the silica gel is stored in an airtight container.
To dry seeds using silica gel, first put seeds into paper packets and weigh them. Add them same weight of silica beads to a jar and place the packets inside, making sure to seal the jar as tightly as possible (Rowe, 1998). Leave the container alone for seven or eight days, after which time the seeds should have dried thoroughly.
When it comes to harvesting seeds, there are two basic categories to keep in mind: wet seeds and dry seeds. Wet seeds are harvested from inside the moist flesh of fruits or berries. This includes plants such as tomatoes, melons, and peppers. Dry seeds are generally harvested from husks or pods that have been left to dry out on the plant. This category includes corn, beans, and lettuce.
Dry Harvesting
Threshing is the process of removing seeds from their coverings, or chaff. There are a variety of ways to do this. A common and simple method involves putting dry pods into a sack (often a pillowcase will work) and stomping or treading on it until the seeds break free (Ashworth, 2002). This works best with harder seeds, as too much force can cause cracking or breakage. A pedal thresher can also be used, and can be made by attaching a studded drum to a stationary bicycle (Hess, 2010). As one person pedals the bicycle, another holds the seed heads against the spinning drum, loosening the seeds which then fall into a collection container.
Wet Harvesting
Harvesting wet seeds has little variation from species to species, and essentially involves cutting the fruit open and scraping the seeds out of the flesh (Ashworth, 2002). For smaller fruit and berries, crushing and mashing works best.
Fermenting
The seeds, pulp, and juice of some species may need to be fermented before cleaning (Rowe, 1998). This natural process allows microorganisms to kill many seed-borne diseases that can affect the next generation of plants. Certain species, such as tomatoes, require fermentation to break down the seed coat in order for seeds to be able to germinate easily (Ray, 2012). While the process varies slightly species to species, generally it involves putting the wet, pulpy seeds and a little water into a clear container for several days until a layer of mold forms on the top of the mixture (Ashworth, 2002). Make sure to keep the container in a warm place (75°F is ideal) out of direct sunlight. Once this mold has covered the surface, add water to the container and stir. The viable seeds will sink to the bottom and can be collected by gently pouring off the top layer. If left too long to ferment, seeds will begin to germinate, so watch them carefully!
Hot Water Treatment
This process works well for both wet and dry seeds. Although it is not necessary for seeds to germinate, it is an effective method for killing seed-borne diseases such as black rot and bacterial canker (Ashworth, 2002). To do it properly, you will need a thermometer, an electric frypan, a large saucepan and a sieve. It is recommended to run through this process without the seeds first, as it is easy to lose track of the exact water temperature which could potentially damage the seeds.
First, heat some water in the saucepan to the appropriate temperature (see table, below). Warm the electric frypan, and then pour about a third of the hot water into it. Set the saucepan with the remaining water into the water in the frypan. Practice regulating the temperature by controlling the heat of the frypan, or by removing the saucepan from the water. Once the desired temperature can be steadily maintained, pour in the seeds, stirring them throughout the remaining time. After the treatment is complete, sieve the seeds and spread them out on a hard surface to dry.
Hot Water Treatment Variations
| Type of Seeds | Water Temperature | Exposure Time |
| Broccoli, Brussels sprouts, kale, Chinese cabbage, cucumber, carrot |
122° F (50° C) | 20 minutes |
| Eggplant, spinach, turnip | 122° F (50° C) | 25 minutes |
| Celery, pepper | 122° F (50° C) | 30 minutes |
| Cauliflower | 126° F (52° C) | 25 minutes |
| Cabbage | 126° F (52° C) | 30 minutes |
| Tomato | 131° F (55° C) | 25 minutes |
| Mustards, radish | 122° F (50° C) | 15 minutes |
| Lettuce | 118° F (48° C) | 30 minutes |
Storing Seeds
Storing seeds properly is very important for long-term viability. Finding the right location for storing seeds can be tricky and may take some trial and error. When preparing to store saved seeds, it is important to keep two things in mind: heat and moisture. If heat or moisture levels are too high, seeds will mold or germinate. If the temperature or humidity often fluctuates, seeds will lose their viability (Ashworth, 2002). When choosing a place to store seeds, calculate the sum of the temperature (in degrees Fahrenheit) and the relative humidity. (Relative humidity can be calculated using a hygrometer, which can be purchased at most hardware stores.) The sum of the temperature and relative humidity should not exceed 100 (Ashworth, 2002). In most cases, high humidity is worse than high temperature, because it allows the growth of bacteria and fungus that will affect the seed’s viability.
The easiest way to combat humidity is by storing seeds in airtight containers. Glass and metal containers are best, as they are moisture-proof. Canning jars with new lids are perfect and are very nearly airtight when the lid is screwed on very tightly. It is important to carefully label each container with the seed species and variety, and the date they were packaged. This will make things easier when you are choosing which seeds to plant in future seasons.
Once seeds are packaged and labeled, store them in a cool, dry, dark place with little or no temperature fluctuation. A root cellar or dry basement is ideal, but, for small operations, storing seeds in a refrigerator can work well, too (Ray, 2012). In the proper conditions, most seeds will stay viable in storage for several years (Rowe, 1998). Some species will only last for one year in storage, such as onions and chives, parsnips, parsley, and any seeds that have been hot water treated. The best way to test viability is by placing ten seeds between two moist paper towels, making sure there is some space between each seed. Put the towels and seeds into a Ziploc bag to keep them from drying out. Place the bag in a warm spot for a couple of days and then check the seeds, remoistening the towels if necessary. Keep v\checking daily for another three or four days, by which time the seeds should have germinated (Hamir, 2002). You will get a rough estimate of the percentage of viable seeds by counting how many out of ten germinated (for example, if 7 out of 10 seeds sprout, the germination rate is roughly 70%).
Frozen Storage
Seeds that are stored in a freezer will last for many years - up to ten times longer than seeds that are kept unfrozen. However, when freezing seeds, it is even more important to ensure that they are as dry as possible. If seeds have more than an 8% moisture level the excess water will expand, rupturing cell walls and cracking the seed casing (Ashworth, 2002). If a seed breaks when it is bent, it is most likely dry enough to be frozen. Hard shelled seeds, such as corn or beans, will shatter when struck with a hammer if they are thoroughly dry, and will mash if they are still too moist (Ashworth, 2002).
Saving Seeds from Biennials
Biennial plants produce seeds during the second year of their two-year life cycle, so in order to save seeds from these plants you must be able to keep them alive through the cold season. Most biennials need to be exposed to a period of freezing weather to mature properly, so wintering them in a greenhouse is not usually recommended (Rowe, 1998). How you choose to overwinter your crops will depend on your climate and weather patterns, so some further research about your specific area is a good idea.
Generally, in warmer climates where the ground does not freeze or freezes only slightly, plants can be left in the ground and tended throughout the winter. If you are growing in a warm climate where there is a slight possibility of freezing, special mulching techniques can be used to cover plants and keep them warm enough to survive mildly cold weather (Ashworth, 2002). Unheated coldframes can provide adequate shelter in moderately cold climates. In very cold climates roots, bulbs, or plants will need to be dug up and stored through the winter, and then replanted in the spring.
Root cellars are designed to store crops through the cold months and are a great place to overwinter your biennial plants (Ashworth, 2002). The trick is not letting them freeze or dry out, so a moderately warm and moist environment is ideal. Temperatures between 32° and 40° F are best, and containers that are regularly filled with water can provide sufficient humidity (Ashworth, 2002).
Roots and bulbs can also be “dug in” (or “heeled in”) to be stored over the winter. This process involves digging up the plants when extremely cold weather arrives and reburying them elsewhere in moist sand, peat moss, sawdust or another material that is moist and relatively inert (Rowe, 1998). Before reburying, remove all vegetation except the smallest leaves and shoots. This will prevent excess moisture loss. When buried these crowns (and only the crowns) should be left exposed, and can be covered with a tent of greenhouse plastic to keep them from drying out (Rowe, 1998). It is recommended to dig plants into a trench in the ground. Trenches are best dug near the foundation of your house, as it will let off enough radiant heat to keep them from freezing completely (Rhoades, 2013). These plants should be kept so that they avoid hard freezes and excessive warmth and dryness. Ideally they will remain at temperatures that hover around freezing until spring (Rowe, 1998).
Once outside temperatures begin to stabilize above 28° F or so, plants should be dug up again and planted in containers (Rowe, 1998). Place the containers in a spot where they can safely harden off for a few weeks before replanting them in the garden. You may decide that it works better for you to replant crops straight into the ground and skip the containers. If you do this, make sure you protect the plants from harsh wind and sun for a few weeks to let them establish new roots without stress.