The storing of seeds entails that important prerequisites are met beforehand. One of the most crucial factors is the degree of dryness of the seeds, which must be appropriate in order to preserve their viability and maximize their life when stored long-term. The longevity of seeds in storage is indeed highly sensitive to moisture content and temperature. Generally, the duration of seed viability (germination capacity) is reduced by half for every 1% increase in moisture content or increase in temperature of 6°C. We also must ensure that the seeds are not infested with insects and other parasites that may permanently damage the seeds. They should then be stored in a dry, cool, and dark location to prevent the emergence of bacteria or fungi, inside adequate storage containers to protect them further from rodents and other insects. To determine whether a location is cool and dry enough to store seeds, the “100 rule” can be used, where the sum of the storage temperature (in degrees F), plus the relative humidity (in percent) should not exceed 100 (temperature F + relative humidity % = <100). Optimal storage conditions will help to preserve and prolong the life of the plant embryo presents within each seed as well as its food source, while slowing down the consumption of the latter by the former. This will also ensure that the seeds that are taken out of storage and planted will be able to use the remaining food reserves to support rapid seedling growth. Depending on the specificities of your climate, some storage parameters and solutions may need to be adapted accordingly (i.e., tropics). Other factors influencing seed storage life such as tolerance to low moisture, genetics, seed coat traits, and oil content should also be taken into consideration. In addition, you will have to make sure to always hold extra seeds of each of the varieties you are maintaining in reserve in case of crop failure or unwanted cross-pollination.
Determining the Moisture Content of Your Seeds
Several techniques and solutions can be used to assess the moisture content of your seeds. Seeds that are elongated and thin can be tested for their brittleness: seeds that are adequately dry when bent will easily snap, while seeds that still contains too much humidity will bend. Bigger or harder seeds can be hit on a hard surface with a hammer: seeds that are dry enough will shatter when hit, but the ones that still have a high moisture content will smash of mush instead. This can also be determined by biting the seeds, where seeds that leave no mark or indent when bitten indicate that they are completely dry and the ones that do are not.
Dryness can be tested more accurately by determining the percent moisture content of your seeds. This can easily be done by recording the weight of a sample of your seeds, placing it inside an oven, and drying it slowly and carefully with low heat until all moisture is removed. The seeds are weighed several times during this process to assess the complete removal of the moisture. The weight of the completely dry seeds is then recorded, and the percentage calculated by using the following formula:
Weight of fresh seeds – Weight of dry seeds
______________________________________________ x 100
Weight of fresh seeds
As a general rule, small seeds are considered adequately dry and ready to be stored when their moisture content does not exceed 5%. Larger seeds are ready at 7% seed moisture.
Other Seed Drying, Moisture Absorption, Insect and Disease Control Solutions
Depending on your climate, seeds that are dry at ambient conditions might sometimes require the use of other, more efficient and adequate solutions in order to remove all their moisture content. Some can be used throughout the first storage phases of your seeds, which will extend their viability in the long run. Others will prove to be useful in locations where high humidity and frequent rains are prevalent. Additionally, specific methods aiming to kill pests and other seed borne pathogens will have to be uses when necessary to ensure that the seeds are free of any bacterial diseases before being stored.
Food dehydrators, drying houses and cabinets
Food dehydrators are particularly helpful when seeds are to be dry and protected during adverse weather or the wet season in the tropics. A food dehydrator sets at a temperature of 30°C will allow your seeds to dry further and evenly. The process must be regularly monitored to ensure that the seeds don’t over-dry and demands to experiment with small batches first.
Drying houses use smoke or radiant heat to dry the seeds. Special consideration must be given to the construction of such houses, as excessive temperatures and lack of ventilation will likely result in damaging the seeds. Seeds are placed on screen trays arranged on top of each other inside a small roofed shelter featuring a chimney. A hole is cut through the floor, big enough for a pipe to pass through and funnel smoke coming from a small insulated fire, which will then dry the seeds. Drying houses can easily be made outside using natural material readily available such as clay, straw and bamboo for the main structure, as well as palm or banana leaves for the roof.[insert drying house illustration here]
Where humidity and rain are an issue, drying cabinets can be used as an alternative whenever seeds need to be dry. They can be built out of kitchen cupboards, with seeds drying on screened shelves in a temperature-controlled environment. Each cabinet is divided into three chambers; a superior chamber containing a heat source (incandescent light bulbs) and a fan, a middle chamber housing the racks, and an inferior chamber that is empty. Air enters from the top chamber through an opening and directed by the fan into a channel located on the side of the cabinet. This channel is connected to the inferior chamber, then allowing the air to circulate around and through the racks of the middle chamber, and back up to the top chamber as it passes through a dust filter. A connected thermostat (optional) can be added, which will be responsible for turning the heat source as needed to regulate the temperature. The amount of air that is exhausted (to remove humidity) versus recirculated (to warm the air) can be controlled with the addition of a valve or diverter into the air intake.[insert seed drying cabinet picture here]
Desiccants such as packets of silica gel can be temporarily placed inside an airtight jar (at a 1g of seed for 1g of silica gel ratio) to help absorb the moisture from the air and maintain a dry storage environment for the seeds. The crystals can also continue to draw out any remaining moisture from the seeds. The color of the silica gel is a reliable indicator that will show how effective the container is at keeping the moisture out. Crystals that exhibit a dark blue color indicate that they are fresh and dry, while pink ones signal that a high level of moisture has been absorbed and that they should be removed from the container. Note that the packets should be monitored closely as seeds that lose too much moisture (falling below 3% for small seeds and 5% for large ones) might become damaged or go dormant. Both small and large seeds reach optimum moisture levels for storage after 1 week inside the container. Silica gel can easily be reactivated and reused by driving out all the moisture from the crystals with the help of an oven or fire (ideally heated at 175° C). The crystals will turn blue again when fully dry.
Ash and rice
Besides its ability to absorb moisture, ash is also an efficient method to control insects and eggs in stored seeds. Their small particles are indeed harmful to the shells and bodies of insects. If a seed variety is known to be more prone to insect attacks than others, a layer of ash can be added at the bottom of the jar containing the seed envelopes or ziplock bags as a preventive measure. If no ash is available, rice can be used as an alternative, provided its moisture content has been driven off beforehand (with the use of an oven or a pan). Neem leaves – known to repel some insects and pests – can also be incorporated if needed. A small amount of salt placed in a small container (with holes in the lid) can be placed inside the jar and serve as an indicator to determine how well the ash or rice is absorbing moisture over time. Salt crystals clumped together will indicate that the ash/rice is no longer able to absorb moisture from the air and should therefore be replaced.
Hot water treatment
Properly used, hot water treatment kills most bacterial disease-causing organisms on or within seeds. This treatment works best with small fresh seeds and is suggested for eggplant, pepper, tomato, carrot, spinach, lettuce, celery, cabbage, turnip, radish, and other crucifers. Seeds of cucurbits (squash, gourds, pumpkins, watermelons, etc.) can be severely damaged by hot water and thus should NOT be treated. Instructions are as follows; the use of an accurate thermometer will be required:
- Step 1 – Wrap seeds loosely in a woven cotton (such as cheesecloth) or nylon bag;
- Step 2 – Pre-warm seeds for 10 minutes in 100° F (37° C) water;
- Step 3 – Place pre-warmed seeds in a water bath that will constantly hold the water at the recommended temperature (see table that follows). Length of treatment and temperature of water must be exactly as prescribed. If water is too hot or treatment is too long, seeds may be damaged.
|Type of seeds
|Brussels sprouts, eggplant, spinach, cabbage, tomato
|122° F (50° C)
|Broccoli, cauliflower, carrot, collard, kale, kohlrabi, rutabaga, turnip
|122° F (50° C)
|Mustard, cress, radish
|122° F (50° C)
|125° F (51° C)
|Lettuce, celery, celeriac
|118° F (47° C)
- Step 4 – After treatment, place bags in cold tap water for 5 minutes to stop heating action;
- Step 5 – Spread seeds in a single, uniform layer on screen to dry.
Note that seeds may be damaged if they are hot water-treated twice. In addition, old or poor quality seed can be injured by seed treatments and cause a vigor reduction overtime. Therefore, it is recommended that a small sample be treated and tested for germination prior to treating the entire seed lot.
Credit: Ohio State University Extension
Before their permanent storage, seeds can go under a temporary freezing phase for several days in order to efficiently kill any insect or egg that is still present. Note that any seed that is taken from frozen storage and transferred to a temperate environment should be allowed to reach room temperature before opening the container in order to prevent condensation.
Home Storage Solutions
Airtight storage with containers
When fully dry, seeds stored in airtight, moisture-proof containers made of glass or metal will retain maximum vigor. Those should be ideally placed in a dry, cool, and dark environment with a stable humidity rate and constant temperatures, which will also better preserve their viability and ability to germinate. An environment with fluctuating conditions might indeed lead to the emergence and development of bacteria and fungi, detrimental for the quality of your seeds. Locations at floor level are usually preferred, as they are usually cooler. The better the storage conditions of your seeds, the stronger the plants in your garden, and the healthier the seeds saved from those plants, allowing to close and perpetuate this virtuous circle.
Canning jars are very versatile. They have the advantage of being more resistant, offering an airtight seal when closed with a rubber seal, and being transparent, which in some cases might be useful when seeds are to be checked without the need to open the containers they are in. To optimize space, seeds of different varieties can be put in separate ziplock bags or sealed envelopes inside the same jar provided they are labeled accordingly.
Long-term frozen and refrigerated storage
Although requiring more resources (freezer(s) run on electricity), long-term frozen storage is the most efficient solution to store seeds long-term while increasing their longevity by better preserving their viability for several years. Provided they are adequately dried and sealed inside containers, seeds stored this way will experience almost no loss of germination and only minimal loss of vigor over these years. It is especially useful when a substantial amount of seeds needs to be stored and further protected. While the storage environment offers by this solution allow better control (temperature), freezers are notably dry and will likely damage any unprotected, moist seeds. The water still presents within the seeds will indeed expand when frozen and break their cell walls. It is therefore critical for the seeds to be completely dry beforehand and stored inside airtight containers. In place of containers, pouches made of laminated layers of plastic and foil can be used, where individual labeled packets of dry seeds are heat-sealed and placed inside. Alternatively, seeds can also be stored inside a refrigerator or air-conditioning units, applying the same requirements and recommendations above-mentioned.
Again, note that any seed that is taken from frozen or refrigerated storage and transferred to a temperate environment should be allowed to reach room temperature before opening the container in order to prevent condensation. One should also limit the number of times seeds are retrieved from the container as temperature fluctuations will gradually reduce the viability of the seeds. Before planting, allow the seeds to slowly absorb some moisture by exposing them to ambient air for a few days. This will increase the chances of good germination once in the field and offer a better adaptation to the environment.
Traditional and Ancestral Seed Storage Methods
Challenges in Storing Seeds in the Tropics
With a high humidity rate, hot temperatures and the prevalence of insects, seed storage in the tropics can be challenging and a cause of rapid loss in seed viability. Humidity is a particularly detrimental factor, as it increases the chances of rotting and irreversible deterioration in seeds. Extra care must therefore be taken when drying your seeds, sealing your storage containers to keep them free from moisture, and choosing your storage location. The solutions outlined above should be adapted and experimented with accordingly with these environmental specificities in mind. For instance, a storage room could also be kept dry with the help of devices such as dehumidifiers, which might however lack practicality in some circumstances as it requires electricity to function. Additionally, seeds could be vacuum-sealed to minimize exposure to ambient humidity and potential remaining insects (killed as a result of reduced oxygen in the container), thereby increasing the longevity of the seeds in storage.
In case your storage location is not optimal or that the ambient conditions are difficult to control, focus should be made on the dryness of the seeds as well as keeping the humidity down in the sealed jars. This will prove to be more beneficial in the long run than focusing on the humidity of the room they are stored in, since well-sealed jars act as their own environment. By order of importance, keeping the temperature relatively low is important, keeping the temperature from fluctuating is more important, and keeping seeds under dry conditions with low oxygen is the most important for prolonging seed viability. Low oxygen content has indeed been linked with reducing free radicals (unstable molecules), which damage living seed tissue.
Other Factors Influencing Seed Storage Life
Beside ambient humidity and temperature, it is good not to overlook other factors that might influence the storage of your seeds. Knowing the tolerance to moisture of the seed varieties you are saving will help you keep them under best storage conditions. Orthodox seeds such as grains and most vegetables can be safely dried to a low moisture content of at least 5% and stored in a dry, cool place. Referred as recalcitrant, the seeds from fruit crops such as mango and avocado will rapidly degenerate when dried and stored for any length of time. The seeds of some plants (known as intermediate) fall somewhere between those two categories, with seeds that can be stored longer than recalcitrant ones but not as long as true orthodox seeds, and under specific storage conditions. Star fruit (carambola) and papaya are such examples, and can be stored successfully under dry but not freezing conditions.
Genetics and some physiological characteristics are also an influencing factor, with crops that are inherently more long-lived than others. Onion seeds are, for instance, commonly known to be naturally short-lived. Seeds that possess a thick seed coat protecting the embryo usually make their storage easier as the seed coat contributes in excluding moisture and oxygen. Same goes with starchy seeds, which usually store longer than oily ones (inclined to cell membrane damage as lipids degrade). In contrast, thin-coated seeds are more prone to rapid moisture uptake and tissue damage, therefore requiring additional attention when storing them.
Header photo credit: Phillip Merritt (CC BY-NC-SA 2.0)