Drying and storing seeds at a local level
Local habitat restoration and species re-introduction projects, community seed banks and community-based initiatives to conserve medicinal plants may all involve short to medium term seed storage. Understanding how seeds age and how correct drying and packaging can slow down the ageing process can help such projects to store seeds for longer. This can help to improve seed security and/or reduce the need to re-collect seeds.
It is important to collect or harvest seeds at the right time. The majority of crop and wild plant species produce orthodox or desiccation tolerant seeds, which are able to withstand drying. By the time of natural dispersal or harvest, with very few exceptions, seeds are desiccation tolerant, and will have reached their optimum storage potential. If seed collections contain a significant proportion of seeds that are not fully mature, use post-harvest ripening methods to improve their storage potential.
Seeds begin to deteriorate as soon as they are harvested or collected. Depending on ambient temperature and humidity, there is a risk that they will be dead by the time they are needed for planting.
For practical purposes, aged seed lots are not very useful. If more than 50% of the seeds have already died, potentially important characteristics will have been lost. These might include genes for disease or drought resistance, early ripening, or other characteristics contributing to diversity and adaptation. Even if they are still alive, aged seeds will:
- Germinate more slowly.
- Produce fewer seedlings overall and more abnormal seedlings.
- Germinate poorly under extreme or stressful conditions, leading to poor field establishment.
- Produce lower yields.
How long can seeds live?
How long a particular seed lot/seed collection will remain alive, and therefore usable, depends upon:
- Seed moisture content - seeds stored in open storage in humid conditions will have high moisture levels and will not live as long as collections that have been properly dried and stored in sealed containers or those stored under dry conditions.
- Storage temperature - seeds stored at higher temperatures will not live as long as seeds stored under cooler conditions.
- Initial seed quality - seeds collected too early or too late will not live as long as seeds collected at the optimum stage.
- Species - some species are inherently short-lived.
The most important factor, and probably the easiest to control, is seed moisture content. Correct drying and packaging can make the difference between seeds surviving for a few months at best to being a high quality resource available for use for many years, possibly decades, into the future.
Measuring seed moisture
Because seed moisture is so important to maintaining seed longevity we need to be able to measure it, or at the very least be able to determine when seeds are dry enough to store.
Scientists often express seed moisture status as moisture content (mc). This is determined by an accurate, but destructive, oven method. There are also various types of portable moisture meters, less accurate, but still destructive. An alternative, non-destructive, method uses digital hygrometers to measure the equilibrium relative humidity (eRH) of the air surrounding the seed in a sealed chamber.
This method relies on the fact that seeds rapidly gain or lose moisture depending on the relative humidity of the surrounding air. Moist seeds in dry air lose moisture; dry seeds in moist air gain moisture. After about 30 minutes there is no further movement of moisture between seeds and air, and seeds are said to be at equilibrium. This methodology is now used routinely in modern seed banks for monitoring seed moisture status.
For those involved in short to medium term seed storage, who need to know whether seeds are dry enough to store, but not necessarily what the exact eRH is, there are acceptable, less costly alternatives. They are all based on the same principle as the digital hygrometers and need to be sealed in a container with the seeds until the moisture in the air reaches equilibrium with the moisture in the seeds.
Seed moisture detection equipment
Dial hygrometers consist of a chemically coated metal coil attached to a needle which contracts or expands as RH changes. They measure on a graduated scale from 0 - 100% RH. They reach equilibrium within a few hours and are accurate to within about +/- 10%RH.
Self-indicating silica gel contains a methyl violet indicator which changes colour from green to orange below about 20-25% RH. It thus gives a very good indication that seed moisture is at a level that will permit safe storage for many years. The disadvantage of these indicators is that they are slower to equilibrate (several days) than the dial hygrometer or indicating strips.
Moisture indicating strips are chemically impregnated cards which change colour as RH changes.
A very cheap and simple salt test can be used by farmers to show if seeds are dry enough to store. Mix common salt with seeds in a glass jar and shake for a couple of minutes. Leave for 10-20 minutes and examine the walls of the jar. If the seeds are still wet the salt will have absorbed moisture and will be sticking to the sides of the jar.
Drying seeds to about 70% eRH (12-15% mc, depending on seed composition) minimises the risk of fungal damage and is generally regarded as a 'safe' moisture level for short term storage. This is also the maximum moisture content for sealed storage (which can help to prevent insect damage).
However, drying seeds even further will slow down physiological aging processes and prolong their useful storage life. Down to about 15% eRH (4-7% mc depending on seed oil content) a simple rule of thumb applies: for every 10% reduction in eRH or 1% reduction in seed moisture content, seed life span doubles (Harrington, 1970).
Seeds at risk of rapid loss in viability
Rate of deterioration slower; seeds may survive for 1-2 years
Seeds could survive for several years
Seeds can be kept alive for decades
Remember also that temperature affects seed longevity. A similar rule of thumb applies: seed longevity doubles for every 5°C reduction in storage temperature. Thus, seeds dried to 70% eRH and stored at 20°C will survive twice as long as similar seeds stored at 25°C. If average local temperatures are relatively high, try to compensate for this by drying seeds to lower moisture levels.
In many countries, farmers traditionally use sun drying to reduce seed moisture before storage. This involves spreading seeds out in a thin layer, sometimes on a tarpaulin. It is important to turn seeds regularly, because high temperatures may cause damage. Drying tables, or other methods of raising seeds off the floor to increase air circulation, will speed up the drying process.
The lowest seed moisture level that can be attained using sun drying depends on the local climatic conditions (temperature and relative humidity) and the species in question. In arid regions, where average relative humidity is low, safe moisture levels for short term storage are readily attained.
In locations where average relative humidity is quite high but falls during the day, it is possible to dry seeds in the late morning and afternoon when the weather is favourable. Cover seeds up when temperatures begin to fall in late afternoon to prevent moisture uptake as relative humidity increases.
In humid regions where relative humidity remains high throughout the day it is virtually impossible to dry seeds to safe moisture levels. It is very difficult for farmers to store seeds for longer than a few months without significant loss of viability and/or insect attack.
Using charcoal to dry seeds
Drying seeds with desiccants
Some traditional seed storage methods suggest placing a layer of charcoal on top of seeds before sealing them for storage. This technique may help to deter insects. However, dried charcoal and other desiccants such as dried rice can also be used to dry seeds before storage. Charcoal and seeds such as rice or maize are readily available, inexpensive potential desiccants that can be found in rural areas throughout the world.
The desiccant must be dry to start with, otherwise it will not be able to absorb much moisture, and the seeds will not dry properly. Being black in colour, charcoal has the potential to absorb solar heat during the day thus causing it to dry below ambient humidity levels. Charcoal exposed to direct sun for about 5 hours will reach a very low moisture level and will have a lot of drying potential. Otherwise, dry the charcoal over a low heat (e.g. over a stove or fire).
Place the dried charcoal in a sealable container and spread a thin layer of seeds above the charcoal. Separate seeds from charcoal with a thin sheet of newspaper, or similar porous material. Alternatively, place seeds in a porous bag or envelope above the charcoal.
Seal the container and leave in a safe place, not in direct sun. Depending on the size of the seeds and how dry they were to start with, it will take about two weeks for the seeds to dry. After drying, seeds need to be sealed in a moisture-proof container to prevent them from gaining moisture again.
How much charcoal to use?
Charcoal drying works because the charcoal absorbs moisture from the seeds. Therefore, the wetter the seeds, the more charcoal is needed. If the seeds are freshly harvested and are still very moist, you would need a lot of charcoal to absorb the moisture from the seeds. It is best to first dry the seeds as much as possible using sun drying. This will remove some moisture and means that you will need less charcoal. Once the seeds are air dry, a ratio of one part seeds to three parts desiccant (by volume) should work well.
Keeping seeds dry
Remember that dried seeds will only remain dry if the air around them is dry. In very arid environments, it may be possible to store seeds in bags for one or two seasons without too much loss of viability. In most environments it is better to store seeds in sealed containers to prevent them re-absorbing moisture. Sealed storage also helps to prevent insect damage.
Glass drinks bottles, plastic tubs and buckets and many other locally available containers can be used to store seeds. The most important thing is that they have a good lid. Test if containers are air-tight by holding them under water. Sealing can be improved by adding a rubber gasket cut from an old car inner tube. Fill the container as full as possible with seeds.
Harrington J.F. 1970. Seed and pollen storage for conservation of plant gene resources, pp 501-522. In: Genetic Resources in Plants - Their Exploration and Conservation. IBP Handbook No. 11. Eds: Frankel O.H. & Bennett E. Blackwell Scientific Publications, Oxford, UK.
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