Sacha Johnson
EHUF 490B
Native Plant Production
Final Paper
Seed Storage Facility Requirements
Propagating plant material from seed has many benefits. Seeds are the product of sexual reproduction, through which the process of meiosis leads to genetic recombination. By propagating plant material from seeds rather than cuttings, there will be increased genetic diversity among offspring, and therefore better overall survival rates in varying environmental conditions. Even in “native species”, non-local plant material can differ in timing of germination, bud-break, growth, flowering, fruiting, seed-set, and leaf-drop (5). Seed propagation is an economical way to produce genetically diverse native plant material for the small scale or low budget nursery.
Seed, which has been properly collected and cleaned, must be stored until appropriate time for sowing. Proper storage of seeds is imperative to seed longevity. Seed viability can be preserved over a long period of time by controlling temperature and moisture content of the seeds.
Seeds are hygroscopic, meaning that they are able to both pick up moisture from the air and release moisture into the air. This means that the relative humidity of a seed storage facility determines seed moisture content, and, conversely, seed moisture content affects relative humidity of the air surrounding them. When stored in air-tight containers, seed moisture content determines the relative humidity of the air within the container. However, air-tight containers are only appropriate for areas where temperatures do not fluctuate. Maintaining temperature-controlled environment requires expensive equipment and probably is not appropriate for low budget nursery operations. In bulk storage or bags that are not airtight, seed moisture content will reflect the relative humidity of within the storage facility. Seed moisture content influences metabolic activities: higher temperatures lead to increased respiration and fungal problems (4).
Temperature
is an important factor, because it determines dew point or the amount of
moisture that the air can hold. Low
temperatures negate the impacts of high humidity while high temperatures
accentuate high humidity. This
phenomenon makes seed storage cheap and easy in the
In the first stages of seed storage, lower seed moisture content by spreading seeds out on drying racks, for example wire mesh stretched over wood frames. Each 1% reduction in seed moisture content, down to 6%, doubles seed life (1). Seeds can then be stored in breathable containers for up to three years, given the right storage conditions. In general, seeds will remain viable at temperatures below 60º F with 60% relative humidity. Each 10% reduction in temperature, down to 32º F, doubles seed life (1).
Although well-ventilated sheds should work quite well in the PNW, it is wise to monitor temperature and humidity (4). This can be done at relatively low costs. Thermometers can be bought at any local hardware store. Hygrometers, which measure the relative humidity in the air, are also readily available at hardware stores. There are mechanical and electronic hygrometers. If your storage shed does not have electricity, a mechanical thermometer and hygrometer would be the appropriate and cheaper choice. The table below details the advantages and disadavantages of electric versus mechanical hygroscopes (3).
|
|
Mechanical Hygrometers
|
Electronic Hygrometers |
|
Cost |
$10 to $20 |
About $35 to $60 |
|
Appearance |
Plastic or wood; often round
with a pointer and dial display |
Plastic construction, LCD
display |
|
Batteries required |
No |
Yes |
|
Sensitivity to humidity |
May "stick' if humidity
does not change for a long time |
Sometimes slow to respond to
changes in humidity |
|
Accuracy |
Reasonably accurate once
calibrated |
Usually accurate in the mid-
to high-humidity ranges; inaccurate in the lower ranges (below 30% relative
humidity) |
|
Ease of calibration |
Some can be physically
corrected (by adjusting the pointer) |
Cannot be physically
corrected |
If temperature and humidity either are too high or too unstable, more expensive equipment can be bought to control the environment: fans and dehumidifiers. This equipment requires electricity, but maintain stable temperature and humidity. Also, with access to electricity, seeds can be stored very efficiently in freezers, as long as they are stored in air-tight containers and have a moisture content lower than 13% (1). Good insulation will also act as a buffer to outside temperature and humidity fluctuations, providing a more controlled environment. Fans will improve ventilation between indoor air and outdoor air.
In conclusion, storage of seeds, so that they remain viable for long periods of time, relies on temperature and humidity. However, each seed is a living organism and reacts uniquely to temperature and fluctuation. Chemical composition, size, surface area, and many other factors alter seed storage requirements. Therefore, the above information is a generalization. Species-specific requirements should be taken into consideration, and storage conditions may need to be adjusted accordingly (6).