Seeds are a vital component of a healthy, profitable farm. In 2018, farms in the United States spent $22 billion on seeds, a 30% increase since 2010. Therefore, proper seed storage is equally important.
Here are some important considerations for proper seed storage.
Short Term Seed Storage
The first and most important consideration for effective seed storage is the length of time that you intend to store the seeds. Less than a year is typically considered short term.
A wide variety of materials exist for short term seed storage including cotton, burlap, multi-wall paper, plastic, and polyethylene bags. These materials are usually porous and are used to simply keep the seeds from mixing together. However, these materials do not provide protection from moisture and therefore, should not be considered for long term seed storage.
If you’re planning on storing your seed for over a year, then you’ll need to consider long term seed storage.
Long Term Seed Storage
When storing seeds for an extended period of time (longer than 1 year), there are several factors that contribute to the viability and vigor of seeds. Viability refers to the percentage of seeds that germinate upon planting. Vigor, on the other hand, refers to the rate at which seeds germinate and grow in the early stages, which helps seedlings overcome disease, weeds, and pests.
When storing seeds long-term, the goal is to maintain the highest level of viability and vigor
Depending on the type of seed, the optimal temperature can widely differ. The general effect of temperature on longevity is that longevity increases as temperature decreases. This is true of even subfreezing temperatures as long as the moisture content of the seeds is below 14% (so ice crystals don’t form).
General Moisture & Humidity Guidelines
Moisture & Humidity
Seed moisture affects the viability of seeds more than temperature. This is why newly harvested seeds undergo a drying process, because it preserves the viability of the seeds in long-term storage. As moisture and humidity increase, fungi and other microorganisms begin to rapidly deteriorate the seed viability.Fortunately, there are “rules of thumb” that you can follow to ensure the maximum viability of your seeds in long-term storage. We’ve included a graphic that illustrates these rules.
Some seeds may do best when stored in a container with adequate ventilation. Others may suffer because of it and will need to be sealed in an air-tight container. The most important factor affecting the need for ventilation is the moisture content of the seeds. But at a minimum, seeds should always be kept dry.
The effects of light on stored seed have been studied (including the effects of different wavelengths of light): Some studies showed a benefit and some showed a detriment: the results are inconclusive and
Seeds stored in glass containers should be stored out of direct sunlight because of the localized “greenhouse heating effect” on seeds. This might seem an unnecessary and overly obvious cautionary note, but in my experience, it has happened accidentally more than once. For example, this can happen if jars of stock seed are taken outside for the purpose of removal of some seeds for planting, or for transporting to another location. Though the jar may be temporarily stored in the shade, the angle of sunlight may change quickly and the jar will be in direct sun causing very rapid heating within the jar.
Though some commercially produced seed is dried in direct sunlight (in dry climates), drying seeds in the sun is a questionable practice in the Mid-Atlantic and South if the air temperature is above 90oF (32oC).
The air temperature at the seed surface is higher because of the conversion of light energy into heat at the seed surface, and the heat is “moist heat” (though this wouldn’t be an issue in dry climates where evaporative cooling occurs at the surface). I’m not aware of any studies on this issue for our region.
Another concern, also not well documented, is that the ultraviolet light from the sun may have a deleterious effect on seed longevity while the seeds are drying (Harrington, 1972). Harrington’s suggestion was based on the known effects of ultraviolet radiation on biological systems (rather than specific data). Whether the ultraviolet exposure is long enough to cause concern, is unknown.
Fungus, Bacteria, & Insects
The process of seed harvest and cleaning removes most debris and insects, but certain fungi, bacteria, and insects make their way into stored seed. Fortunately, the same conditions that are favorable to seed preservation inhibit fungi and bacteria and kill insects.
Bacteria — Bacteria do not have a significant role in seed deterioration because free water is required for bacterial growth, and if the moisture content of the seed is high enough to support bacteria, the seed is more likely to succumb to deterioration due to other causes such as fungi, respiration, heating or premature sprouting.
Fungi — Most seed storage fungi are inhibited when the relative humidity is kept below 65%. At this relative humidity the moisture content of starchy seeds is about 13%, and oily seeds about 7%. The major effects of fungi are to:
- decrease viability;
- produce toxins that affect seed viability and germination;
- increase heat production – important in large seed lots; and,
- cause discoloration, mustiness, and caking.
Insects — In hot, humid climates such as the Mid-Atlantic and South, mites, weevils, flour beetles, and borers can be a serious problem in stored seed, but if the seed is dried to 8% moisture content and the temperature reduced to (64 to 68oF (18 to 20oC), insects should not be a problem. At a moisture content of 15% and a temperature of 86 to 95oF (30 to 35oC), they can become very destructive. Mites will not survive when the relative humidity is below 60% (Bewley and Black, 1985).