USA
February 22, 2023
Most people are aware that seeds need to be kept cool and dry to maintain their quality in storage. But the real questions are: How cool, and how dry, do the storage conditions need to be? While the answers to these questions differ between species of plants, there are some general guidelines that apply to all types of seeds.
The two most significant factors that affect seed quality in storage are temperature and relative humidity. Seeds need to be stored in conditions that carefully balance temperature and humidity to maintain seed quality and reduce seed deterioration.
Scientists also know that the sum of relative humidity (as a percentage) and temperature (in °C) must not exceed the number 100 for safe seed storage. So, if the temperature is high, relative humidity needs to be kept low, and vice versa.
My graduate work at Oregon State University focuses on the best storage conditions for hemp seed (Cannabis sativa L.) My advisor, Sabry Elias and I are exploring different ratios of temperature and relative humidity to determine how they affect hemp seed quality in storage.
Researchers in Oregon are testing storage conditions that help hemp seeds grow best. Shown here, a normal hemp seedling that sprouted during the Field Emergence vigor test. Vigor tests determine how well plants grow after they germinate. Credit: Grace Fuchs
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Seed quality refers to a combination of two criteria: viability and vigor. Viability relates to the seed’s ability to germinate under optimal conditions. When you buy a packet of vegetable seeds for your garden and you see the germination percentage on the bottom, that number was produced in artificial, lab-created “optimal conditions.”
As we know, neither the weather outside nor pre-planting conditions is always optimal. You might spill your boiling coffee on the seed packet before you plant them. Or perhaps there is unexpected frost a few days after planting. Although your seeds that are exposed to less-than-ideal conditions may germinate, the seedlings could be deformed, non-uniform, or have slow and uneven growth. Because of all these variabilities, viability of a seed doesn’t always correlate with how the plant will grow.
Vigor relates to the seed’s ability to grow into a normal seedling under a wide range of adverse conditions. It also factors in the speed of germination, and the uniformity of a given group of seeds. Therefore, vigor tests can give a more complete picture of the seed’s overall health and field performance than viability tests.
There is uncertainty among many hemp seed growers about how to maintain the quality of their seed in storage. Many of them wonder about the ideal environmental conditions at which one can safely store hemp seed. They also want to know how long the seeds can be stored.
To provide much-needed clarity for hemp seed growers, I tested three storage environments. Each had different ratios of temperature to relative humidity. We evaluated the differences in seed quality over 18 months.
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Even though a seed may germinate, the storage conditions of the seed can influence how well seedlings grow. Shown here, a normal hemp seedling growing well on the left, with two abnormal seedlings on the right. This indicates that the seedlings on the right emerged from seeds that were stored in less-than-ideal conditions. This can influence the yield of hemp for farmers. Credit: Grace Fuchs
I also looked at how five different hemp varieties responded to these storage conditions over time.
I performed seven initial quality tests (including both vigor and viability tests), then placed the seeds in their respective storage environments. Every four months, I removed the seeds and evaluated their quality using the same seven tests. I repeated this process for 18 months (to collect enough data for statistical analysis.)
When conducting vigor tests, instead of only checking whether the seed germinated, I categorized the seedlings into three groups:
- normal seedlings (with well-developed root, shoot, and leaf structure),
- abnormal seedlings (underdeveloped, stubby, or shrunken structures), and
- un-germinated seeds.
The preliminary results of my study showed that storability differs significantly between hemp seed varieties. Seed lots that began with a lower initial quality deteriorated at a faster rate than seed lots with higher initial quality. Additionally, these lower quality varieties retained high viability (they will germinate) but showed reduced vigor (they won’t grow well).
For example, one lower quality variety showed 90% germination rates after 18 months of storage at room temperature. This is a very high retention of seed viability, but a test showed that vigor had dropped to 15% by the end of the study.
Grace Fuchs removing sealed boxes of hemp seed from the oven after their stress exposure. The Accelerated Aging vigor test exposes seeds to 100% relative humidity for 48 hours at 43°C (very hot and humid!) Seeds are then planted under optimal conditions, and normal vs. abnormal seedlings are evaluated after one week. The research is part of a study to help industrial hemp farmers increase yield by storing seed in optimum conditions. Credit: Grace Fuchs
This highlights the importance of examining vigor in addition to viability when differentiating between quality of seed lots. Would you buy a packet of vegetable seeds that said “Vigor: 15%” at the bottom, knowing that only 15% of the seeds will perform like you expect them to?
Based on the results presented in this study, hemp seed that is stored at 5°C, 10°C, and 22°C, with their respective relative humidity can maintain quality for 18 months. This could be good news for hemp seed growers. They may be able to store their seed at room temperature instead of a cooler – as long the storage area has low relative humidity. (In my case, the relative humidity of the OSU Seed Lab is about 30%). However, variety and initial seed quality can affect the longevity in storage over time.
With all this talk about storage, I want to propose a question: When does seed storage begin?
Technically, seeds are being stored on the plant before they are harvested. After harvest, they are stored by the farmer.
The moment the seeds reach what is called “physiological maturity,” they have reached maximum quality and dry weight. From this point forward, their quality is completely dependent on external factors. This is because they are no longer attached to the mother plant by the funiculus (think of this as the seed’s tiny umbilical cord). The seeds will continue to dry to a point called “harvest maturity,” when they are dry enough to be safely harvested without mechanical damage.
In addition to harvesting hemp seeds at the correct time, it is important that growers take precautions to reduce damage during both the harvest and the cleaning process. The best way to promote almost all types of seed longevity in storage is by beginning with the highest possible quality of seeds.
Answered by Grace Fuchs, Oregon State University
This blog is part of Crop Science Society of America’s Seed celebration. Why celebrate seeds? Anyone who plants a seed is investing in hope. That’s one of the attractions of seeds. For the gardener, it could be hope for a beautiful flower, or perhaps a delicious zucchini squash. For our farmers, seeds are the hope of this year’s yields of produce, cash crops or forage. No matter the size or shape of the seed, they all can bring forth new life. At Crop Science Society of America, we hold seeds in very high regard. Please visit our Seed webpage for news stories, blogs and more information about seed research and facts.
About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.
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