Spring is here and you have already planted your seeds in the garden. Can an unexpected frost kill your seeds? And can seeds stored in the garage survive a cold winter?
Sub-zero temperatures do not kill seeds (even tropical seeds) as long as they are properly dried. For seeds from cold climates, freezing or cold stratification before germination helps break seed dormancy, improve plant health, increase cold tolerance, and produce bigger and more leaves, flowers, and fruits.
In this article, we will look at how the moisture content of the seed and the species of the plant seed affect germination.
Does freezing kill seeds?
Freezing temperatures do not kill seeds as long as they are dry.
Dried seeds can be stored at extremely low freezing temperatures for years without damage. On the other hand, seeds with moisture can be damaged when it is below 32°F (0°C).
The cold the seeds can tolerate is directly related to their moisture content because frozen water can burst cell walls, resulting in freezing injury and cellular damage.
A study using lettuce seeds found that seeds can tolerate extremely low temperatures relative to the moisture content in the seeds (Junttila & Stushnoff, 1977).
Dry seeds with a very low moisture content of 5-13% can survive any temperature as low as that of liquid nitrogen (-321°F or -196°C).
Seeds with some moisture content can tolerate between 40°F and -4°F (between -40°C and -18°C).
And, for seeds fully soaked in water, they can be stored at a temperature as low as 23°F (-5°C) without cellular damage.
| Water content in seeds | Lowest temperature without damage |
| 5-13% | -321°F (196°C) |
| 16% | -40°F (-40°C) |
| 20% | 0°F (-18°C) |
| Seeds fully soaked in water, germinated | 23°F (-5°C) |
For home gardeners, keeping dry seeds in the freezer will not kill them and it is actually the best way to store seeds for years.
That also explains why dry seeds that are sown in the garden or dropped naturally on the ground before winter or frost will survive the frozen ground and would likely germinate in spring when temperature increases and the snow melts to break seed dormancy.
In fact, the Global Seed Vault at Svalbard, Norway stores crop seeds from around the world at 0 °F (-18 °C) in refrigeration units and cryogenic chambers.
Does freezing kill tropical seeds?
Ultra-low temperatures also do not kill warm-season seeds, including tomatoes and cucumbers, as long as they are dried.
This is proven by the study of freezing tomato seeds (Shevchenko et al., 2021) and cucumber seeds (Markovskaya, Sherudilo & Sysoeva, 2007), both are seeds of warm-season plants.
In the tomato seed study, the seeds were first dried to a low moisture level of 8-10% before freezing them for a month at the ultra-low temperature of liquid nitrogen 321 °F (−196 °C).
When the frozen seeds were thawed and returned to 77 °F (25 °C), all of them germinated and even developed into plants with better yield. More on this later.
Does freezing benefit seeds?
Freezing can help some seeds to germinate and improve plant growth, yield and plant health.
1. Germination
Freezing seeds before germination is a technique called “cold stratification”.
It mimics the natural cycle of seeds remaining dormant under freezing temperatures on the ground in winter so that they do not germinate prematurely. When spring comes with rising temperatures, moisture, and light, seeds break out of their winter dormancy and germinate.
But to germinate, seeds generally require at least 32°F (0°C), or ideally above 50°F (10°C) for cold-season seeds or above 60°F (16°C) for warm-season seeds.
For example, for the seeds of alfalfa, a cold-season plant, it takes 15 days to germinate near freezing (32°F or 0°C) and only 3 days to germinate at 50°F (10°C) (University of Minnesota Extension, 2022). For the seeds of cucumber, a warm-season vegetable, it germinates rapidly germinate at temperatures between 63-77 °F (17-25 °C), but often fail to germinate below 53 °F (11.5 °C).
2. Crop yield and plant health
Freezing seeds also help increase crop yield and plant health.
In a study, giving the cold stratification procedure to tomato seeds before germination resulted in a much higher yield and more healthy plants than those that were not frozen before (see Table 1).
The freezing temperature is believed to have decreased the viral and fungal load of the seeds, resulting in healthier plants. For tomatoes, tomato yellow leaf curl virus and tomato mosaic virus are often transmitted via seeds.
| Yield change | Plant number change | |
| Seven Cultivar | 268% | 3333% |
| Potiron Ecarlate Cultivar | 208% | 50% |
| Druzhba Cultivar | 71.9% | 0% |
3. Leaf and flower formation
Freezing seeds before germination can even improve plant growth, producing more and bigger leaves and flowers.
Freezing seeds of melon and squash at sub-zero temperatures for only a few hours or a day before germination all resulted in more and larger leaves and flowers (see Table 2).
It is believed that freezing induces changes in the plant’s metabolism, such as higher content of ascorbic acid and chlorophyll, resulting in higher resistance against low temperature.
It is also believed that exposing seeds (even warm-season plants such as cucumber) to freezing temperatures can help increase the production of plant sugars, and subsequently increase the plant’s tolerance against the cold (Markovskaya, Sherudilo & Sysoeva, 2007).
| Plant | Results after cold stratification |
| Melon | 44-67% bigger leaves, 16% more flowers, 37-42% higher yield, more leaves |
| Squash | 10-20% increased yield |
| Cucumber | intensive flowering, early fruits, 40-70% increased yields |
How to cold stratify seeds?
Putting your seeds in the freezer for a few days or even just a few hours before germination is a great way to cold stratify seeds.
But, you must make sure that the seeds are adequately dried. To check their moisture content, you can perform the “bend test”. If the seeds bend, they are too moist and not ready for cold stratification. If they immediately snap, they are dry enough for freezing. For tiny seeds, use tweezers to perform the bend test.
If the seeds are still moist, leaving them in the sun for a couple of days would be enough to dry them.
Do all seeds need cold stratification?
Plant seeds that can benefit from a pre-germination cold stratification are believed to have cold climates as their natural environments, namely woody plants (e.g. apple, plums), and herbaceous perennials (e.g. lavender, sage, sedums, milkweed species, perennial sweet peas, wild rose).
For warm-season seeds, cold stratification may not have any effect on them. But, there are also studies (e.g. on tomatoes, cucumbers) that they can benefit from the pre-germination cold treatment as the freezing temperature can kill the fungus and virus on the seeds.
In any case, as long as the seed is adequately dried, freezing them before germination will not kill them.
Conclusion
For seeds that are dried, freezing seeds is a great way to store them. Cold stratification does not kill seeds and can even be beneficial in developing into plants with better growth and better health.
Happy gardening!
Related
5 Ways To Break Seed Dormancy: Which is the Best?
How Long Do Seeds Last? (And How to Store Them)
Top 12 Fruits That Grow True To Seed
References
Junttila, O., & Stushnoff, C. (1977). Freezing avoidance by deep supercooling in hydrated lettuce seeds. Nature. 269: 325–327
Jaganathan, G. K., Dalrymple, S. E., & Pritchard, H. W. (2020). Seed Survival at Low Temperatures: A Potential Selecting Factor Influencing Community Level Changes in High Altitudes under Climate Change. Critical Reviews in Plant Sciences, 39(6), 479–492.
Parker, K. (2022, January 27). Seed stratification: What seeds require cold treatment. University of Illinois Extension.
Markovskaya, E. F., Sherudilo, E. G., & Sysoeva, M. I. (2007). Cucumber Seed Germination: Effect and After-effect of Temperature Treatments. Seed Science and Biotechnology.
Shevchenko, N., Miroshnichenko, T., Mozgovska, A., Bashtan, N., Kovalenko, G. & Ivchenko, T. (2021). Does Cryopreservation Improve the Quality of Tomato Seeds? Biol. Life Sci. Forum 2022, 11(1), 6;
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