A lot of specialty seeds need light to germinate, but it never occurred to me that vegetable seeds might need light until a couple of months ago, when I read that lettuce needs light. I have always covered my lettuce seed, and it germinated just fine. How can that be if they need light?
How much light do they need? Does a thin covering of soil allow enough light to reach them?
I decided to do some tests to figure things out, and that just made the picture murkier. Here are my test results and information from a number of scientific studies. The answer is not what you think.
Key Takeaways
- Lettuce seeds can germinate in the dark.
- The light requirement for germination depends on the cultivar.
Testing the Light Requirements of Lettuce
This seems simple enough. I used two-year-old, McKenzie, ‘Prizehead’ lettuce for all of my tests. I set up some pots with Pro-Mix potting soil and placed 5 seeds on top. I then covered the seeds with 0, 1/4″, 1/2″, and 3/4″ of potting soil. This was done so that the final amount of soil in each pot was identical. The pots were watered and set under lights, waiting for germination.
The viability of seeds can be easily tested using a seed viability test.
| Amount of Soil Over Seeds | Percent Germination |
|---|---|
| 0″ | 80% |
| 1/4″ | 40% |
| 1/2″ | 0% |
| 3/4″ | 0% |
The results seem to indicate that lettuce seeds need light, which is what most gardeners report.
But it is also possible that the small seeds germinated in the dark but could not reach the surface. If this were true, they might have germinated just fine, but never grown properly. More testing was needed.
Testing With the Baggy Method
I germinate most seeds using the advanced baggy method. One advantage of this method is that you can easily observe the germination process, and since germination occurs without being buried, any issues with soil on top of the seeds are eliminated.
I set up two baggies, each with 30 lettuce seeds. One was left on my grow room potting bench in full light all day. The second one was placed in a drawer under the bench, ensuring similar temperatures for both sets of seeds. These are the results.
| Light Exposure | Percent Germination |
|---|---|
| full light | 50% |
| dark drawer | 47% |
The results indicate that lettuce seeds either don’t need light or need very little light to germinate.
Since these results contradict common knowledge, I tried to rationalize the difference. It is possible that the “dark drawer” was not dark enough. Perhaps it let in enough light to trigger the seeds to germinate, after all, the bench is located in a very bright grow room. It should also be noted that the baggy in the drawer was taken out of the drawer once a day to count germinated seeds. This only took a minute, but it still exposed the seeds to light.
One more experiment was warranted to test “true” darkness.
Testing Complete Darkness
I set up another baggy with 30 seeds just like the previous experiment. This time, the baggy was placed inside a well-sealing metal container. That was placed inside a black cloth cooler, which was placed in a dark closet. The seeds were left for two weeks with zero light exposure.
| Light Exposure | Percent Germination |
|---|---|
| Full darkness | 20% |
The amount of germination in full darkness was lower, but there was still some germination.
Conclusions About Lettuce Germination
What do these experiments show?
- The seeds I tested germinate in the dark, but light increases the rate of germination.
- Even small amounts of light are enough to improve germination.
- Covering the seed with a thick layer of soil prevents germination or stops the growth of the germinated seed. Too much soil over small seeds is never a good idea.
These results do NOT agree with the common claim that lettuce seed needs light to germinate.
What Does Science Say?
Most studies about lettuce germination refer to a very old study by H A Borthwick et al, 1952, that looked at the effect of far-red and red light on germination, using Grand Rapids lettuce. References to their work claim that they showed lettuce seed needs light to germinate, however, their data clearly shows that their dark control had 8.5% germination. This is much lower than seeds given light, but it’s not zero germination.
I suspect that the common belief stems at least in part from a misinterpretation of these results. People just rounded off 8.5% to 0%.
A more recent study looking at ‘Levistro’ (green) and ‘Carmoli’ (red), lettuce cultivars, found that 99.5% germinated in the dark. This lettuce is a loose-leaf type of Lactuca sativa var. acephala.
Lettuce Germination is Temperature Dependent
Germination is also affected by temperature.
Does Lettuce Seed Require Light for Germination?
My experiments and the research studies don’t present a simple answer. The reason for this is that there is no simple answer.
Grand Rapids is a well-known cultivar that’s been grown for a long time. When studied in 1952, it germinated very poorly in the dark.
Over the years, breeding has changed lettuce. In an effort for better and more reliable germination, some cultivars of lettuce have been bred so they no longer need light for germination. Grand Rapids is a good example of this. Newer varieties are less dependent on light.
A more recent assessment of Grand Rapids varieties found the following germination rates in the dark (the reference provides a longer list of these).
- Waldman’s Dark Green (older cultivar) – 47%
- Two Star – 70%
- Black Seeded Simpson – 90%
Other types of lettuce don’t have such a strong requirement for light.
What does all this mean?
- The statement that lettuce seed needs light to germinate is a myth. Some cultivars need more than others, and some cultivars will benefit from light, but to say lettuce needs light is wrong.
- Even cultivars that germinate better with light have some germination in the dark. Each cultivar has different requirements.
- When light is useful, a very low level of light will improve germination.
- If you are growing seeds with unknown light requirements, it is still a good idea to either leave them uncovered in high humidity conditions or cover them with minimal soil. They are still small seeds with a limited ability to grow through the soil.
The Red vs Far-red Effects on Lettuce
This has nothing to do with the above discussion, but it is so interesting that it’s worth adding to this post.
The work mentioned above by Borthwick et al. showed that red light triggered germination of Grand Rapids lettuce, while far-red light inhibited it. When such seed was given alternate red and far-red light, it was the last light treatment that determined germination. Red and far-red light work like on/off switches for germination.
Sunlight includes both types of light, but leaves absorb only the red light. The far-red light travels through leaves, reaching the seeds on the ground. Since seed lying on the ground under plants only gets far-red light, it stays dormant. When the canopy opens up, red light reaches the seed, initiating germination.
This control process keeps seeds from germinating in areas that don’t have enough direct sunlight for them to grow properly. Instead, they wait until space opens up for proper growth.
Thanks to M. Neff, L. Sanderson, and D. Tedor for describing this in their write-up of a great school project for teachers.
