Growth and germination rates of hydroponic lettuce seedlings vary with light exposure time.

When growing lettuce from seed in hydroponics, I was wondering how much light I should expose it to after it germinates to speed up its growth, so I experimented.

Lettuce seems to be a long-day plant and needs a moderate period of darkness, so we prepared 50 seedlings each of lettuce that was not exposed to light between 9:00 p.m. and 5:00 a.m. (16-hour lettuce) and lettuce that was exposed to light all day (24-hour lettuce), and observed the growth process.
Fifteen days have passed since sowing, and although there are individual differences, we can conclude that the 24-hour lettuce grew 1.5 to 2 times larger on average than the 16-hour lettuce.
I’ll explain the results in order.

Germinate lettuce seeds by germinating them in the vegetable compartment of the refrigerator.

I used regular sunny (leaf?) lettuce seeds that I bought at a 100-yen store. Lettuce seeds.
I put moistened kitchen paper in a plastic case, scattered the seeds on the paper, covered it with a lid, and stored it in the vegetable room.

It is said that by storing the lettuce in a dark and damp place, the lettuce seeds will think they are planted in the soil and will germinate. I’m sorry if I’m wrong.
The next step is to sow the germinated seeds into the sponge medium.

Prepare seedling containers and lights for the same conditions.

Before sowing, we made containers for seedling growth.

There are four LED bulbs attached to the back of the lid.
The inside is filled with water, and Otsuka House’s concentrated liquid fertilizer is added to keep the EC value at 1.3. The air pump is branched to bring in oxygen.

The only difference is that the outlet in the left container has a smart plug engaged in it, and as mentioned earlier, the lights go off between 9pm and 5am, and it is completely dark inside.

Start seedling growth by floating a styrofoam with sponge medium set on it.

The germinated seeds are then sown into the sponge medium.

Plant the germinated seeds one by one in the middle slit of a sponge filled with water.
In the photo, I used a special sponge, but you can also use a cut-up 100-yen dish sponge.
I was thinking of using special seedling pots to make replanting easier, but I didn’t have enough pots for 100 seedlings on each side, so I just sandwiched them in the styrofoam.
We floated the styrofoam on the surface of the water mixed with nutrient solution and started to grow seedlings.

The germination rate and growth rate differed depending on the exposure time of the light.

After about 10 days, I guess, I started to see a gradual difference in growth.

On the left is a container of 16-hour irradiated lettuce and on the right is 24-hour lettuce.
It is a little difficult to see in the photo, but the lettuce that has been exposed to light for 24 hours is growing larger.
At the same time, however, there were many seeds that had not germinated.
In concrete terms, 35/50 (70%) of the seedlings germinated, while the remaining 15 seedlings did not produce any leaves.
On the other hand, the lettuce seedlings irradiated for 16 hours on the left had a germination rate of 42/50 (84%), although the growth rate was inferior to that of 24 hours.
Since the seedlings are still young, we don’t know how they will change in the future, but it may be better to continue to expose them to light if we want to grow larger seedlings, and to provide a dark period if we want to increase the germination rate.

Future growth experiments

I will be experimenting to see if a dark period or a 24-hour period is better for growing strong plants without overgrowth.
I also bought a plant growth light, so I’d like to see how much the growth rate changes with regular LED bulbs.