Even plants are affected by their youth

Anyone debating whether to use fluorescent lighting or LED lighting for indoor gardening, or ​trying to decide on which ratio of red to blue light to use, may well be advised to pay attention to ​this recent South Korean study, especially when it comes to the ​lighting ​requirements of seedlings. In this 2018 study, the researchers briefly grew 2-week old cherry tomato seedlings under different combinations of red and blue light versus a fluorescent light control for 4 weeks before transfer to a regular greenhouse (i.e. natural sunlight) with some striking results.

Seedling light treatment
Abbreviation
0 DAT ^
37 DAT ^
68-75 DAT ^
Fluorescent (+HPS**)
control
less growth
less growth
​smaller harvest
​100% Red LEDs
​0B
​more growth
medium harvest
​87% Red : 13% Blue LEDs
​13B
​more growth
largest harvest
74% Red : 26% Blue LEDs
26B
​65% Red : 35% Blue LEDs
​35B
​53% Red : 47% Blue LEDs
​47B
medium harvest
41% Red : 59% Blue LEDs
59B
less growth
less growth
​smaller harvest

More...

​^DAT : Days After Transplantation - This refers to the number of days ​following transfer of the plants to the greenhouse.
**One discrepancy to note is that according to the study, fluorescent lighting and HPS lighting were used ​together as the control condition for the experiment. However, our examination of the light spectrum provided for the control condition suggests that the HPS component was negligible as the control spectrum looks identical to that of fluorescent lighting alone. Therefore, our inclination is to assume that the control condition in this study is actually more equivalent to fluorescent lighting only. However, either way, the main conclusions from the study still hold true and underline the fact that LED lighting, applied at the seedling stage at least, can generate better results ​for indoor growing.

​At various points during the grow, the seedlings / plants were checked for stem length, leaf size, and other growth characteristics to reveal some interesting insights. After the 4 weeks under the different LED / fluorescent light treatments, the young plants grown under intermediate red : blue light combinations ​performed better than ​​the seedlings grown under fluorescent lighting (​as well as some of the extremes ​red or blue light conditions).  At 37 days after transplantation (DAT) into the greenhouse, ​​it was also clear that plants exposed to ​most Red : Blue LED lighting combinations ​as seedlings showed significantly ​better growth than the fluorescent light-exposed plants. Finally and probably of most importance to the indoor gardener, at 68-75 DAT when the fruit was harvested from the plants, the differences between the various conditions were quite ​striking with ​most red : blue light combinations yielding significantly more fruit (up to 1.8x more) than the control plants.

What implications does this study have ​for indoor gardening?

​In the case of cherry tomato ​plants at least, too much blue light applied at the young plant stage, can be detrimental to overall cherry tomato plant growth and fruit production, while ​higher levels of red light ​applied to seedlings ​appear to correlate with better growth. Of course using too high a percentage of red light can also have negative consequences for the cherry tomato plants. Ultimately, an intermediate combination of about 74% red light to 26% blue light or at an approximate ratio of 3 to 1, applied at the seedling stage, appears to be best for maximum ​cherry tomato plant growth and for maximal fruit harvest.