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.
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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 |
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.