How much light should tomato seedlings get? If you give them too little, they will become tall, skinny, unhealthy plants, a condition called leggy. If you give them too much the leaves will be damaged. How much is the best amount?
The new science of growing tomato seedlings will surprise you.
Key Takeaways
- Tomato seedlings grow best with a PPFD of 250 to 300.
- A DLI of 10 to 13 works best.
- All plant seedlings need 8 hours of darkness.
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Executive Summary
Gardeners struggle with leggy seedlings which is mostly due to a lack of light either from windowsills or budget LED lights.
New LED lights are very bright compared to the older models or LED shop lights. Are they too bright? How much light do you need to grow tomato seedlings?
Research has determined that the ideal light for tomato seedlings is between 250 and 300 PPFD with a DLI of 10 to 13. That is lower than suggestions from most online sources.
I measured the light on my sunny west-facing window, zone 5, April 10, on a sunny day and got a PPFD of 750. Keep in mind that although the light level is high, it is only available for part of the day. That is why the DLI is so important.
This spring I grew my seedlings using a new MetryGrow LED light at a PPFD of 450 and a DLI of 23. This will give good growth, but I can reduce my light costs and produce better plants if I lower the amount of light a bit. You can see the seedlings produced by this light in this video.
The terms PPFD and DLI are not familiar to many gardeners but if you grow under lights you must understand them better. They are fully explained in the following sections along with references to the science that supports the above claims.
The Quality and Amount of Light
The focus of this post is to discuss lights used by gardeners to start tomato seedlings indoors and I’ll focus the discussion on LED lights and window sills.
The quality and amount of light depends on these factors:
- wavelengths of light.
- light intensity.
- duration and total amount of light.
It is important to understand each of these to better understand the effect of light on plant growth.
Wavelengths of Light – Getting the Right Color Spectrum
The electromagnetic spectrum is divided into separate bands of energy. They range from low to high frequency and include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Plants use bands of infrared, visible light, and ultraviolet.
The claim that plants are also affected by electroculture is a myth.
Even though plants can use a wide range of frequencies, certain ones, especially in the visible range, are most important for plants. Scientists call this spectrum the “PAR spectrum“. It consists mostly of blue and red light, but also includes some green and yellow.
Most of today’s higher-quality LED grow lights mimic the PAR spectra. Non-grow lights and cheaper LED lights, even if they are called grow lights, may not do that. You can identify a quality grow light by the fact that its marketing material shows you the spectra of the light produced by the lamp.
Older terms used for fluorescent lights, such as cool white, and 5000K (i.e. kelvin), do not apply to LED grow lights, and gardeners should stop using them.
Light Intensity
In addition to having the right kind of light, it is also important to have the right amount of light. Scientists measure the quantity of PAR light using the PPFD (Photosynthetic Photon Flux Density) value, which has the unit of micromoles per square meter per second (µmol/m²/s). PPFD quantifies the amount of photosynthetically active radiation (PAR) that reaches a plant or a specific area.
In simple terms, PPFD is the amount of “good” light that reaches a plant. A low value is equivalent to a shady condition and a high value indicates a sunny location.
The goal of growing any plant is to provide a PPFD value that is high enough to produce good growth, but not so high that it damages the plant.
The marketing material for a good quality LED grow light will provide the PPFD values in the form of a PPFD map This map will show the value at specific spots under the light, with the light at a specific height. In this example, you can see the value at specific spots at a height of 12″, 14″, and 18″.
Note that the data for PPFD maps are collected with the light inside a grow tent or chamber. The inside walls of the grow tent are reflective, resulting in higher values of light than one gets in a normal room, resulting in values that are higher than found in a normal room or grow area.
What is the PPFD of sunlight? That depends on where you live, the time of day, and the cloud cover. At the equator, at noon, without clouds, sunlight has a PPFD of 2000 (roughly equivalent to 10,200 foot candles or 108,000 lux). In New York, the summer PPFD ranges from 1500 on a sunny day to 300 on a cloudy day.
Duration and Quantity of Light
The PPFD value indicates the amount of light hitting the plant at any given point in time, but it does not account for the amount of time the light is on.
The effect of light on photosynthesis is cumulative. So a high-intensity light for a short period can produce the same growth as a lower level of light that shines for a longer period. For this reason, it is important to know the total amount of light in a 24-hour period, which is known as the DLI or Daily Light Integral.
The DLI is a useful way to quantify the total amount of plant-usable light (i.e. PAR light) that a plant receives each day.
Best Light Intensity for Tomato Seedlings
What light intensity is best for seedlings? The PPFD of midday sunlight can be as high as 2,000 and our plants grow outside all summer long. It is reasonable to think that tomato seedlings need a high value like this. Keep in mind that this value is only reached at midday. It is much lower In the early morning and evening, and it’s also much lower on cloudy days.
Proper LED grow lights can easily produce values over 1,000 at one foot under the center of the light and these lights are recommended for growing tomato seedlings.
What does science say? A recent study looking at different LED light intensities of 60, 150, 240, 330, and sunlight found that a value of 240 gave the best growth. A value of 240 produced a larger root system and more total plant mass, while keeping plants shorter. Both the 60 and daylight produced taller seedlings that were of a poorer quality. The lights were on for 12 hours in this study producing a DLI of 10.4 for the 240 value.
The research also found that 240 μmolm-2s-1 produced the highest chlorophyll content, indicating that the plant was functioning at its best at this light intensity.
Another study found similar results and concluded that “300 μmol m−2 s−1 PPFD are recommended for ‘Micro-Tom’ cultivation at the vegetative growth stage to improve the RUE”. RUE is a measure of how efficiently the plant uses light.
Other Interesting Facts?
Is Nighttime Darkness Important?
What happens if tomato seedlings are given light continuously?
This was tested by exposing seedlings to a PPFD of 300 for 12 hours. The control plants were then given darkness for 12 hours while test seedlings were given various light levels, including white LED light at 150 or 300. Plants that received light during the night showed injuries and the degree of these injuries increased with increased light levels.
Plants should be given at least 8 hours of darkness.
Is Full Spectrum Light Important?
Another study looked at cherry tomatoes and the effect of different colors of light at 320 PPFD. A combination of red and blue, or red, blue, and green, performed better than a single color or white light. Similar results were found for peppers, in another study.
Given the same total amount of light, plants do better with just red and blue, than with white because they get more light energy of the wavelengths they can use efficiently.
This testing also debunks the myth that green light is not used by plants.
Best DLI for Growing Tomato Seedlings
This study looked at different combinations of light intensity and duration to determine the best DLI and concluded that it is 13.2 mol·m−2·d−1. Plant quality decreased when the DLI was over 14.3.
The best growth was produced with a PPFD of 200 and a duration of 18 hours. However, from a gardener’s perspective, the difference between 18 hours and 12 hours was not significant and as a general rule, gardeners should provide at least 8 hours of darkness.
This DLI value is lower than reported by other blogs, which suggest a DLI of 22 to 30, but they don’t provide a source for the numbers.
I’ve germinated tomato seeds under fluorescent lights. These were a combination of cool white and warm light (i.e. the cheap type of tube, one of each type in the fixture.) The fluorescent tube was kept at about 2 inches from the top of the seedling. The seedlings grew well, with no visible ill effects: nice green color, no signs of leaf burn or etiolation.
I did this to test soil for pesticides, per your recommendations, and not to grow tomato plants.
I also use fluorescent lights to keep alive the plants that I bring indoors for the winter. (Too many plants and not enough sunny windows.) I keep the tubes fairly close to the plants. (some tubes are hung vertically.) These too are regular cool/warm white bulbs, and they do the job. I get flowers Pelargonium (commonly called geranium) and Bougainvillea, but not on Lantana, but my goal is tokeep the plants alive, not to have growth or flowers.
For me, this is a cheap, simple solution that works well for me.
I always enjoy reading Mr.Pavlis’s advice. Reading all the light science is a little confusing to me but interesting. I have been starting my tomato seeds for the last 30 years in barrels behind my garage with plexiglass lids, whether it’s cold, hot whatever, I always end up up with the best tomatoes in my gardening circles. It takes a lot of monitoring so they don’t burn or freeze but it’s rewarding. I don’t need grow lights, heating pads etc. and supply probably 8 friends with my plants. I call it my Polish greenhouse ” since I am Polish and grew up in a coal mimng town where I learned to improvise.