How Bright Should My Hydroponic Lights Be?

In the post Do I Need Grow Lights I measured the amount of light in each of my grow rooms, hoping to figure out which room would produce the best plant growth. At the time I was hoping to not have to invest in a grow light; man was I wrong!

A Short Warning

LUX, or lumens, to determine plant growth is widely inaccurate due to the different types of lights and the wavelengths light types each produce. Plants need certain wavelengths, or colors, to grow. No matter how bright your lights are if they are missing those wavelengths then then the plants will just not grow.

This post will not consider the wavelength or colors of lights. I’m trying to keep it as simple as possible so that anyone can follow along, even without fancy gadgets like quantum meters which can easily cost US$500. Future posts will explain wavelengths in detail. Beginners Guide To Hydroponic Lighting has a list of all the posts on how to measure and set up your lights.

You need the umol’s value or the umol/s/m2: in essence umols are how bright your light is. This value is also known as “photosynthetic photon flux” or PPF. Note that PPFD is a different value, taking into account the distance from the light.

Too low of a light strength on your plants may lead them to not growing, and too high may burn or kill them.

How To Calculate Bright Your Light Is

Most people don’t have fancy tools to measure the amount of light in their homes, such as a PAR meter or a quantum meter. Thankfully, you can get free apps on your phone that measure lumens easily.

You can use the Plantekno website to convert LUX to umol/s/m2. Note: many grow lights will tell you the PPF or umol/s/m2.

Natural sunlight has a PAR value of 900-2000μMol/m2/s when directly overhead, varying with location and season. For a grow light to be effective, it will have PAR values of 500-1500 μMol/m2/s.

In comparison, I measured my kitchen window light at 250LUX or 4.6umol/s/m2. No wonder my plants kept dying! As you’ll see below, its way below the minimum light requirements for lettuce to grow.

But how bright should my light be?

Be aware that not all aspects of how much light plants need are fully researched yet : Lefsrud at McGill University has extended upon NASA research and one thing he noted was common incorrect assumptions. For example, many people consider lettuce to a shade loving plant and tomatoes to enjoy full sun. However when shone with 5000 umol’s, the tomatoes suffered whilst the lettuce showed no signs at all! The exact opposite of what was expected.

Research Gate website had several listed studies that mentioned the number of umol’s used for ideal plant growth:

Minimum umol’s Required
Lettuce100-200
Watermelon250
Tomatoes400-500

According to the company Valoya the amount of umol’s a plant needs will vary depending upon its growth stage: seedling, vegetative, flowering/fruiting. They claim that lettuce needs approximately 80 umols during seedling, 150 for vegetative stage and 200 when it begins to flower.

Valoya’s Comparison of umol’s for Lettuce Growth

I found a really great article from the Department of Horticulture and Crop Science: this PDF covers a huge variety of plants, and the recommended lighting and hours lit for each of the growth stages, and what level nutrient solution is ideal.

Here is a small sampling of some common plants and their needed light intensity (taken from above PDF link):

Lettuce

# Light HoursLight Intensity
(umols)
Propagation12-20150-250
Vegative12-20250-450
Flowering12-20250-450

Strawberries

# Light HoursLight Intensity
(umols)
Propagation12-20250-450
Vegative12-20 250-450
Flowering<12 250-450
Fruiting12-20 250-450

Tomatoes

# Light HoursLight Intensity
(umols)
Propagation12-20250-450
Vegative12-20450-700
Flowering12-20 450-700
Fruiting12-20 450-700

Where To From Here?

Notice how I mentioned the number of light hours needed for plants in the tables above? Different plants need different lengths of light exposure. You can’t just leave your grow light on 24/7! The next post in this series is How Long Should My Hydroponic Lights Be On? It explores Daily Light Integral or DLI, the measurement that takes into account growth light strength x time its on for optimal growth.

Beginners Guide To Hydroponic Lighting has a list of all our posts on lighting, written to help a beginner learn the basics and get started right through to more advanced topics.

Do I Need Grow Lights? is the previous step in this series. Not everyone has a PAR meter or Quantum meter; they can be expensive. So how else can you measure the light your room gets? Whilst not very accurate, this can give you an idea of how bright your rooms are; helpful in finding out if you need to invest in a grow lamp!

How Long Should My Grow Lights Be On For Hydroponics? is the next step in this series. You know how bright your light should be, but how long should it shine? Plants need time to sleep and rest too, just like you. This post explores how to determine how many hours of light your plant needs (it can vary a little depending upon how bright your light is).

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