How Long Should My Grow Lights Be On For Hydroponics?

I need sunlight daily to get Vitamin D, but if I get too much at once I will get sunburnt and potentially cancer! It’s better to get an optimal amount of light (such as when wearing sunscreen) over a couple hours.

Daily Light Integral

Just like us plants have optimal light brightness levels, as we calculated in the post How Bright Should My Hydroponics Light Be? Different stages of plant growth will require more or less light strength and lighting time. A seedling might need very low light strength over a short time (its a baby so it needs to sleep a lot), whereas a fruiting plant will need all the extra energy it can get through higher light strength and longer lighting times.

The best way to tell if your plants are receiving enough light to grow is by measuring the Daily Light Integral, or DLI. The DLI is a measure to of how many moles/m2/day our plants get. Notice how we will be converting from umols, or micromoles, to their higher unit. This is a bit like grams to kilograms, or ounces to pounds.

If you already have the PPFD or umols/s/m2, such as whats advertised when buying lights online, then you can use a simple online calculator to calculate your required number of hours to the DLI. If you don’t know how many umols/s/m2 your room or light is, check out the post How Bright Should My Lights Be?

According to Wikipedia having the correct DLI can help your plant leaves to grow thicker, increase flower and fruit yields, have more and stronger roots, as well as have more leaves and heavier biomass (they produce more leaves and less stems and stalks).

According to Specmeters:

DLI
Lettuce (butterhead)14-16
Tomatoes (seedlings) 6-8
Tomatoes22-30

Examples

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.

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

My kitchen had on average 250 LUX from the window – it’s a dark room considering direct sunlight has 900-2000umols. Using the Environmental Growth Chambers calculator, 250 LUX converts to 4.75 umols/s/m2.

If I was growing tomato seedlings, I would need to divide my result (4.75) by the number of hours to reach the ideal DLI of 8. In this case I would need my plants to receive approximately 1.5 hours of sunlight per day. An adult tomato needs 22-30DLI, so as it grows I would slowly increase the number of hours its exposed to sunlight to 6 hours a day.

But I will be using my grow light on my tomatoes. This light produces 565umol/s/m2. Using the same calculator I can tell that for seedlings I only need to expose them to 4 hours a day of light, whilst adult plants would need ideally 14 hours a day.

Quick Reference Chart

This PDF has a wonderful table of many, many, plants that compares different life stages, the strength of light recommended, and how many hours those plants require at such a stage.

Where To From Here?

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.

How Bright Should My Hydroponics Light Be? is the previous post in this series. It explores how much light certain plants needs to grow. I also show you how to convert to the more common used light measurement value umols or PPF, which many grow lights use. Understanding these values, and how they get them, will help you make an informed purchase.

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