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In the accompanying article to this one, titled Plants Do Math At Night. How Long Should You Let Them Sleep?’, we looked at why giving plants a daily rest period is critical, and some of the important biological functions they perform when the lights are out. In this article, we continue the conversation into grow lights, and more specifically we address the question of how long you should be leaving your grow lights on for each day.

Now, there’s a lot of science on this topic, and being the plant nerds we are, it took all the restraint we have to refrain from turning this into a thesis dissertation. Instead, we’re offering the following:

  1. A 3-Step Process that you can use to calculate the optimum light intensity for your plants, and
  2. A free online spreadsheet that you can use to help with your own math.

Let’s jump in.

Step 1 – Understand The ‘Ideal’ Hours Of Light Per Day For Your Plants

Short vs Long Day Plants – How Their Light Needs Differ

In the plant world, scientists refer to plants as being either “long” or “short” day plants. 

  • LONG-day plants need short periods of darkness to flower. Common examples of LONG day plants include basil, cilantro, parsley, dill, mint, and tomatoes. 
  • Conversely, SHORT day plants need long periods of darkness to flower. Common examples of SHORT day plants include avocado, mustard greens, marigold, zinnia, and strawberry. 

So theoretically, if you wanted to emulate a plant’s ideal growing environment, then all you’d need to do is look up its ideal day length and use that to set the on/off timer on your grow light.

The problem with this approach however is that it’s likely to lead to your plants flowering quickly. For some plants, e.g. tomatoes and peppers, that’s a good thing – because the flower is what ultimately produces fruit, but for other plants, such as lettuce and cilantro, flowering is the beginning of the end. Leaves will also change shape, their taste can become bitter, and at this point, most gardeners will pull them out and start over. 

In addition to classifying plants as either short or long days, we can also classify them based on whether flowering is desirable, or undesirable.

Using Light Duration To Promote (or Avoid) Flowering

So for plants where the flower is desirable, you want to give your plants their ideal light duration. For plants where flowering is undesirable, you should do the opposite. We find it is useful to break the plant world down into four quadrants at this point.

  Long Day Short Day
Flowering Desirable

Provide a long day, since we want flowers:

  • Fig
  • Peach 
  • Tomato

Provide a short day, since we want flowers:

  • Avocado
  • Chamomile
  • Coffee
  • Cosmos
  • Marigold
  • Strawberry
  • Zinnia
Flowering Undesirable

Provide a short day, to avoid flowering:

  • Arugula
  • Basil
  • Cilantro
  • Dill
  • Lemon Balm
  • Lettuce
  • Mint
  • Parsley

Provide a long day, to avoid flowering:

  • Ginger
  • Sage

Is There A Limit To How Long You Can Leave Lights On For?

Short answer: yes. Even the long-day plants, where flowering is desirable, do have their limits. As we explained in ‘Plants Do Math At Night. How Long Should You Let Them Sleep? plants do require a daily respiration period of at least 6 hours per day (for seedlings) and ideally 8-10 hours for more mature plants. Therefore we do not recommend providing more than 14-16 hours per day of light, even if you are growing long-day-loving varieties and trying to induce flowering/fruit from them. 

Step 2 – Understand The ‘Ideal’ Daily Light Integral (DLI) Requirements For Your Plants

Not only do plants have an ideal duration of light each day, but they also have varying needs in terms of the ‘volume’ of light that they accumulate per day. This ‘volume’ of light is measured via a term called ‘Daily Light Integral’ or DLI. (check out Grow Lights For Indoor Plants – How To Measure It, and Understanding Watts, PPF, PPFD, and DLI if you’d like to learn more about this topic). 

Decorative indoor plants like the popular pothos, snake plants, or monstera might be content with a DLI of 1-4 mol/m2/day, but most edible plants will need a DLI somewhere in the 10 – 30 mol/m2/day range.  

Side note: DLI is a measure of how much light energy falls on a surface (say, a plant leaf) in a 24 hr period. Where you and I have a certain number of calories we need each day to keep us going (some of us more than others), plants all have their own DLI requirements. Failing to give a plant sufficient DLI over an extended period will have the same effect as failing to give you enough calories. At best, you’ll get hangry. At worst, you could perish. 

Step 3 – Calculate The Ideal Light Intensity (PPFD) Requirements For Your Plants

Once we know a) the total volume of light that a plant needs to be happy, and b) the ideal time/duration in which we should deliver it, then we can simply divide one number by the other in order to calculate the ideal delivery rate of that light. This is exactly the same math you would use if I told you we had a 10-gallon bucket and it needed to be filled over a 5 hr period. You’d simply divide 10 by 5 to deduce that you needed to be filling the bucket at 2 gallons per hour. 

Where we measure the rate of water flow in gallons (or liters) per hour, we measure the rate of flow for light in PPFD (photosynthetic photon flux density) in moles per meter squared per second, or mol/m2/s. 

We’ve summarized this math for you in the table below. If you’re interested in a plant that’s not listed here, please leave a comment and we’ll do our best to find an answer for you and add it to the table. 

Other Considerations in Determining How Many Hours a Day Are Needed for Indoor Plant Grow Lights

Other considerations to keep in mind when determining how long to leave your grow lights on for indoor plants include:

  • Delivering the target DLI in too short a time period requires a high rate of flow (PPFD). Just like you can get sunburned, plants can also get ‘light burned’. Subjecting plants to excess PPFD is likely to cause their leaves to go brown.
  • If you are having trouble getting a plant to flower (and you want it to flower) try very gradually to modify the day length. Gradually shortening day-lengths can ‘trick’ plants into thinking that the end of the growing season is approaching and winter is coming. They’ll want to get their fruit produced before that happens.
  • Plants need to sleep. They perform important respiratory functions at night. Seedlings should have at least 6 hours of darkness per day, and more mature plants at least 8-10 hours a day. 

We hope this guide to indoor edible gardening and grow lights has been useful. The next logical step from here is to take what you’ve learned about ideal PPFD and use that to determine the placement and type of grow light you need. If you’d like to learn more about this topic we recommend reading Setup and Placement of Grow Lights For Plants – How To Get The Distance Right? (coming soon) as well as signing up for our email list below. 🙂

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