Do Seedlings Need Light? (What About Darkness?)


Most seeds do not need light to germinate.  But what about after a seed sprouts and turns into a seedling?

So, do seedlings need light?  Seedlings need light to continue growing after germination.  This light can come from both natural sources (sunlight) and artificial sources (grow lights).  Seedlings need to get enough light each day that is bright enough for photosynthesis in order to produce energy.

Of course, it is also possible for seedlings to get too much light.

In this article, we’ll talk about how much light to give seedlings.  We’ll also talk about some different types of light you can use.

Let’s get started.

Do Seedlings Need Light?

Seedlings need light to continue growing after seed germination.  The reasons is that light is necessary for photosynthesis (energy production from light, water, and carbon dioxide).

seedlings
Seedlings need light in order to continue growing into mature plants.

This light can come from natural or artificial sources, either indoors or outdoors.  However, the light must be bright enough to support the growth of the plants.

If the light source is not strong enough, seedlings will not grow properly.  According to the University of Maryland Extension, plant stems will stretch themselves out and lean towards a weak light source (especially tomato and cabbage seedlings).

This stretching will make seedlings thin and “leggy”.  It can also stunt their growth later in the season.

A lack of sunlight also hinders chlorophyll production.  Chlorophyll is what makes plants green and helps them with photosynthesis.

Without enough light, plants will turn light green and then yellow, due to a lack of chlorophyll.  They will eventually die due to a lack of energy.

Do Seedlings Need Sunlight?

Strictly speaking, seedlings do not need sunlight.  You can provide them with artificial light and keep them indoors until it is warm enough to plant outside.

sunlight through trees
Sunlight is the best light source for plants, but they can survive without it, even as seedlings.

 However, bright sunlight is the ideal light source for seedlings.  If you can only provide partial sun (such as on a sunny windowsill), you might be better off using artificial grow lights until you can transplant the seedlings outside (more on grow lights later).

Do Seedlings Need Darkness?

Seedlings do need darkness.  According to the University of Maryland Extension, vegetable plants need 6 hours of rest in the dark.

It makes sense when you think about it – in nature, plants are not exposed to constant light.  After the sun goes down, they experience darkness for several hours.

As seedlings grow into more mature plants, darkness continues to be an important factor.  For example, according to the Oregon State University Extension, some plants need a specific time in the dark to begin flowering.

Photoperiodism refers to the time that a plant is in darkness vs. light.  There are different types of plants, based on their photoperiodism:

  • Short Day Plants – also called long night plants, these only form flowers when day length is less than 12 hours (that is, day is shorter than night).  Short day plants include chrysanthemums and poinsettias.
  • Long Day Plants – these plants only form flowers when day length is more than 12 hours (that is, day is longer than night).  Long day plants include lettuce, spinach, and potatoes.
  • Day Neutral Plants – these plants don’t really care about day length!  They flower with no regard for day length.  Day neutral plants include tomatoes, corn, and cucumbers.
female cucumber flower
Cucumber plants are day-neutral and will flower regardless of day length.

Can Seedlings Get Too Much Light?

It is possible for seedlings to get too much light.  In fact, according to the University of Minnesota Extension:

“Plants exposed to too much light may result in scorched and bleached leaves.”

https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants

This damage can hinder a plant’s growth for the rest of the season, leading to stunted growth or reduced yield.  Remember that both natural and artificial light can burn seedlings if they are too bright!

Keep in mind that plants vary in their light needs.  Some shade-tolerant plants will burn more easily than ones that like bright light (such as tomatoes).

tomato leaf roll curled leaves
Tomato plants prefer lots of bright light.

Also, a mature plant can generally tolerate more light than a seedling.  So, be careful when using grow lights for your seedlings – a small change in the distance from the light can make a big difference!

You can learn more about lights for seedlings in this article from the University of New Hampshire Extension.

There is one more thing to remember about light for seedlings.  When the time comes to transplant seedlings to the garden, be sure to harden them off.

Hardening off seedlings means giving them time to get used to outdoor conditions.  This gradual transition to different light, wind, and temperature changes gives plants a chance to acclimate to the outdoors.

You can learn more about hardening off seedlings in my article here.

When To Give Seedlings Light

Give seedlings light as soon as they start to sprout.  In nature, seedlings get light as soon as they break through the soil surface, so that is what you should aim for!

tomato seedling
Give seedlings light as soon as they emerge. This is how it happens in nature!

A seed contains enough energy and nutrients to get the plant started.  With this stored energy, the seed can germinate and grow these parts:

  • a stem (shoot)
  • a radicle (small embryonic root)
  • cotyledons (seed leaves)

(You can learn more about seedlings and how they start growing in this article from Wikipedia.)

However, after this initial growth, the plant needs to start producing its own energy.  It does this through photosynthesis, which requires light (either from the sun or from an artificial source).

Remember that leaving your seedlings on a windowsill might not give them the bright light they need (in a pinch, the south side of the house is better).  Seedlings on a windowsill might not get enough hours of light per day, especially in the shorter days of winter and early spring.

sunlight
Seedlings might not get enough light on a sunny windowsill, but you can use grow lights to make up the difference.

The solution is to use artificial lights (grow lights) to help your seedlings grow until the days are longer and warmer.

How Many Hours Of Light For Seedlings?

A seedling may need anywhere from 5 to 18 hours of sunlight per day, depending on how bright the light source is.

For example, the University of New Hampshire Extension suggests that a tomato seedling might only need 5 hours of light per day if the light source is full, bright, direct sunlight with no shade.

On the other hand, the University of Vermont suggests that a plant may need 12 to 14 hours of light per day if bright indoor grow lights are used.

incandescent light bulb
Using a bright light means that plants won’t need such a long exposure to the light.

At the extreme, the University of Illinois Extension suggests a maximum of 18 hours of light per day for seedlings.  This duration would be necessary for dim indoor grow lights.

The table below summarizes the duration of light for seedlings, depending on the light source.

Light
Source
Duration
Dim Indoor Grow
Light or Shaded
Outdoor Area
up to 18
hours daily
Bright Indoor Grow
Light or Indirect
Outdoor Sunlight
12 to 14
hours daily
Bright Sunlight
Without Shade
5 or more
hours daily
Duration for various light sources for seedlings.

Of course, you can always combine different sources to give seedlings enough light in a day.

For example, you can leave seedlings on a bright windowsill during the day (perhaps for 12 hours).  Then, to provide extra light, you can turn on grow lights in the evening (perhaps for 2 to 6 hours, depending on how bright they are, and how bright the sun is).

It might help to come up with a light schedule for your seedlings.  You could keep track of things like:

  • When to turn grow lights on and off (you could use a timer for this)
  • When to take seedlings out of sunlight (for hardening off)
  • Light source intensity (if you are experimenting with different types of bulbs and plants)

Let’s take a closer look at the types of light you can use for seedlings.

Types Of Light For Seedlings

There are three common types of light bulbs you can use for seedlings:

  • Incandescent Lights
  • Fluorescent Lights
  • LEDs

We’ll get into more detail about these types of light soon.  But first, we’ll talk about the colors of light and which are best for seedlings.

What Color Light Is Best For Seedlings?

A light that provides many colors across the spectrum would be best for seedlings.  Although plants do need both red light and blue light, that is not the whole story.

According to Garden Myths, plants also need green and yellow light, in addition to red and blue light.  Light does not need to be white, and it does not need to match the sun’s spectrum exactly.

Incandescent Lights For Seedlings

An incandescent light gives off light by heating up a metal wire filament.  Unfortunately, the heat generated makes an incandescent light less energy efficient than LEDs or even fluorescent lights.

light bulb
An incandescent light bulb gives off lots of heat, and is not energy efficient.

In addition, incandescent bulbs don’t usually last as long as LEDs or fluorescent lights.  It will cost more to replace them and to run them than other type of lights.

Are Incandescent Lights Good For Seedlings?

By themselves, incandescent lights will not be able to provide the balanced light spectrum that seedlings need for growth.  According to the University of Vermont Extension, incandescent light bulbs give off mostly red light.

Red light is indeed important for plant growth.  However, as we saw before, plants also need blue, green, and yellow light to grow.

In a pinch, however, incandescent lights are better than nothing.  They can also give off enough heat to keep seedlings warm if your seedling heat mat stops working and you need to buy a new one.

(You can learn more about how to keep seeds warm in my article here.)

Fluorescent Lights For Seedlings

A fluorescent bulb gives off light when an electric current excites mercury vapor gas, producing UV light that makes a glow in the phosphor coating on the bulb.

fluorescent light bulbs
Fluorescent lights give off less heat than incandescent bulbs and are more energy efficient.

A fluorescent light bulb is more expensive than an incandescent light bulb.  However, the longer life and lower electricity cost (due to less heat produced) will make it more cost-efficient in the long run.

Are Fluorescent Lights Good For Seedlings?

If you use “full spectrum” fluorescent bulbs, seedlings will get more of the various types of light that they need.  A full spectrum fluorescent bulb imitates the spectrum of daylight, making for a more “natural” light for plants.

By themselves, most fluorescent lights that are not full spectrum will not be able to provide the balanced light that seedlings need for growth.  According to the University of Vermont Extension, fluorescent light bulbs give off mostly blue light.

Although blue light is necessary for plant growth, plants also need red, green, and yellow light to grow.  If you combine both incandescent and fluorescent bulbs, you will get a more balanced overall light source for plants (both blue light and red light).

Fluorescent lights are still better than nothing.  They can also give off a little heat (less than incandescent bulbs, but more than LEDs) to keep seedlings warm in a pinch.

LEDs For Seedlings

An LED (light emitting diode) gives off light when a current flows through a semiconductor.  One or more LEDs make up an LED lamp.

LED lights
LED lamps are energy efficient, giving off almost no heat.

An LED light tends to be expensive, but they last a long time, meaning you won’t need to replace them often.  LEDs are also very energy efficient, since they don’t give off much heat.

This means you save on both electricity and replacement costs when you use LEDs.

Are LED Lights Good For Seedlings?

Modern LED lights are available in “full spectrum”, meaning that they provide all of the different types of light that plants need for growth.

These full spectrum LED grow lights are good for seedlings, since they can provide red, blue, green, and yellow light.

You can learn more about LED grow lights from Gardener’s Supply Company.

How Far Should A Light Be From Seedlings?

The cooler the light, the closer it can be to plants – as long as it is not too bright.  Lights that burn hot or bright are more likely to burn seedlings.

According to the University of Vermont Extension:

  • Incandescent bulbs give off the most heat, so they should be at least 12 inches above seedlings.
  • Fluorescent bulbs give off less heat than incandescent bulbs, so they can be 4 to 6 inches above seedlings.
  • LED bulbs give off even less heat than fluorescent bulbs, but they give off lots of bright light.  As such, they should be at least 12 inches above from seedlings.

Do Seeds Need Light To Germinate?

Most seeds do not need light to germinate.

green lettuce
Some seeds, such as lettuce, need light to germinate.

However, some seeds do need light to germinate, including:

  • Celery
  • Dill
  • Lettuce

You can learn more about light and its effect on seed germination in my article here.

Conclusion

Now you know what kind of light seedlings need and how to provide it.  You also know about the dangers of too much light for seedlings.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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