When Should You Expose Microgreens To Light?

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It can be hard to know precisely when to expose microgreens to light. Luckily, growing these little plants becomes straightforward once you are armed with a bit of information.

When should you expose microgreens to light? Most microgreens should grow in the darkness for three to five days before being exposed to light. Faster growing types of microgreens will need two to three days in darkness, while slower-growing types will need five to eight days.

Where your microgreens fall in that range depends on the specific type of microgreen you are growing along with growing conditions. In addition to this critical initial period of darkness, microgreens need a mix of light and dark as they continue to grow.

What Is The Right Time To Expose Microgreens To Light?

Microgreens need from two to eight days in darkness before being exposed to light. The amount of time before microgreens should be exposed to light is called the blackout period. Some easy to grow types of microgreens and their blackout period’s are:

  • Arugula: Day 1-5 
  • Broccoli: Day 1-4
  • Brussel Sprouts: Day 1-5
  • Cauliflower: Day 1-5
  • Chia: Day 1-4
  • Cilantro: Day 1-6
  • Cress: Day 1-3
  • Kale: Day 1-4
  • Lettuce: Day 1-5
  • Red Clover: Day 1-4
  • Sunflower: Day 1-4
  • Wheatgrass: Day 1-3

Beets are an example of a slower-growing microgreen as they need around five days of blackout time. A good rule of thumb is to check the microgreens after two days and assess their stage of growth. If the seeds have germinated, they are ready to be exposed to light.

The exact amount of blackout time your microgreens will need depends on more than just the type of plant. Temperature, moisture level, and seed quality play a role in determining the growth of microgreens. 

If the temperature is room temperature or around 70 degrees Fahrenheit, the seeds will germinate, or begin to grow, fastest. Straying too far from that benchmark temperature will increase the amount of blackout time needed. 

Moisture is also required for germination and affects how long before microgreens should be exposed to light. The particular amount of moisture and soaking your microgreens require varies widely depending on the type of plant. 

Seed quality is a fancy agricultural term for if the seeds are free of disease and genetically sound—the better your seeds, the faster the growth rate and the shorter your blackout time.

Do Microgreens Need Darkness?

Microgreens need darkness during their first few days. This time simulates a seed being planted in the ground. A common mistake among beginner microgreen growers is to try and skip this phase. Some microgreen seeds will not begin to grow without the beginning blackout period. The darkness also ensures the stems of the microgreens grow longer. In the dark, microgreens lengthen as they search for a light source. This will result in a more tender finished product. 

All plant seeds need darkness for optimal germination to happen. Scientific studies have proven that light disrupts the chemical reactions necessary for the seed to begin to grow and ‘hardens’ the seeds.

Microgreens will also need some darkness after germination to mimic nighttime. Once the seeds have germinated, and the plant is no longer in its blackout period, microgreens should get around 12-15 hours of light a day and around 9-12 hours of darkness. 

You can use natural light if you don’t have a setup with grow lights. Just be sure to put your tray near a well-lit window and give them at least 6-8 hours of light. This is a great reference if you are only able to provide natural sunlight. A few people recommend a couple of days of twenty-four hours daylight right before harvesting, but most people do not recommend this approach. 

Some growers disagree with this commonly held wisdom and report that microgreens grow just fine under continuous light. Their position is that microgreens are okay in these conditions as only flowering plants require periods of darkness. One thing you should consider is that some lights need to be turned off for a set period of time every so often.

How Much Light Do Microgreens Need Each Day?

After the blackout period, microgreens should get around 12-15 hours of light a day. The amount of light your microgreen will need partially depends if they can get natural sunlight or are placed under a grow light. 

If the plants are in direct sunlight or you are using grow lights, your microgreens may need less light. If you are using artificial light, there is a wide variation in how strong various lights are.

Without the correct amount of light, microgreens will not develop properly as photosynthesis will not occur. The shoot will not thicken, and the plant will not turn green. The microgreens will also appear pale. The microgreens might also grow spindly and tall. 

On the other hand, if microgreens get too much light, the leaves may develop dry or burnt pieces. The leaves may also develop spots if the light is placed too close to the microgreen due to too much heat.

What Type of Light is Best for Growing Microgreens?

Indirect sunlight is the best type of light for microgreens just out of the blackout period. Depending on the size of your operation and the weather where you live, indirect sunlight may not be the best option for you. 

Alternatives include:

  • Direct sunlight
  • Incandescent lighting
  • Fluorescent lighting
  • LED bulbs
  • LED grow lights.

READ ALSO: Microgreen Lighting Guide: Choosing The Right Light

Direct Sunlight

Direct sunlight naturally provides a lot of light to enable photosynthesis in your microgreens. As mentioned above, direct sunlight may be too much light or heat for your plants. If you see spots, dry places, or burnt pieces, move the microgreens into indirect sunlight. On the other hand, make sure the window or opening lets in enough light sunlight so that the microgreen can grow.

Incandescent Light

Incandescent light is generally not a good choice for microgreen growing. It produces a lot of heat relative to light, making it inefficient. The excessive heat can also damage microgreens. 

Fluorescent Light

Many growers choose fluorescent lighting for its low upfront and recurring costs. By using less electricity, the lights will cost you less money over time. The exact type of fluorescent lighting will not significantly impact microgreen growth. 

Fluorescent lights that use smaller lighting tubes will be more efficient electricity wise and still produce enough light for a relatively small operation. Larger tubes will be more inefficient but will emit more light.

LED Lights

LED bulbs put off little heat and have many other advantages. LED bulbs are more expensive to purchase and, as such, tend not to be used by beginning growers. They also do not effectively simulate sunlight as they do not produce the light of the correct wavelength. 

Some sources say this is not an issue for microgreens, while others stress the importance of a full spectrum of light. LED grow lights combine LED lights to solve this problem. By using different lights, all of the needed wavelengths of light are produced.

Growing Microgreens with Just the Right Light

With just a bit of research, you are well on your way to having the right amount of light for your little microgreens. Understanding how light can damage the plant if overdone and when to introduce it is key to success in this endeavor. Thankfully, you’re all set now!

Microgreens Corner
Microgreens Corner
We are Janette & Jesper, and we love microgreens.


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