How Long Should Seedlings Be Left in Darkness?

Growing seeds into seedlings in a way that they can thrive properly is often a process of trial and error. There are many factors that influence how well a seedling will grow, but one of the biggest to consider is how much light seedlings need to grow tall and strong rather than leggy and scrawny. How long should seedlings be left in darkness? 

Seedlings should be left in darkness for approximately 6 hours a day. Although the amount of time will vary from plant to plant, most seedlings should receive approximately 14-16 hours of light each day.

This article will explain why seedlings need a particular ratio of light to darkness in order to grow successfully. It will also look into the characteristics of why some plants thrive better with more time in the dark than others. Lastly, it will provide some tips on how best to expose seedlings to light when they are ready and suggest some tools to make the process easier.

Understanding When to Expose Seedlings to Light

When seeds are first being planted, the majority of them should not be exposed to any form of light. This is due to the fact that being bathed in light too early on will stop germination in its tracks, and the seed’s growth will be stunted. Being kept in total darkness in these early stages is much more beneficial.

Just like any other living thing, seeds will only thrive if the conditions are just right. They will become seedlings and begin to sprout when they are planted in the perfect combination of temperature, moisture, and light. 

Seedlings are like humans in that they need time to rest and rejuvenate their energy every night to have the strength to keep growing. This means that they definitely do not need to have light shining on them 24 hours a day. 

Some seed varieties will require larger amounts of light than others while they are in the process of germination. However, for the most part, seeds should remain void of light until they begin to sprout.

Once the seeds have germinated and become seedlings, they will burst through the soil to reach the air. This is the time to learn whether they will continue to need dark, damp conditions to prosper or if they should see the light for most of the day. It will largely depend on what type of seeds are being planted.

Which Plants Require More Darkness?

There are two main categories to consider when understanding how much light a particular type of seedling needs while it is growing.

These categories are:

  • Whether the plant is a long-day plant or a short-day plant
  • Whether the plant will flower quickly if exposed to light or not

Short-day plants require more darkness than long-day plants. Additionally, certain plants will flower more quickly than others when exposed to longer periods of light. Take these factors into consideration when choosing the right environment for your plant.

The amount of time for each individual plant to be out in the light or in the darkness can be deduced by looking at these categories further.

Taking a peek at the information on the seed packets may also provide some further advice on what types of plants they are—long-day or short, quickly-flowering or not.

Long-Day vs. Short-Day Plants

The first aspect to look into when deciding how much light a seedling should receive is whether that seedling will grow into a long-day plant or a short-day plant. These two varieties of plants are aptly named because this aspect is based on how many hours in the day the plant should be in the light or darkness. 

A long-day plant is going to need to spend most of the day in the sunshine (or some other sort of artificial lighting) to reap the best growth results. A short-day plant will need only a few hours of light and is content to grow mostly in darkness. 

All plants need to have time to respirate without being exposed to light, and those 6 hours of darkness each day are typically enough time for that process to take place. Whether the plants spend the rest of the hours soaking up light or remaining in the shade will depend on if they are a long-day or short-day plant. 

Good Flowering vs. Bad Flowering

The second aspect to look at when putting a seedling under a light source is whether the plant will grow flowers quickly when exposed to that light or not.

There are some cases where the plant blooming at a fast pace can have a good outcome. A tomato plant, for example, will produce fruit from its flowers, so planting the tomato seedlings in an area that receives a lot of light will be beneficial. 

In other situations, a plant sprouting flowers early can have negative outcomes. If a leafy green plant, like lettuce, is exposed to an overabundance of light too soon, it will start to flower prematurely. As opposed to the tomato plants, this is not a good thing. When lettuce starts to grow flowers, it usually means that it has gone bad. 

Understanding whether the plant is going to flourish and produce blossoms under light is an important step in deciding how much time the seedlings should be left in the darkness.

How to Expose Seedlings to Light

No matter what type of plant is being grown, it will need at least a few hours of light every day once it begins to sprout out of the earth.

But what is the best technique for providing the seedlings with that light? After all, adequate lighting is crucial for seedlings to have a successful initial growth period after breaking the surface.

Start Growing Seedlings Indoors

The first step is to start the seeds growing inside. Even if the climate where they are being planted is warm, letting them germinate and sprout a bit before moving them outdoors full-time is important. 

This is even more necessary in northern climates where the growing season is shorter and the chilly weather lasts longer. Starting the seeds indoors can help ensure that they get a full growing season, both in terms of warmth and exposure to light.

Consider Purchasing Grow Lights for Indoor Seedlings

The next question to consider is—if you’re planting the seeds inside where they will not be in the sun—what is the best way to make sure they get plenty of light?

The answer to this is purchasing grow lights, as Investing in some good-quality grow lights can make a major difference when starting seedlings out away from the elements.

A major benefit of using grow lights to aid plant growth is that they trick the seedlings into thinking it’s later in the year than it actually is. The plants will work harder because the light makes them believe it is already time for them to start sprouting. Then, when it is actually time to transplant them into the garden, they will be ready to bloom fully.

How to Use Grow Lights on Seedlings Indoors

Here are a couple of tips that may be helpful when utilizing grow lights to help seedlings flourish:

  • If using LED lights to warm the plants, they should be placed about 8-12 inches (20-30 cm) away from the seedlings.
  • If using fluorescent lights to warm the plants, they should be placed about 5-6 inches (13-16 cm) away from the seedlings.
  • The lights being used to warm the plants should mimic normal sun patterns, meaning they should be running during daylight hours and turned off during nighttime hours.
  • Attach the grow lights to a timer that can be programmed to turn on and off at specific times of the day. This will ensure that the plants consistently receive the right amount of darkness and light.

Final Thoughts

How long seedlings should be left in darkness will largely depend on the main factors listed above. Keep in mind that every plant variety is going to be completely unique, especially when it comes to plants that produce fruits and vegetables. These types of plants will have very different needs compared to generic flowering plants or greens that are for decoration and not food production. 

No matter what type of plant is going to be grown at any particular time, it is certain to thrive if it is given the right atmosphere, attention, and care.

Alexander Picot

Alexander Picot is the founder of and its lead content writer. He created the website in 2022 as a resource for horticulture lovers and beginners alike, compiling all the gardening tips he discovered over the years. Alex has a passion for caring for plants, turning backyards into feel-good places, and sharing his knowledge with the rest of the world.

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