When To Put Light On Germinated Seeds


Seeds need light to germinate, but seedlings need it more!

Knowing how and when to put light on germinated seeds is crucial for gardening success, so I put the details right here, which includes:

  • Whether seedlings need sunlight or artificial light
  • How much light seedlings need
  • How to properly use grow lights for seedlings

Don’t be in the gardening dark when it comes to creating happy seedlings. Learn how to increase your garden’s beauty, health, and abundance by using light on germinated seeds the correct way!

Do Seedlings Need Light? Sunlight?

Seedlings are delicate and require light to thrive so that stems can grow thick and leaves can flourish.

Sunlight is terrific for seedlings, but getting enough for even growth can be difficult. 

seedlings with sunlight

Many gardeners start seeds in a greenhouse or use artificial lighting indoors to boost plant growth, so they are robust enough to survive for transplant later.

Setting trays near windows or hoping for cloudless days, so your greenhouse is bright enough won’t be sufficient to keep your seedlings growing vigorously in most regions.

That is why smart gardeners rely on artificial illumination from special lights to boost growth consistently.

How Much Light Should Seedlings Get?

Seedlings do best when they get 12 to 16 hours of light a day. By providing artificial light, seedlings don’t have to expend energy stretching to reach natural sunlight.

Without the need to stretch, seedlings won’t get leggy and instead put that energy into forming a sturdy stem and lots of foliage.

Grow lights are easier to manage when growing seedlings as opposed to relying on natural sunlight. 

You can set a tray under grow lights and have every seedling get maximum light, or you can continually monitor, move, and turn trays set near windows to catch sunlight throughout the day.

When Do I Put My Seedlings Under Light?

You need to put seedlings under the light as soon as they begin to sprout. 

Many gardeners put their newly seeded trays under lights to not miss a second of potential exposure once they germinate.

Catching early sprouting and getting them under light is the key to keeping seedlings healthy and actively growing.

Sunlight provides red through violet wavelengths, but some may be stronger during certain seasons of the year, which can alter how your seedlings grow if you have them in a greenhouse or near a window.

seedlings under red grow light

Grow lights come with a wide variety of light-spectrum choices, so you can select the ones that suit your growing needs best.

Blue wavelengths of light trigger the plant to grow a robust root system, which gardeners refer to as the “Veg” stage. Seedlings excel with blue light as dense roots help support and feed the plant throughout its life.

Red wavelengths are more important later in a plant’s life as exposure stimulates the urge to flower and fruit. Using red light is how commercial growers can force flowering off-season so you can enjoy tomatoes any time of year.

For no-fuss results, go with a full-spectrum grow light as it gives the seedlings everything they get in nature from the sun. Seedlings growing under full-spectrum lights do well, and have good yields, especially if you never transplant them outdoors.

Using A Grow Light To Start Seeds

Seeds need light to germinate, so starting seeds under a grow light brings many benefits. 

Relying on sunlight to start seeds is problematic. The day could be cloudy, some seasons only provide a short window of light, or windows may be in the wrong location to illuminate your plants all day.

Grow lights let you control exposure time, heat output, and location while reducing the chances seeds won’t germinate or fail to thrive once they sprout.

Grow lights come in two styles:

  • Hanging grow lights dangle from the ceiling or underside of shelving and keeps your gardening space clear
  • Free-standing grow lights look like a lamp with a flexible head that you sit on a table or stand on a floor to direct the light onto your planting trays

Grow lights work with three types of bulbs:

  1. Fluorescent bulbs are good for beginning grow-light gardeners as their heat, and light output are sufficient for starting seeds, especially vegetables, indoors.
  2. HID (High-Intensity Discharge) bulbs are best for large growing rooms with higher ceilings as the light coverage is vast
  3. LED bulbs are very energy-efficient, but you pay more upfront per bulb over other types. Heat output of the bulb is low, and the wavelength spectrum can suit specific growing stages

The most popular combination is a hanging LED grow light system. Hanging lights allow you to easily adjust how far the light is from your growing plants, while LED lights stay cool and can run for thousands of hours before burning out.

How Far Should Seedlings Be From A Grow Light?

The distance from grow light to tops of seedlings will vary depending on the type of bulb you’re using.

LED lights need to be mounted 12 to 36-inches above the tops of your plants, depending upon the wattage. 

Lower 200-watt bulbs can be a foot or so above seedlings, while a 1000-watt bulb needs a minimum of three feet. Each bulb will state a recommended height, which you should start with, then alter higher or lower as you watch how the seedlings are faring.

HID lights should start no closer than 12 inches from your seedling canopy, raising them higher as the wattage increases.

Fluorescent lights can get the closest, with a distance of only six inches or so above seedlings a safe starting point. If you see seedlings stretching thin to reach for the light, lower the fixture an inch or so daily until it stops.

A grow light meter is a handy tool to help you monitor and pinpoint the perfect light output reaching your seedlings.

How Long Should Grow Lights Be On For Seedlings

Seedlings under grow lights do well when getting 14-18 hours of light exposure a day.

But remember that more light isn’t better. Some people mistakenly think they are doing seedlings a favor by keeping the lights on 24/7, but all plants need a period of rest.

During nighttime hours, plants stop photosynthesis while respiration continues. Plants rely on circadian rhythms to manage their days and seasons just as humans do. 

During darkness, the glucose made through photosynthesis pushes throughout the plant into cells, where it’s stored as starch for later use as energy for continued growth.

As long as your seedlings are getting at least six hours of darkness each day, full light the rest of the day is perfectly fine.

In Summary

For avid gardeners, setting up a grow light system to germinate seeds and prep seedlings is a logical next step to improve crop production for both vegetables and flowers.

When you follow the information in this lighting guide for seedlings, you can bypass years of frustrating trial and error and get straight to the tools and knowledge necessary for gardening success!

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