Most seeds will not germinate without sunlight and will perform best with 12 to 16 hours each day. Indoors, place seed containers in a sunny, south-facing window and give the container a quarter turn each day to prevent the seedlings from overreaching toward the light and developing weak, elongated stems.
Seedlings need 14-16 hours of light every single day, without fail. Don't leave them on for 24 hours a day though. Like us, they need to rest at night. So plan to keep your artificial lights on for 14-16 hours during the daytime, and turn them off overnight.
Not all seeds have the same light requirements. Most seeds germinate best under dark conditions and might even be inhibited by light (e.g., Phacelia and Allium spp.). However, some species (e.g., Begonia, Primula, Coleus) need light to germinate (Miles and Brown 2007).
Different plants have different light intensity needs, but most seedlings grown for the garden will need higher intensity light to flourish. In general, the leaves should be about 2 - 4 inches away from the light source (assuming use of a fluorescent bulb – see below).
Grow lights should be left on for 12 to 16 hours a day. You may be tempted to leave the seedlings under lighting 24 hours a day. But this does not make them grow faster. It can actually hinder the success of your seedlings.
For best overall results, BIOS recommends that you grow your seedlings under an 18/6 light cycle using Icarus LED lamps, set 24-36 inches from the leaf canopy, and under a blue light spectrum for their first 10-14 days. 18/6 simply means 18 hours of light and 6 hours of darkness.
Vegetative Growth & Flowering
During the seedling phase, when plants are tender, lights are suspended higher. Generally, LED lights should be about 24-26 inches during this phase, and then lowered to 18 to 22 inches during the vegetative and flowering stages of growth.
For many years, fluorescent shop lights were the go-to seed starting bulbs for gardeners, but LEDs have quickly replaced them as the standard choice. LEDs are usually more expensive than fluorescent bulbs, but they are very energy-efficient and long-lasting.
The majority of seeds grow most effectively in the dark. Light, which is essential for seedling growth, may actually hinder the germination process. The three primary and necessary conditions for a seed to germinate are water, oxygen, and temperature. Thus, Light is not necessary for a seed to germinate.
Most of the seeds germinate best in dark environments. The presence of light tends to inhibit their growth. The light decomposes carbonic acid gas and expels oxygen which leads the seed to harden. These gases are key factors that promote germination.
All seeds need water, oxygen, and proper temperature in order to germinate. Some seeds require proper light also. Some germinate better in full light while others require darkness to germinate.
Once sprouted, most vegetable seedlings and other garden plants require at least 12 hours of good light per day, along with 8 hours of darkness. However, seedlings will grow the best when provided 14 to 16 hours of light.
Some seeds don't need light to break through their seed casings and sprout. Most seeds germinate best with controlled amounts of UV generation, but there are seeds that germinate in the absence of light. Furthermore, there are plants that get enough just light in highly shady areas of a garden or even in darkness.
A: Stick with ordinary fluorescent lights. They are readily available, reasonably priced, and work well for seedlings.
The only seeds that will germinate faster with light are the ones mentioned above in this article (or other seeds that require sunlight to germinate). Most seeds do not need grow lights until after they germinate. Most seeds germinate faster if they are warm.
The parts of the plant exposed to the excess light will show physical symptoms. You might see the leaves droop before changes in color appear. Then pale or brown spots, or yellowing leaves can occur, even as the veins remain green and looking healthy.
Light intensity can cause severe damage to your seedlings to the point where they die or are stunted.
The Best Color Light for Seedling Growth
Initially blue light is important to help plants grow strong and sturdy. Seedlings need blue light in order to grow into hearty, robust plants.
A: In general, you should not leave grow lights on 24/7. Plants need a light-dark cycle to develop properly.
Seedlings should be watered once a day or every other day, depending on how much sun and heat they get. Remember that seedling roots are fairly close to the surface and they're growing in a small amount of media, so they don't need a deep soak the way larger plants do.
Plant stems become “leggy,” meaning stems become long and thin and appear to be reaching toward the source of light. A lack of sufficient light causes the plant to grow long spaces on stems between the leaf nodes (the point where a leaf grows out from the stem).
Pale green and yellow leaves that drop off are a sure sign of inadequate sunlight. Also, with variegated plants, the leaves will revert to a solid green color so the chlorophyll within can do its job. If your typically colorful plant has lost its color, move it to a sunnier place and see its color return.
To be effective, grow lights really need to be on for at least 8-10 hours a day. This can vary up to 16 hours, depending on the conditions. That's why LED grow lights are so popular – when they have to be left on all day, it's best to use an energy efficient type! Always position a grow light above the plant.