They perform important respiratory functions at night. Seedlings should have at least 6 hours of darkness per day, and more mature plants at least 8-10 hours a day.
Once those seeds sprout, proper seedling care is essential, and that includes supplying the growing plants with the right amount and quality of light. It is true that seedlings need lots of bright light to grow healthy and strong—but they also need a period of darkness in order to thrive.
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.
Light intensity can cause severe damage to your seedlings to the point where they die or are stunted.
Seedlings need more light than full grown plants, ideally as much as 16-18 hours a day. Additional light may be required for seeds started during the winter months. Affordable plant lights can be found at many garden supply stores for less than $100.
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).
Seedling Daily Light Needs
Leave the grow lights on for 14 to 16 hours per day to provide light for the young seedlings. It's best to turn the light on early in the morning and turn it off around dusk.
Can you burn plants with LED grow lights? Even though LED lights are the safest for your plants, they can still hurt them and even burn their leaves if they're too close. LED lights can even cause photo-bleaching in plants that don't need plenty of light throughout the day.
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.
This is why some LEDs give off a purple glow, unlike full spectrum lights that emulate daylight. But as we know, LEDs, like those available for home lighting, come in more neutral white colors that will also work for seed starting.
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.
A: In general, you should not leave grow lights on 24/7. Plants need a light-dark cycle to develop properly. It's believed that they truly do “rest” during periods of darkness, and probably use this time to move nutrients into their extremities while taking a break from growing.
Cluster your seedlings into a confined place such as a plant house or cold frame. Now fill gallon-sized bottles with hot water and place these into the cold frame with your seedlings. Leave enough space around the bottle for the heat to escape and to avoid overheating nearby seedlings. Tightly shut the door or lid.
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.
Where Should Grow Lights Be Located? For seedlings, LED grow lights should generally be mounted between 24-36 inches above the plant canopy – however, this depends on the power (wattage) of the light source.
Red light stimulates vegetative growth and flowering (but if a plant gets too much, it will become tall and spindly). Blue light regulates plant growth, which makes it ideal for growing foliage plants and short, stocky seedlings (but too much will result in stunted plants).
Since sunlight is what plants naturally use to grow, the best color light for seedlings is the color that best mimics sunlight. The light from the sun includes all colors on the spectrum, so a full spectrum light is ideal.
Many plants don't need light until they've sprouted, so you'll start using grow lights once your seeds begin to germinate and the first leaves poke through the soil.
Plants require periods of both light and darkness to keep their metabolism and blooming phases in balance, and when they get too much light, those patterns are disrupted. While some plants need less darkness time than others, too much light will impede any growth schedule over time.
As an unofficial rule of thumb, many growers work on the basis that: You need 20-40 watts of power per square foot. Lower wattage LEDs of around 200 watts should sit between 12-20 inches from the top of the plant. Higher wattage LEDs of 1000 watts and above should sit between 36-46 inches from the top of the plant.
Light Intensity
Over time, this can deplete the plant's water reserves and leave insufficient water for vital processes such as photosynthesis. If the plant's capacity to dissipate heat is depleted, or if the amount of heat generated exceeds the plant's ability to dissipate it, the plant will suffer heat damage.
For most home gardeners, a plug-in shop fluorescent or LED light fixture is the perfect type of light to use for seed starting. When shopping for fluorescent lights, look for high-output, full-spectrum (sometimes called “daylight”) bulbs.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
The best time to water plants is in the morning or evening.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.