How long should grow lights be on for cannabis seedlings? How Long To Keep Lights On My Seedlings? Seedlings don’t require too much light to grow. In fact, a light, that is too strong, can sometimes do more harm than good in this early stage. Many cultivars grow seedlings under fluorescent light tubes, which are relatively weak and optimal suited for this purpose. Because of the rather weak fluorescent light it is normally recommended, that you place the fixture with the light as close as 5cm above your seedlings. Know that the further away your grow light will be, the more stretched-out and longish your plants will grow in their search for light. Avoid having your seedlings become too stretched-out, because they would then easily tip over. Find the best distance to the light for your seedlings so they won’t stretch too much. If you use other types of grow lighting for your seedlings, such as HID lights, be aware, that this type of lights can often produce a considerable amount of heat. In this case you need to be careful and start out with a light distance of about 80cm above your seedlings. Gradually have your plants accustomed to the HID light and lower it a few cm each day until you reach a distance of 40-60cm depending on the amount of Watts. You can also do a simple heat test when you hold your hands under the light. If it feels too hot for your hands, it will also be for your fragile seedlings. For optimal seedling growth, get a grow light with a “cold” spectrum, that will be beneficial for early vegetative growth. These types of lamps have more blue in their spectrum as compared to flowering lamps, that will have more red. As for the best light cycles for your seedlings, you can go with the common 18/6 light schedule, which is 18 hours of light per day, with a 6 hour darkness period, although some growers use 20/4 or even 24/0. You can experiment which light schedule would give you the best results.
Types of Bulbs for seeding You can choose between incandescent, fluorescent, LED, and high-intensity discharge (HID) bulbs, each of which has its own pros and cons. Choose the grow light that works best for the type of plants you want to grow and where you plan to grow your seeds. Incandescent Bulbs These include halogen bulbs and are the type of light bulbs still used in most homes (although they're getting harder to find now that stores are carrying only more efficient bulbs, such as CFLs and LEDs). Incandescent bulbs are a good source of red light, but a poor source of blue, meaning that plants will likely become spindly when grown under incandescent light.
Incandescent bulbs, and especially halogen bulbs, also produce a lot of heat in relation to the amount of light they give off; plants growing too close to the bulb can be easily burned. Generally speaking, these are not the best type of lamps for growing seedlings. Fluorescent Bulbs These types of bulbs produce two to three times more light than incandescent bulbs for the same amount of energy and are the most inexpensive lights for indoor gardening. However, they usually require bulky external ballasts (like, for example, overhead shop lights) so aren't as easy to work with as incandescent and LED bulbs. Cool white bulbs are a good source of blue and yellow-green light, but are a poor source of red light. Plants grown under cool white bulbs will be stocky or even slightly stunted. Warm white bulbs emit plenty of orange and red light, but less light in the blue and green spectrum. These bulbs, when used alone, result in tall, spindly plants. If you are growing seedlings under two-bulb fluorescent fixtures, you can usually achieve a good color balance by combining one cool white and one warm white bulb. stacked grow kit A set of stacked shelves with fluorescent T5 lamps (or LED lamps) makes it easier to grow a large number of seedlings. Full-spectrum fluorescent bulbs produce a balance of cool and warm light that replicates the natural solar spectrum, although these are less energy efficient than other fluorescent bulbs and tend to produce more heat. But, given the wider range of light frequencies emitted by these bulbs, they are a good choice for growing seedlings. T5 lamps are fluorescent lamps that are 5/8" of an inch in diameter, making them much less bulky than typical fluorescent bulbs. These are the lamps you're most likely to find in grow light kits. When using fluorescent lamps, be sure that all plants get ample light. For a typical seedling tray, that means using 2 bulbs, ideally with a reflector hood over them to focus all the light on the seedlings below. LED Grow Lamps Unlike other bulbs which produce light across a broad spectrum, light-emitting diodes (LEDs) give off light within a narrow band. LEDs designed for growing plants emit light in the two bands that plants need - red and blue. The result is a purple glow that not everyone likes. LEDs are mercury-free and won't shatter like glass. These bulbs are long-lived (up to 5x longer than fluorescent lamps) and very energy efficient, but they cost considerably more than fluorescent bulbs. Research is still ongoing to determine which combination of light frequencies are best for plant growth and how LED grow lights compare to fluorescent bulbs in producing healthy seedlings. I haven't seen a definitive answer on this one yet but more and more companies are producing LED grow lights and seed-starting kits, and even commercial growers are slowly moving in that direction. High-Intensity Discharge Lamps These lamps are used by commercial growers and serious horticulturists. These energy-efficient bulbs generally emit twice the amount of light (lumens) as a fluorescent bulb. However, the bulbs and special fixtures are considerably more expensive than those
needed for incandescent or fluorescent lights. They also tend to be high-wattage bulbs, so you need to be sure your electrical system can handle the load. Some of these lights burn so brightly that they must be located in a special room and you’ll need to wear eye protection when working around them. Metal halide grow lights emit an intense, bluish-white light that is excellent for growing plants. The foliage stays green and vigorous, and plants are usually stocky and strong. Metal-halide lights are currently the number one choice for serious indoor gardeners. Mercury vapor lamps emit a bluish, relatively well-balanced, high-intensity light. High-pressure sodium bulbs are usually used to promote flowering and fruiting but, when used exclusively, they produce leggy, weak-stemmed plants.
What's the best option? For the average home gardener starting seedlings indoors, a fluorescent or LED lamp will usually be the best choice to ensure that your plants get the quality, intensity, and duration of light they need to stay in peak condition.