Cannabis seedlings are delicate, with an embryonic root system, minimal nutritional needs, and modest lighting requirements. Optimising the lighting for seedlings can therefore be a tricky affair, especially for the less experienced grower. With insufficient lighting, your seedlings may stretch too much and end up with long thin stalks incapable of supporting themselves. Using too much light can stunt and shrivel your delicate seedlings… Let’s shed some light on the most valuable lighting practices you need to know to give your plants the best start!
When and how to put cannabis seedlings under light?
How much light do cannabis seedlings need?
Cannabis seedlings have only modest light requirements. Their new, delicate leaf tissue is easily damaged by excessive light or heat. Most growers with modern LED fixtures tend to dim their lights and/or raise them a suitable distance above the seedlings.
Others will use the soft gentle light from purpose-designed fluorescent or LED seedling lights. Place your cannabis seedlings too close to a powerful light and growth will come to a standstill, or worse yet – your seedlings will start to weaken and eventually die.
Cannabis seedling stage how-to guide
How many hours of lighting do seedlings need
When growing autoflowering cannabis seeds indoors, most growers use a cannabis light cycle with 20 hours of daily light. This can be used all the way from seed to harvest.
Some growers also use an 18/6 or a 24/0 cannabis light cycle. Read our in-depth guide on autoflower light schedules for more information.
When growing feminised seeds indoors, growers use anything from 18-24 hours of daily light. This is often used for the first month or two of the plant’s life. When daily light hours are reduced to around 12 i.e. 12 hours of light followed by 12 hours of darkness, the cannabis flowering stage begins.
Best light schedule for your cannabis seedlings
Cannabis is a flexible plant that adapts to changes. There isn’t a clear right and wrong when it comes to the best light schedules for cannabis seedlings. Dutch Passion have seen excellent results from growers using a variety of light schedules, generally between 18 and 24 hours of daily light for their cannabis seedlings.
Some growers implement sunrise and sunset modes on their LED lights. This is where the light is gradually increased over e.g. 30 minutes during lights-on, and gradually decreased over e.g. 30 minutes from full power to zero during lights-off.
But in general, 20 hours of daily light from seed to harvest is recommended for autoflowers. And 18-24 hours of daily light is recommended when growing feminised seeds.
Best light intensity for your cannabis seedlings
PPFD light levels of around 100-300 µmol/m²/s are typically used for cannabis seedlings. Good grow light manufacturers will explain the power setting and corresponding hanging heights for each stage of growth.
Some growers invest in light meters. These are not cheap, but they do allow the serious grower a great grow room gadget that will allow them to precisely map the light intensity in each area of the canopy at various hanging heights. This in turn allows them to constantly keep their plants in the optical sweet spot from cannabis seed to harvest. This results in optimised yields, especially if the plants can also be kept in the nutrient sweet spot from cannabis seed to harvest.
Artificial light vs sunlight for cannabis seedlings
Indoor cannabis growers tend to control their lights schedules with precision. But outdoors, cannabis plants see daylight hours change continuously from month to month. On top of daylight-hours variability, outdoor cannabis plants also must cope with variable light intensity. Both from season-to-season and day-to-day as the weather alternates from sunny to cloudy.
Key benefits of putting your seedlings under grow lights
Indoor cannabis growers have the ability to control the grow environment with great precision, that’s one of the advantages over outdoor cannabis growing.
Indoor grow lights ensure your cannabis seedlings bathe in optimised light levels with stable warm temperatures while they are young and at their most vulnerable. It’s worth noting that many outdoor growers prefer to germinate their cannabis seeds indoors and allow the seedlings to grow in the safety of an indoor environment before the plants are placed outdoors later on.
What kind of grow lights are best for cannabis seedlings
Cannabis seedlings grow well under any grow light which includes blue-spectrum light. Many growers use their main grow light, set at reduced power output and perhaps at a higher hanging height than usual. Some use specialist seedling lights, these are available in LED and fluorescent fixtures. These days, many prefer the long-lasting LED lights with their stable light output over many years.
HPS lights used to be the most common type of grow light and are still a budget option, they give good results for seedlings. As do ceramic metal halide (CMH) lights.
If using either HPS or CMH lights you may have the option to use a blue-rich bulb, these are not suitable for bloom but allow good growth for seedlings and plants in vegetative growth where the requirement for blue light content is higher. All the serious LED suppliers ensure that their light spectrum contains sufficient blue for seedlings/veg and sufficient red content for heavy bloom.
How to use artificial grow lights for cannabis seedlings
With such a diverse array of manufacturers and different technologies (HPS, CMH, LED, fluorescent, plasma etc) available it is difficult to give precise recommendations. The best advice is to buy your grow light, preferably LED, from a competent manufacturer who is able to provide expert advice on power levels and hanging heights at each stage of growth.
Online grow forums also feature plenty of growers that may be using a grow light like yours. Try some googling to see how other growers use similar grow lights to yours. But remember that cannabis seedlings require only modest light levels for the first couple of weeks.
As soon as the cannabis seed germinates you can put it in your preferred grow medium and get your lights on.
It all depends on the power and whether it is adjustable. A 10-20W LED seedling light can be placed a few centimetres/inches above your plants. A non-adjustable 600W HPS may be 1+ metres (3-4 feet+) above your plants. A 600W LED could be positioned just 40cm from your seedlings if the power level can be reduced to a few percent of maximum power. Check the manufacturers recommendations. If the info can’t be found look for recommendations from similar lights or search the grow forums.
It varies from grower to grower and depends on the prevailing outdoor weather conditions. If the weather outside is still frosty it makes no sense to move your cannabis outdoors. Keep them indoors where they can continue to thrive. Outdoor growers that start their plants indoors typically give them anything from 2 weeks to 2 months of indoor growth before planting outside.
Lighting for cannabis seedlings FAQ
Here are some of the most common questions we get about cannabis seedlings and how to provide the best lighting. The following article listing the top-10 most common mistakes made with young cannabis plants is useful reading.
Yes, and you need to. As soon as the seedlings have pushed out their cotyledon (baby) leaf pair they are ready for photosynthesis to begin. Switching your light on allows the process to start.
Not necessarily. Many growers have a personal preference for e.g. a grow cycle with 18 hours of daily light. Others may prefer 20 hours of daily light of their seedlings.
Cannabis seedlings grow best when their light contains sufficient blue light content. Those with spectrum-adjustable LED grow lights can select blue-rich light for seedlings and veg growth. Once in bloom, light intensities can be pushed higher with a red-rich light spectrum to promote heavy yields.
Yes, as can excessively strong light from any type of grow light including HPS, CMH or plasma. One of the most common lighting errors, especially from less experienced growers trying to master a new high-power LED grow light is the tendency to damage seedlings with too much light.
Those with adjustable lights dim them for seedlings. This provides the seedlings with sufficient light. It’s also a cost-effective way of growing seedlings, often using just a few Watts of power. That’s far cheaper than running a non-dimmable 600W HPS at a 1m+ hanging height.
Your cannabis seedlings won’t grow and will gradually lose vigour the longer they remain under punishingly intense light. The dark green colour turns into paler light green/yellowish tones, sometimes accompanied by tiger prints and curling leaves. Eventually your seedlings will die if left under damagingly strong light.
Healthy cannabis seedlings are the best foundation for a killer harvest
Cannabis seedlings need relatively little attention once established in friendly conditions. The first 2 weeks, or so, of life above ground is often referred to as the seedling stage. After that, growers may consider their plants to be in vegetative growth.
Give your plants a good start as seedlings and you are well placed for successful veg and bloom. As always, choose from the best cannabis seeds and enjoy the next grow!
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