The seedling stage of cannabis follows on from germination. You will be able to tell when it has begun when your shoots start to develop a second set of leaves. These will look different to the embryonic cotyledons. They will be bigger, with a long serrated edge – a look that is much more associated with cannabis plants. However, these new leaves will only have one “finger”, instead of the 5 or 7 normally seen in adult leaves.
After this your cannabis plants will start to grow a third set of leaves, these will tend to have three fingers. This will be followed by more and more leaves, with each one beginning to resemble the expected cannabis leaf shape more and more. As mentioned, the most common and expected is a 5 or 7 finger leaf, but do not be surprised if you end up with 9 or even 11 fingers!
The seedling stage is usually over when you start getting leaves with their maximum amount of fingers. Other indications will include a stem with a thickness of around 4-6mm and a height of around 3-4 nodes. During this stage the seedling will also try and develop a strong root base, this will allow for strong future development and nutrient intake. Depending on where you grow, the seedling stage can take roughly 2-3 weeks indoors and 4-6 weeks outdoors.
As an extra tip, it is usually advisable to set up a small, weak fan to blow on at your seedlings. This will help them gain fresh air and encourages strong growth. It does this by giving the plant something to resist. As the fan blows at your cannabis plants, your seedlings will recognize that they need to put extra effort into growing and developing their structural integrity to withstand possible future sources of resistance.
You will know the time to transplant your seedling to a larger grow medium when you start seeing signs of its developing root base. Chances are your seedlings will be growing in individual, small pots. As their roots grow they will start to exit out the holes in the bottom of these pots. This shows that they need more room and are ready to be moved. Be very careful transplanting them though, roots are very fragile and any damage can have very serious implications into your cannabis plants' ongoing health.
There are a lot of cultivators out there who put their seedlings under fluorescent tube lighting. Due to the weakness of florescent lighting they need to be placed quite close to your seedlings – about 5 cm above them, (don't worry, they do not produce much heat, so this is fine). This causes your cannabis seedlings to grow broad – otherwise the plants need to "search" for light and you end up with long plants with thin, weak stems that will easily fall over. This is fine if you are willing to offer your plants support, but it is not a good foundation to lay if you want healthy, vibrant plants in the long run.
Some cannabis growers use HID lighting for their seedlings. HID lights produce a much better quality of light for growing cannabis, leading to stronger, healthier plants. The offset is that they produce a lot more heat - something your marijuana seedlings are very vulnerable too. For this reason it is important to make sure your lights are at the correct height above your plants. Start by having your HID lights 80cm above your plants, lowering them by 5cm each day until you are in the region of 40-60cm (depending on the wattage of your bulbs). Also, always perform the 'hand test' to see if things are getting a bit to hot.
In terms of color-band, it is best to go for a bulb within the blue spectrum. This will encourage strong and healthy stem and leaf growth.
During the seedling stage, most cultivators will use an 18/6 photoperiod. It is also possible to do a 20/4 period, with some cultivators going as far as using a 24/0 period to really boost growth.
Generally speaking, seedlings are pretty self sufficient. However, as stated in our germination article, using a seedling/germination soil is recommended. This will have the finer, small amounts your cannabis seedlings can use. You should note, cannabis seedlings are extremely vulnerable to overfeeding, any excess will cause root burn, killing off your plants.
As a result, novice cultivators are strongly advised from adding anything to your soil other than clean, distilled water (pH of 6.5-7). Your seeds should have everything they need and will need minimal extra input from you. Just make sure they have the water and light they need and they will do fine. Once your seedlings have grown, they will enter the vegetative stage of their life cycle.