Cannabis seedlings – How to avoid stunted growth

Content updated in June 2022

How to germinate Cannabis seeds: a guide for beginners

In this article, you’ll find information about how to take care of your seedling, some problems that may arise, and how to take care of them.

Cannabis seedlings like relatively moist conditions. Temperatures should remain warm and lights should be placed near the seedlings and moved up as the plants grow. Cannabis seedlings don’t need extra nutrients and their needs will grow as the plant gets bigger and stronger.

Seedling stage (1 to 3 weeks)

After seed germination, the Cannabis plant enters the seedling stage. When the seed opens, the first pair of small round leaves that appear are called “cotyledons”. After that, the little Cannabis plant starts producing serrated leaves, not the characteristic digitated leaves, only single serrated leaflets or “fingers”.

As the plant grows, new leaves with more leaflets start to develop until the plant produces digitate leaves with serrated borders we can all identify as Cannabis fan leaves. When these leaves appear, the plant enters the vegetative stage. This process may take up to three weeks.

Environmental conditions for Cannabis Seedlings’ growth

• Warm temperature: Seedlings are very sensitive to temperature, they thrive with temperatures around 72 – 79° F (22 – 26º C). The best temperature for seedlings is 77°F – 25°C. During the dark period when the lights are off, a temperature around 65°F – 18°C can help reduce stem elongation and reduce internode spacing
• Relative humidity: A very dry environment may cause your seedlings to dry up easily. Keep humidity levels around 60-65%. Burnt leaf tips may be a sign of low humidity levels.
• Low light intensity: Young plants can’t process high-intensity light yet. Dimmed Led lights or fluorescent low-wattage lights are better for this stage. Light with a high percentage of blue spectrum light is good for this stage, as it promotes vegetative growth without excessive stem elongation
• Adequate light distance: the distance between the plant and the grow lights depends on their intensity and temperature. Start with a distance of about 8 inches (20 cm) if you are growing with low-intensity lights. Keep high-intensity led lights 3-4 feet (0.90-1.20 cm) away from the seedlings or dim if possible. When growing outdoors, just try to place your plants where the sunlight reaches them and keep an eye on them so the soil never gets to dry up.
• Light/Photoperiod: Start with 18 hours of light and 6 hours of uninterrupted darkness.
• Nutrition: Seedlings need none to a low dosage of nutrients or they may experience nutrient burn. If you are growing on soil, choose light mixes and avoid extra fertilization in the first week or two. After that, you can start your fertilizing plan.
• Pot size: Once germinated, place your seed/seedling in a small pot or cup with drainage for better results. Then move your seedling to a bigger pot once it has grown a few cm or if you see the roots coming out through the drainage holes. 
• Watering: Water your seedlings with a sprayer to avoid overwatering. Use clean water only.
• pH levels: Keep pH levels between 5.5 and 6.5 for your seedling to absorb all the nutrients in the soil mix.

Always keep an eye on these factors, if they are not taken care of, your plant may experience stunted growth and finally die. Let’s see the typical problems a seedling may have.

Cannabis seedlings growing slow

Why are my seedlings growing so slow?? There are many factors that may lead to stunted growth in the seedling stage, let’s take a look at some of them.

Problems with watering and pot size

Underwatering and overwatering may cause stunted growth and the signs look pretty similar. Small seedlings growing in big pots may experience overwatering problems, as roots need to breathe oxygen to develop and their root systems are yet incapable of absorbing that much water. When the initial pot is big rather than a small cup, there’s a high risk of overwatering as you try to fill the entire medium with water. In this case, the seedling looks droopy and weak, because it’s slowly drowning. If this is the case, just add less water and only around the seedling, let the plant absorb some of it before watering again. That’s why starting in a small cup, jiffies, or plugs is advised. There you can control the amount of water that the plant is taking and the roots are allowed to breathe normally and grow fast.

On the contrary, if the pot is too small for the seedling, growth would be stunted as well. The roots need space to develop and your risks of having problems with watering are high. In this case, just transplant it to a bigger pot to avoid “root bound”. Poor drainage also leads to these problems.

Damping-off – Fungi disease

Overwatering can lead to “damping off”, which is a very common disease caused by fungi. Pythium, Botrytis, and Fusarium are some of the fungi species that cause damping off, they all appear with excessive humidity and cool temperatures below 68°F – 20°C. A damping-off seedling shows spots on the base of the stem and looks weak and thin. In a matter of a day or two, the stem bends just above the topsoil, and the seedling dies. When the cannabis seedling fell over, there was nothing to do to save it.

To avoid damping off, do not overwater or fertilize your seedling for the first week and avoid excessive relative humidity. If one seedling already died from damping off, take it out as soon as you can before the fungi spread to the other plants. For sterilizing the environment, use a hydrogen peroxide 3% solution.

Problems with nutrients

Nutrient toxicity usually happens when the growing medium has a high concentration of nutrients for the seedling stage and the plants are not able to absorb them. You’ll notice burnt tips and a dark green color in the leaves if this is the case. This is common with designed soil mixes with a high % of Nitrogen or slow-release nutrients.

Nutrient deficiency may happen either because of the growing medium not having any of them or because it has too much of them, so pH levels are off and those nutrients are not available for the plant to uptake, despite being present in the mix. The most common at this stage is Nitrogen deficiency, which shows older leaves turning yellow and then brown and crispy.

Sometimes, this happens also when the plant is in a tiny pot and has already used all the nutrients in the soil. If this is the case, the best idea is to transplant it into a bigger pot containing a soil mix and apply nutrients for the vegetative stage. Remember to monitor watering schedules for better results.

Humic and fulvic acids are a good alternatives to nutrients for the first week or two. Humic and fulvic acids facilitate and enhance micro and macronutrient uptake, and their translocation within the plant. They also promote root growth, and microbial activity and act as biostimulants enhancing plant metabolism. The buffering capacity of the soil is improved and it has better water retention. Humus is a great source of humic and fulvic acids and many products derived from leonardite are also very popular Humic and fulvic acids products are recommended for every soil medium, including coco coir, soil mixes, and hydroponics systems as well. Follow the manufacturer’s instructions when adding these products, because an excess of humic and fulvic acids may slow down the plant’s growth.

Problems with temperature

If temperatures are too high or too low, seedlings are bound to experience stunted growth. This shows with leaves tips turning up and curling. If this is the case, lower the environment temperature and monitor for changes. As stated before, keep the environment temperatures between 72 – 79 F (22 – 26º C). The best temperature for seedlings is 77°F – 25°C. During the dark period when the lights are off, a temperature around 65°F – 18°C can help reduce stem elongation and reduce internode spacing

Problems with light

Too much light or not enough light also leads to stunted growth. Increase the light intensity as the seedling grows.
Seedlings not receiving enough light grow very tall and weak stems, with few leaves. The stem just grows tall and white. If this is the case, your growing light needs to be closer to the plant. Always do the “hand test”, placing your hands on the top of your plants for 30 seconds. So, if it feels like too much heat for your hand, it’s probably too much for the plant, so move up your lamp until it feels warm but not hot. LED lights may not emit a lot of heat but they may burn the plant anyway if placed too close. Follow the manufacturer’s suggestion for keeping the correct distance. When seedlings are receiving too much light, leaves are curled and seem burnt. If this is the case, move your lights up and also do the “hand test”.

Cannabis seed shell stuck on the seedling

A Cannabis seed just sprouted but the seed shell is stuck on the seedling. What should I do? The seed shell normally falls off when the first pair of round leaves (cotyledons) are formed. Sometimes, this shell is very hard and gets attached to the already-formed cotyledons. You can spray it with clean water to soften the shell and leave it for a few more hours. If you decide to take it off, you can try spraying it a little with water and gently take the shell off using sterilized tweezers. Be very careful not to damage the seedling!
Notice that in the following pics, the seedling finally manages to get rid of the seed shell by spraying water on it, but the soil mix seems muddy and not of good quality. Overwatering may cause stunted growth in seedlings

When to transplant cannabis seedlings

When should I transplant my seedlings? If the seedlings are in cups, jiffies, or plugs, in a matter of a week they should be ready to transplant to the next container. When the seedling’s leaves reach the border of the cup and/or the roots reach the bottom and start showing through the drainage holes, it’s time to transplant. Don’t expect the seedling to keep growing if you don’t transplant it, remember each plant grows as much as the containers allow her. A cannabis seedling in a cup for too much time is going to experience stunted growth. For an autoflowering plant, lost weeks in a small container can result in smaller adult plants.

Be extra careful when transplanting and try not to damage or touch the fine roots. Have the next pot ready with the chosen medium and water immediately after the transplant.
The seedling should continue to grow every day if everything went right. When there’s root damage, the seedling may take a few days to repair and grow more roots before resuming growing the stem and leaves. Be patient!

Take care of your Cannabis seedlings!

At the seedling stage, Cannabis plants are very sensitive and vulnerable, their root system is not yet developed and they need special care, little watering, and humidity. A healthy seedling has a short stem and green leaves.

Remember to take notes about this stage and monitor your environmental factors. You can do this with our free app Grow with Jane! Follow this link to download it today. Get the Grow with Jane app.

Hopefully, your seedling will rapidly grow and enter the next stage of a healthy plant. As said before, when the digitated leaves with serrated borders we can all identify as Cannabis fan leaves appear, the plant enters into the vegetative stage.

Vegetative stage in Cannabis plants: a guide for beginners

Grow your own. Grow with Jane!