How to grow healthy cannabis seedlings and avoid stunted growth

Content updated in September 2025

Learn how to grow strong cannabis seedlings and prevent stunted growth with beginner tips on lighting, watering, media, nutrients, and safe transplant steps.

When does the seedling stage begin and end? 1 – 3 weeks

The seedling stage begins immediately after seed germination, when the cotyledons open. Single serrated leaves appear first, then leaves with more “fingers.”

Once the true fan leaves form and growth accelerates, the plant enters a phase of vegetative growth.

The seeding stage usually lasts 1 to 3 weeks.

As the plant grows, new leaves with more leaflets start to develop until the plant produces digitate leaves with serrated borders, which we can identify as Cannabis fan leaves.

When these true leaves appear, the plant enters the vegetative stage. This process may take up to three weeks.

Why is this stage so important?

Seedling care sets the yield potential. Good light, steady temperature, proper humidity, and gentle watering build a dense root system and short internodes.

Mistakes here slow growth and often carry into veg.

Cannabis seedling setup: temperature, humidity, light and pH ranges

Temperature for seedlings

Aim for 72–79°F (22–26°C). The sweet spot is 77°F (25°C).

During lights off, ~65°F (18°C) helps shorten internodes.

Humidity

Keep RH near 60–65%.

Dry air causes wilting and burnt tips.

Use a small humidifier if RH drops.

Light intensity (gentle start, increase gradually)

Young plants need gentle light. Use a dimmed LED or low-watt fluorescent light and increase intensity gradually.
Target roughly 100–200 PPFD in early seedling, then 200–300 PPFD by late seedling.

Photoperiod (Light hours)

Use 18/6 hours of light and complete uninterrupted darkness.

Light distance (adjust to plant’s feedback)

Follow the manufacturer’s chart and watch leaves.

If plants stretch, move the lights closer. If leaves taco or bleach, raise the fixture.

Airflow and ventilation

A gentle, oscillating breeze keeps plant stems sturdy and reduces the risk of mold.
Exchange room air to control temperature and relative humidity (RH).

Outdoor placement

Prevent dry media by checking moisture often and shading at midday.

Nutrition (very mild at start; coco from day one)

Use a light starting mix. Avoid fertilizer in weeks 1–2 when growing in prefertilized soil.

In coco and inert media, supply a very mild nutrient solution from day one, according to the chosen nutrient schedule.

Pot size (start small, up-pot on root cues)

Start in small containers with drainage.

Transplant to a bigger pot once roots reach the cup’s edge or peek through holes.

Watering (light, localized for first 5 days)

Moisten the root zone with a sprayer or small ring around the stem. Avoid saturating the whole pot for the first 3 days.

pH levels

Keep pH ~5.5–6.5 so roots can access all nutrients.

For coco, aim for pH 5.7–6.

For soil, aim near pH 6.2–6.5.

Seedlings growing recommendations

Choosing the Right Medium and Container for Seedlings

Pick a light, airy seed-starting mix with good drainage and low nutrients.

Add perlite to improve porosity.

Buffer coco coir with Calcium and Magnesium and feed a very mild solution from day one.

pH control is essential.

Ideal container size and type (avoid overpotting)

Start in 0.25 – 0.5 L cups or plugs with holes.

Transplant to 1 – 3 L pots once roots colonize the starter. 

Overpotting keeps the media wet too long.

Watering Cannabis Seedlings

How often should you water?

Water by need, not by calendar.

Lift the pot to judge weight and let the top 1–2 cm dry slightly between waterings.

Importance of drainage and avoiding waterlogged soil

Use containers with side and bottom holes, add perlite, and empty saucers.

Roots need oxygen to grow.

Lighting for Cannabis Seedlings

Recommended PPFD 

Early seedling: 100–200 PPFD.

Late seedling: up to 200–300 PPFD.

Signs of light stress in seedlings

Leaf taco, bleaching, and crispy tips signal too much light or heat.

Stretch signals too little light.

Adjusting light intensity as plants grow

Increase PPFD gradually every few days if plants look compact, green, and happy.

Lighting schedule and photoperiod growing guide, including recommended PPFD for each stage.

Feeding seedlings

Do seedlings need nutrients?

In soil, most seedlings do not need nutrients in weeks 1–2.

In coco or rockwool, supply a gentle, balanced feed from day one.

Nutrient burn in young plants

Dark green leaves and burnt tips mean excess nutrients.

Dilute the feed or give a light flush and resume at a lower strength.

Nutrient growing guide for cannabis plants

Transplanting Seedlings Safely

When to transplant cannabis 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. 

Transplant when leaves reach the cup’s rim or roots show at drainage holes.

Do not delay, especially with autoflowers, since lost seedling time reduces final size.

Remember, each plant grows as much as the containers allow it. A cannabis seedling in a cup for too long is going to experience stunted growth.

Tip: For an autoflowering plant, lost weeks in a small container can result in smaller adult plants.

How to minimize stress during transplant

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 goes 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!

Tips: Transplant with a moist, intact root ball.
Pre-moisten the new mix so it feels evenly damp and airy.

Handle the cup, not the stem.

Water in with pH-correct solution.

Ensure strong drainage.

Why are my seedlings growing so slowly?

Issue: Cannabis seed shell stuck on the seedling

A Cannabis seed just sprouted, but the seed shell is stuck to 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.

Fix: 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 taking the shell off using sterilized tweezers. Be very careful not to damage the seedling!

Tip: 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

Issue: Overwatering and underwatering both stunt growth.

The roots need to breathe oxygen to develop, and their root systems are yet incapable of absorbing that much water.

A tiny plant in a big pot stays wet too long and “drowns”.
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.

Signs of overwatering and underwatering

Overwatering shows droopy, heavy leaves and slow growth.

Underwatering shows wilting and dry, crumbly media.

Fix: If this is the case, just add less water and only around the seedling, let the plant absorb some of it before watering again.
For the first 3 days, water only a small ring around the stem and let the top layer dry slightly before the next watering.If the pot is too small, roots circle and stall growth; transplant to a larger container with good drainage.

Tip: 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.

Issue: Damping-off – Fungal disease

Overwatering can lead to “damping off”, which is a very common disease caused by fungi.
Cool, wet media invite fungi species like Pythium, Botrytis, or Fusarium.

A damping-off seedling shows brown spots on the base of the stem and looks weak and thin at the soil level. 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.

Fix: Prevent damping off with moderate watering, 60–65% RH, clean tools, and warm temps. To avoid damping off, do not overwater or fertilize your seedlings for the first week and avoid excessive relative humidity. 

Tip: Remove infected and dead seedlings and sanitize surfaces with 3% hydrogen peroxide to avoid spread.
Do not use “garden soil” or compost for your seedlings, as they are more prone to cause damping off.

Issue: Stretchy seedlings (leggy plants)

Low light or excessive distance causes stretch. Seedlings not receiving enough light grow very tall and weak stems, with few leaves. The stem just grows tall and white.

Fix: Increase PPFD slightly or lower the light to reduce internode length.

Tip: 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.

Issue: Yellowing or discolored leaves – Nutrient deficiency or toxicity

Early yellowing leaves point to underfeeding in coco, old media, or pH issues.

Nutrient deficiency arises when the medium lacks nutrients or when pH is out of range, causing lockout; the most common is nitrogen deficiency, where older leaves yellow first, then turn brown and crispy.

Nutrient toxicity occurs when the medium is too “hot” for seedlings.

Often, soil mixes high in nitrogen or slow-release fertilizers; leaves turn very dark green with burnt tips. Fix: 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.
Check pH/EC and adjust feed strength accordingly.

Issue: Leaf curling or deformation

Heat, strong wind, or overwatering often causes curl.

Fix: Soften airflow, and let the medium breathe..

Lower the environmental temperature and monitor for changes.

Tip:  Do not point oscillating fans directly at the plant to avoid windburn.

Issue: wrong temperature

If temperatures are too high or too low, seedlings are bound to experience stunted growth. This shows with leaf tips turning up and curling.

Fix: If this is the case, lower the environmental temperature and monitor for changes. As stated before, keep the environmental temperatures between 72 – 79°F (22 – 26ºC).

Tip: 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

Issue: Poor airflow & low CO₂ (stale air)

Poor airflow and low CO₂ (stale air). Stagnant air limits CO₂ at the leaf surface, slowing photosynthesis and daily growth.

Fix: Refresh the air frequently. Run an exhaust to exchange air every 1 – 3 minutes with an open intake for slight negative pressure, add a gentle oscillating fan, remove humidity domes after day 3 – 4, and outdoors place seedlings where a light breeze reaches them.

Pro Tips for fast and healthy seedling growth

Using mycorrhizae or beneficial bacteria

Dust the planting hole with mycorrhizae or add a beneficial inoculant at transplant to boost roots and nutrient uptake.

Adding humic and fulvic acids

Humic and fulvic acids are a good alternative 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 acid 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.

Humic and fulvic acids improve nutrient uptake, root growth, and water retention.

Use at a low dose per label in soil or coco. Too much can slow growth.

Strengthening stems with airflow

A soft, constant breeze trains stronger stems and reduces stretch without stressing the plant.

Monitoring root development

Check for roots at the pot’s holes and track pot’s weight day to day. Quick, even drying, and steady weight change signal active root growth.

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!