Everything You Need to Know About the Seedling Stage

June 10, 2020

Marijuana plants need the correct level of care at all stages of their growth, even the seedling stage. Good care is essential to healthy growth and high-yielding flowers, loaded with cannabinoids and terpenes. The journey starts with obtaining and germinating seeds that contain stable, reliable genetics. Once the taproot has wriggled out, your next moves are crucial to the overall success of your grow. But how do you successfully grow marijuana from seed? How do you protect cannabis seedlings? How much do you water cannabis seedings? Do they need anything special in terms of light, heat and humidity? How much do they need feeding? And, finally, what size pot do you need for marijuana at the seedling stage?

How do you grow marijuana from seed?

When growing in the wild, a pollinated female cannabis plant will die at the end of its cycle. It will slowly collapse to the ground, taking with it all the fresh, seed-filled buds. These seeds can remain dormant for an extraordinary length of time, and a few lucky ones will find themselves in just the right position to grow into the next generation of plants. If they end up near enough to the soil and if they don’t get eaten by birds, a little rain or a heavy dew will be all it takes to wake them up

Once they’re exposed to enough water, an interesting chain of events occurs. 

  • The seam between the halves of the seed coat splits. 
  • The taproot breaks free of the shell and bores into the soil. 
  • As this root takes hold, the stalk and the seed husk stand and rise above the soil.
  • The two halves of the seed husk slowly open, gathering energy like biological solar panels
  • This energy is used to produce chlorophyll.
  • Beneath the ground, tiny hairs called feeder roots emerge from the taproot. 
  • They stretch outward, anchoring the young plant, preparing to gather moisture, oxygen and nutrients.
  • In the seam between the halves, the first pair of leaves (sucker leaves) appear.
  • The emergence of these leaves is one of the most rewarding sights in the entire cycle. Tiny, spreading green sails that mark the beginning of the seedling stage.

The second set of leaves will be bigger than the sucker leaves, with a barely noticeable serrated edge. The third set of leaves will have 3 fingers. It’s not until the later sets that we see the iconic, multi-fingered shape of the marijuana leaf.

How do you protect marijuana seedlings?

As your seedling grows, you’ll need to keep it away from insects and other pests. An easy way to do this is to plant the seeds in red solo cup, with a ‘biodome’ made from plastic wrap and an elastic band. Check out Kyle’s Planting Your Seeds video for full details.

You can also use a transparent, plastic dome. A cheap way of doing this is to cut a clear plastic bottle in half, retaining the bottom part. Add a few air holes for ventilation and pop it over your seedlings.

These bio-domes, whether home-made or store-bought, also serve to keep the humidity level high. The seedlings will absorb as much moisture from the air as possible, as the roots will not be fully developed.

The young plants also need watering, but keep the soil moist, never drenched. They also need 24hrs light, but not too intense. If the light source is too weak or too distant, the young plants might stretch too much. You want to keep them short and sturdy as they grow into veg – a lanky plant will struggle to stay upright!

How much do you water cannabis seedings?

Overwatering your seedlings is a common mistake, especially when using pots. If the soil is too wet for too long, the roots will be unable to absorb oxygen and will drown. It’s like holding your head underwater! Under-watering is just as bad – dry roots are very soon dead roots. Make sure you water correctly at every stage of your plant’s life. Check out Kyle’s Feeding and Watering video for full details.


It’s too easy to over-water your marijuana plants. You could be worried the soil is too dry (when it’s perfectly damp below the surface soil). Your drainage holes could be blocked. Your pot could be too big. It’s worth exploring how these mistakes occur in the hope they can be avoided

What size pot do you need for marijuana seedlings?

During the seedling stage, the root system is growing, but still very small. An over-large container needs a lot of water to fill, which means a lot of pressure on the tiny room system – the same pressure divers feel the deeper they swim.

As Kyle Kushman advises, a solo cup or small pot is perfect for your seedlings – especially with a bio-dome to allow secondary moisture absorption through the tiny leaves.

As the seedling grows above the soil, the roots will be expanding under the soil, eventually outgrowing the small pot. This is the point at which it needs to be transplanted to a larger pot before it becomes root-bound. A root-bound plant is an unhealthy plant, and it won’t survive. The roots, squashed and under pressure, will be unable to uptake nutrients, water or oxygen. The plant will first wilt, then it will die.

During their weeks in the small container, there are two things you need to remember.

1. Observe the runoff and make sure the drainage system (holes in the bottom) is working.

2. If you have put the seedlings in a large pot, there is a way of watering that can avoid drowning the roots. Water only around the edges of the young plant. When it appears dry, poke a finger under the soil, watering only when nearly dry. If you can manage this, if you can move your seedling straight to its final home, you can avoid the stress of a second transplant later down the line

Planning ahead is essential. Generally, roots grow at a faster rate the longer the plant remains in a container. Hence, you want to minimize the number of times they are transplanted. You should take this into consideration when planning the pot sizes.

Do cannabis seedlings need good drainage?

Your container should not accumulate excess water so make sure it has drainage holes. In most cases, this should be enough. The growing medium you use will also affect the rate at which water is drained. Clay-based soils, for example, have a high water retention rate and you should avoid them where possible.

One way to prevent drainage issues is to add perlite to the soil. This amendment helps keep the soil light and airy, making it easier for water to pass through.


In trying to avoid giving the plants too much water, overcompensating may present another problem – under-watering.

Plants release moisture through the leaves. This creates a vacuum inside the plants, which forces the roots to absorb water, collecting nutrients and oxygen at the same time. Dry soil has severe consequences. Never let the soil completely dry out.

What sort of nutrients do cannabis seedlings need?

Depending on your potting mix, you may have to give your plants extra nutrients. Make sure you know which soils come pre-loaded, how long the nutes will last, how much more to give them and when. As there are so many different feeding methods, we cannot generalize. Talk to the guys at the hydro store and always read the label! Planning ahead is vital – it will help you make sure you have the nutes to hand exactly when the plant needs them.

Cannabis seedlings and nutrient toxicity

What is wrong when you notice tips of the leaves appearing burnt, darker in color? One explanation is nutrient toxicity – an excess of nutrients in the soil.

How do you avoid nutrient toxicity?

A classic newbie mistake is to erroneously use ‘slow-release’ soils. These are ‘hot’ soils containing high amounts of nutrients, far too much for seedlings. If you accidentally use hot soil, you can flush the excess nutes by ‘pure’ watering regularly. Your little babies should recover in a few days.

When selecting the growing medium for seedlings, opt for a high-quality potting mix ideal for seedlings. Your hydro staff should be able to advise you.

Cannabis seedlings and nutrient deficiency

Yellowing, folding leaves towards the bottom of the plant could be a sign of a nutrient deficiency. Unless you identify the source of the problem, your young plants will wilt and die.

How do you avoid nutrient deficiency?

Always use a high-quality potting mix, appropriate for seedlings. If planted in a soilless medium with no nutrients, you’ll need to provide the right kind of nutes in the right amount. Remember, as the seedlings grow in size, their nutritional requirements also increase. Very young seedlings don’t require any additional nutes and can be grown in an inert medium.

If your medium is too acidic or alkaline, you might also see symptoms of nutrient deficiency. If the pH level of your grow medium is outside optimal parameters, plants lose their ability to absorb minerals. Like trying to drink through a squashed drinking straw. You should measure the soil pH daily or once every two days. If adjustments are needed, you can use pH buffers (up or down) in the water you provide when watering the plants.

Generally, the recommended pH range for soil is 6.0 – 6.8. For soil-less and hydro, it should sit between 5.5 and 6.5. Some experts narrow it down to 6.3 – 6.7 and 5.5 – 5.7 respectively.

Make sure your seedlings are in containers appropriate for their size. Root-bound plants, for example, cannot absorb nutrients efficiently.

What sort of light, heat and humidity do marijuana seedlings need?


Marijuana plants prefer a warm temperature in the range of 70 – 85°F (20 – 30°C). If you notice the leaves starting to curl upwards, forming a canoe shape, they could be suffering from heat stress. The answer is to increase the distance between the light and the plants by raising the lights or lowering the plants.

As for seedlings, they do not need a huge amount of light energy, but the distance from the lights needs to be short enough to avoid excessive stretch. What they really need, at this point, is humidity.


With regards to humidity, the general recommendation for seedlings is 65 – 80% RH. The rationale behind this is that the plants will absorb moisture from the air, allowing the roots to focus on growing and becoming stronger.


A fluorescent light (CFL or tube) is perfect for seedlings, but you’ll still need to use an MH lamp during the vegetative stage and HPS throughout the flowering period (unless you have a state of the art LED set-up).

Because fluorescent lamps don’t not produce too much heat, your seedlings can sit just 2 to 4 inches under the light. Be careful to maintain the same distance as they grow.

A light cycle of 18 hours on, 6 hours off is ideal, some people keep them on 24 hours on. Also, try to keep the temperature a little below 80°F – the optimum temp for healthy growth.

Basic care for cannabis seedlings

Your seedlings don’t require a great deal of attention. The main bulk of the work, they do themselves. All you need to provide is a suitable home, a good light source as close to the pot as possible and occasional checks. Your main worry will be that the seedling grows too quickly and squishes itself on the bio-dome!

This is an essential stage of your plant’s life – get the basics right and your seedling will have all it needs to survive into the vegetative stage, and beyond into flowering.

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