Seedling Phase in Cannabis Plants: Grow Healthy Cannabis Seedlings

The seedling phase is the first stage of growing cannabis, and arguably the trickiest. Cannabis seedlings are fragile, and minor slip ups can severely stunt their growth, or worse; kill them altogether. With the right know-how, however, even beginner growers can master the cannabis seedling phase and grow big, healthy, and high-yielding plants in no time.

Growing healthy cannabis seedlings is key to a heavy harvest. Unfortunately, giving cannabis seedlings a solid start to life can be tricky, especially for beginner growers. After all, no one is born a great cultivator, and there’s no shortage of misleading or confusing information online about how to care for cannabis seedlings.

So, how can you be sure you’re giving your weed seedlings everything they need? What are the ideal conditions for cannabis seedlings, how often should you water them, and do they need nutrients? Keep reading for answers to all your seedling-related questions.

Start by investing in the right genetics

Starting your grow off with hardy genetics suited to your growing experience and conditions is the first step to ensuring you grow healthy seedlings and, later, vigorous vegetative or flowering plants. 

In general, the cannabis you harvest is only going to be as good as the genetics you’re growing. And after taking the time and effort necessary to germinate, grow, harvest, dry, and cure cannabis, nothing could be more dissatisfying than ending up with sub-par flower.

Hence, we always recommend starting your grow with the best genetics. When picking a strain for your home cannabis grow, consider:

  • The effects, aromas, and flavours you most enjoy. We’re lucky to live in a time when there’s literally thousands of strains on the market, each with unique traits. Before starting your next or first grow, take the time to shop around for a strain that tickles your fancy.

Next, create the perfect environment for your cannabis seedlings

Once you’ve picked the perfect strain for your home cannabis garden, the next step to ensuring you grow healthy cannabis seedlings is taking the time to provide them with the perfect growing conditions.

We recommend doing this before you germinate your seeds so your seedlings can get the best start to life from the moment they pop out of the soil. To guarantee you’re growing your weed seedlings in the right conditions, consider:

  • Your grow space. Cannabis seedlings have unique needs and are therefore best grown in a dedicated grow space. Propagators, for example, are an inexpensive grow tool that allow you to isolate your seedlings in a space where you can easily control conditions like lighting, temperature, and humidity. Alternatively, keeping your seedlings under a clear plastic dome also works. For the best results, we always recommend starting cannabis seedlings indoors.
  • Containers. Feminized cannabis seeds are best started in small seed starter trays, plastic cups, or 1 l plastic pots. Whatever container you choose, make sure it drains well – stagnant water in pots damages roots and attracts moulds as well as pests. Note that autoflowering cannabis seeds should be planted directly in their final pots as they don’t respond well to being transplanted (see ‘When to transplant cannabis seedlings’ below).
  • Soil. Cannabis seedlings perform best in aerated, well-draining soil that’s light in nutrients. Do not grow cannabis seedlings (or even mature cannabis plants) in potting soils made for ornamentals or vegetables, as these soils are packed with fertiliser and will burn your delicate seedlings. Ideally, we recommend growing cannabis in organic soil with a pH of 6-7. Check out this article to learn how to buy or make the perfect soil for cannabis.
  • Lighting. A good light source is central to growing great weed, and no man-made lamp can compare to the power of the sun. However, sensitive cannabis seedlings are best started under cool, low-watt CFL lights. Place your seedlings under lights as soon as they sprout out of their soil to maximise their growth and keep them from developing leggy stems(see ‘How to plant cannabis seeds’ below). Just like during the later vegetative stage, seedlings perform best under a 18/6 light cycle.
  • Temperature and relative humidity. Cannabis seedlings grow best in warm, humid conditions. For the best results, keep the temperature in your grow space between 20-25 °C (68-77 °F) and relative humidity at 65-70%. Cannabis seedlings take up water through their leaves while their root systems develop. Once their roots have grown, you can bring down the humidity in your grow space or begin acclimatising your plants to the outdoors.
  • Airflow. Cannabis seedlings develop strong stems when exposed to a light breeze, so consider running a small oscillating fan on a low setting close to your seedlings. Do not point the fan directly at your young plants; instead, keep it pointed a few centimetres from their tips.
  • Protection. Cannabis seedlings are very sensitive and need to be protected from anything that can cause them physical damage, such as falling objects, strong winds, birds (which love to eat sprouting seedlings), and pets or other burrowing animals.

How to properly germinate cannabis seeds

Once you’ve picked the perfect strain for your home cannabis garden and taken the time to create the right environment for your seedlings, it’s time to pop your beans.

To give your seedlings the best start in life, we recommend germinating them using the popular “paper towel method:”

  1. Line a plate with 2 layers of moist paper towel.
  1. Place your seeds on top, followed by another 2 layers of moist paper towel.
  1. Cover with an upside-down plate and use a spray bottle to regular mist your seeds. Be sure to keep their paper moist at all times and keep your seeds in a warm area of the house.
  1. Your seeds should sprout within 7 days (although this can take up to 14 days in rare cases) and are ready to plant once they’ve developed a 2 cm-long taproot.

Note that cannabis seeds may develop hard outer shells as they age. If you’re trying to sprout old seeds, it may help to soak them in water for 12-24 hrs before germinating them between paper towels.

How to plant germinated cannabis seeds

Once you’ve germinated your seeds, its time to plant them in soil. As we touched on earlier, cannabis seedlings like aerated, well-draining soil with few nutrients and a pH of 6-7. Indoors, cannabis can tolerate and benefit from slightly more acidic soil with a pH of 5.5. Remember that it’s best to plant your germinated seeds in small containers, such as seedling trays or plastic cups.

Planting cannabis seeds in smaller containers makes it easier to ensure that your seedlings develop strong roots that have properly populated their growing medium. Moreover, it makes it easier to manage the moisture level of your soil, as larger pots might encourage overwatering.

When planting your seedlings, first fill each of your containers with moist soil. If you wish, you can enrich your soil with rooting bacteria/fungi such as Trichoderma or mycorrhiza. These are readily available from garden centres and grow shops.

Use your finger to prod a 1 cm-deep hole in the centre of each container, place a seed in each hole with its taproot facing down, then cover with a very light layer of soil. Make sure to keep the soil moist but never soaked and, as soon as your seedlings pop out of the ground, make sure to place them under a good light source.

What’s the best light for cannabis seedlings?

We recommend growing cannabis seedlings under low-watt CFL lights to avoid stressing them. CFL lights can be hung 15-20 cm from the tops of young seedlings and run on an 18/6 schedule without burning even young plants.

Many growers also see great results starting their seedlings next to a sunny window. If you plan to do this, make sure your seedlings get 6-8 hours of direct light, and move them under your grow lamp or outdoors once they’ve developed 2-3 leaves (see ‘When to bring cannabis seedlings outside’ and ‘When to move cannabis seedlings under HID or LED grow lights’ below). For the best results, keep your seedlings out of direct midday sunlight for the first 7-14 days of their life.

Can you germinate cannabis seeds in soil?

Most cannabis growers prefer to germinate their seeds using the paper towel method outlined above, and for good reason – it’s easily the most reliable way to effectively sprout seeds. However, it is also possible to germinate cannabis directly in soil. Just remember that there’s more room for error when doing so.

To germinate cannabis seeds in soil, plant them directly in small containers. Be sure to keep the soil from drying out and wait anywhere from 7-14 days for the seedlings to pop their leaves out of the ground.

How to properly water cannabis seedlings

Properly watering your cannabis seedlings is key to their growth and survival. Unfortunately, knowing when and how to water seedlings can be tricky, especially if you’re new to the game.

A lack of water can cause cannabis seedlings to dry out, which hinders the growth of their roots and can quickly result in their death. Overwatering, however, can be just as damaging to weed seedlings, also stunting the growth of their roots while simultaneously exposing them to pathogens such as Pythium, Botrytis, and more.

Unfortunately, there is no one-size-fits-all approach to watering weed seedlings. Instead, you’ll need to learn to read your plants and their conditions to gauge when they need water. Of course, this is something you’ll only learn with time and experience, including the negative yet inevitable experience of losing a few seedlings to over or underwatering.

As we mentioned earlier, cannabis seedlings like moist and humid growing conditions. Hence, its best to keep their soil moist at all times. However, avoid watering your seedlings so much that their soil feels/looks wet or soaked.

Instead, touch the soil in your containers and only water your seedlings when it feels physically dry. More often than not, the top layer of soil in your containers may look dry, while the soil beneath remains moist.

When you decide to water your seedlings, do so very sparingly and by applying water in a circle a few centimetres from their stem. This will encourage their roots to grow out in search of water and promote the development of a larger, healthier root system from early on.

Moreover, mist your seedlings regularly. While their roots are still growing, cannabis seedlings take up most of their water through their leaves.

Do cannabis seedlings need nutrients?

No! Cannabis seedlings need no nutrients and should never be fed with fertiliser. Light organic soil has all the nutrients needed to grow healthy seedlings.

When to transplant cannabis seedlings

Once you’ve germinated and planted your seedlings, it’s important you know when to transplant them. Ideally, you’ll want to transplant your seedlings once their roots start to reach the edges of their containers.

Transplant them too soon, and you may stress your seedlings, stunting their growth or killing them altogether. Wait too long, on the other hand, and your seedlings may become rootbound – this happens when a plant’s roots reach the edges of its container and start becoming entangled, eventually growing into a tight ball that restricts any further growth.

When exactly you decide to transplant your seedlings will depend on how fast they are growing as well as the size of your containers. If you’ve started your seedlings off in starter trays, they’ll likely be ready to transplant once they’ve developed their first ‘true’ leaves – that is, leaves with at least 3 leaflets. If you started your seedlings in larger containers (such as plastic cups or 1 l plastic pots), it may take several weeks for them to require transplantation.

A good way to know that your plants are ready for transplanting is to look at the drainage holes at the bottom of their containers. If you see their roots popping out of the soil, this is usually a sign that your plants are ready for a bigger home.

What pot size should you transplant seedlings into?

Once they’ve outgrown their starter containers, cannabis seedlings can be planted in 3 l plastic pots. While you might be tempted to pop your young plants into larger containers, its best to stick to smaller containers at this time. Again, smaller containers make it easier to track the growth of your plants’ roots and further helps to prevent overwatering.

Once they’re in 3 l pots, you won’t need to transplant your seedling for a few weeks. Track their growth and transplant them as soon as their roots start popping out the bottom of their containers.

Once they’ve outgrown 3 l pots, you can move your cannabis seedlings into larger smart pots (the exact size of which will depend on your growing style as well as the size of your grow space). These pots are made out of fabric or porous plastic and promote the growth of strong root systems.

Because they are made of breathable material, smart pots allow your plants roots to breath easily – essential for fast, healthy growth. Moreover, your plants’ roots will be trimmed naturally once they grow to the edge of their pots and come into contact with air and light, encouraging them to grow further down into their medium and preventing them from becoming rootbound.

Do not transplanting autoflowering seedling

Note that the above information on transplanting only applies to feminized or regular cannabis seeds.

Transplanting inevitably stresses plants, no matter how careful and gentle you are. When growing feminized or regular cannabis seeds, you’re able to compensate for this stress by extending your plants’ vegetative growth phase. Even outdoors, where your plants flower in response to natural light cycle changes, they’ll still have plenty of time to recover from the stress of being transplanted.

Autoflowering cannabis varieties, however, flower naturally after a few weeks and have much shorter lifecycles than feminized or regular cannabis plants. On average, autoflowering plants usually begin to flower 4 weeks after germination and can go from seed to harvest in as little as 10 weeks.

This leaves very little margin for your plants to recover from stress, which is why its best to start your auto plants directly in their final containers. Moreover, autoflowering plants shouldn’t be trained, as they also won’t be able to recover from the stress of most training techniques.

How long does the seedling phase last?

In general, growers stop considering their plants seedlings once they’ve developed 3-5 or more true leaves. How long it takes for this to happen, however, varies greatly depending on the conditions in your tent or garden, as well as the growth rate of your plants.

Every cannabis plant is unique and grows at its own rate, and the seedling phase may last anywhere from 1-4 weeks.

When to move cannabis seedlings outdoors

Even if you plan to grow cannabis outdoors, its best to start your seedlings inside to keep them protected. However, once they develop 2-3 true leaves, we recommend acclimatising them to the outdoors to streamline their growth.

Do this by gradually bringing your plants outside. Start by exposing them to 2 hours of morning or afternoon sunlight for 1-2 days. Then bring them outside for 3-4 hours, and so on. After 7-10 days, you should be able to bring your plants outside for the whole day.

If it’s really hot in your area, consider keeping your seedlings out of direct midday sunlight until they’ve developed 5 leaves and seem hardy. Also, make sure to keep close tabs on your soil to ensure its stays moist in hot conditions.

When to move cannabis seedlings under HID or LED grow lights

While CFL lights are great for starting cannabis seedlings, they are far from ideal for mature plants. Hence, you’ll want to move your seedlings under a more potent grow light as soon as possible to stimulate more rapid growth.

You should be able to this once your seedlings have grown 3 true leaves but take care to avoid burning your seedlings by keeping them a fair distance from your grow lights at first. HID and metal halide lights, for example, are best kept 50 cm from the tops of your plants. LEDs, on the other hand, can hang 30 cm from the tops of seedlings.

How to prevent damping off in weed seedlings

Have you ever checked on your healthy seedlings in the morning, only to find them keeled over the edge of their containers a few hours later? You’re not alone. Damping off, a phenomenon caused by fungi and bacteria, is easily the biggest killer of cannabis seedlings and a problem that affects growers of all skill levels.

Unfortunately, the main causes of damping off are silent killers like Pythium, Botrytis, and Fusarium, which often lie dormant in soil and flourish in warm, humid conditions. Worse yet, there’s really nothing you can do prevent your seedlings from damping off other than following a strict watering routine and avoiding overwatering at all costs.

How to prevent stretchy seedling growth

Another common problem facing even experienced cannabis growers is stretchy, leggy seedlings. Overly leggy seedlings tend to tip over, resulting in stunted and uneven growth.

Luckily, unlike with damping off, there’s plenty you can do to keep your seedlings from stretching too much. Mainly, you’ll want to grow your seedlings under well-placed CFL lights from the minute they first stick their cotyledons out of their soil.

Because CFL lights are cool and of low wattage, you can keep them very close to your seedlings without stressing them. Usually, a well-placed CFL bulb is enough to keep your cannabis seedlings from developing long, leggy stems.

Cannabis seedlings FAQs:

1. How often should I water my cannabis seedlings?

Cannabis seedlings should be watered whenever their soil starts to dry out. Stick your finger into your seedlings’ pots and only water them when the soil feels dry to the touch.

2. How much water should I give my seedlings?

This will depend on the size of your containers. Ideally, you’ll want to give your seedlings enough water to moisten their soil without soaking it. When in doubt, it’s best to give your seedlings a little less water, as overwatering attracts pathogens that can cause –damping off and other issues.

3. Do cannabis seedlings need nutrients?

No! Never feed your seedlings with fertiliser as you’ll only burn their delicate root systems.

4. What pots are best for cannabis seedlings?

Feminized or regular cannabis seedlings are best started in starter trays, plastic cups, or 1 l plastic pots. Autoflowering cannabis seeds, however, should be started directly in their final containers, (preferably plastic or fabric smart pots).

5. How much light do cannabis seedlings need?

Cannabis seedlings grow best under 18 hours of light. Use cool, low-watt CFL lights at first, as these will help prevent leggy growth without causing light stress.

6. When should cannabis seedlings be placed under lights?

Cannabis seedlings are best placed under CFL lights as soon as they pop out of the ground. This will maximise their growth and keep them from developing leggy stems. Once they’ve developed 2-3 leaves, you can begin acclimatising your seedlings to the outdoors or HID/LED grow lamps.

Mastering the cannabis seedling stage is key to growing healthy, vigorous plants and reeling in a heavy harvest. Unfortunately, the fragile nature of cannabis seedlings makes them tricky to manage, especially for beginner growers. Hence, its important to take the time to create a good environment for your seedlings well before you germinate your seeds.

Using propagators, high-quality soil, cool CFL lights, and following a strict watering routine are some of the pillars of growing healthy seedlings. At the same time, losing a few seedlings is inevitable and a great learning experience for rookie growers.

Now that we’ve walked you through what we consider essential to growing healthy weed seedlings, be sure to share your own tips and tricks in the comments below. Happy growing!

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.

Comments

15 thoughts on “Seedling Phase in Cannabis Plants: Grow Healthy Cannabis Seedlings”

  1. ciao marco,
    ho letto il “rapporto sulla coltivazione femminizzata di N13 Kush” e l’ho trovato molto utile ed interessante.
    Dove posso trovare un rapporto su HINDU KUSH REGULAR? Ho da poco comprato 10 semi da Sensi. Ne ho germinati due ed un seme ha già fatto una piantina con 4 foglioline nel germinatore (artigianale: sopra un piccolo vasetto con la terra ho messo un coperchio di plastica che lascia passare molta aria e pare che funzioni). ciao e grazie

    1. Mark Smith - Sensi Seeds

      Good morning Luciano,

      I hope you’re having a great day, thanks for your comment.
      At the moment, our Hindu Kush Regular is still being created, but will be released shortly!
      Great news with your seedlings, I wish you the best of luck 🙂

      Have a good one,

      Mark

      1. I purchased 5 seeds of which onl 3 came up after initial sprouting. 60% growth rate. I feel that’s pretty low. Is this normal?

  2. New grower here, waiting on seeds to arrive. In the meantime purchased some twine, blood meal, potting soil & coco. also purchased aerogarden Harvest Model, 10.5 ”x 6.9”x 17.8” Hydroponics Mylar Grow Tent with Grid Observation Window for Indoor Growing. Im sure some may be too early yet, but if there is anything else I may need, advice, suggestions, tips welcomed

  3. I had bad experiences with lights, they were to strong, now I am looking at wing reflectors which are quite practical and it says they don’t burn the plants (if properly installed ofcourse)

    I am currently looking at the Wing reflektor enforcer medium, any opinions?

    Don’t know what to choose, I’m also thinking about the Bat Cool Tube (125-x-490mm) … I would be glad for any opinions ☺️

  4. MastroAldo70

    Sono al secondo tentativo di avviare la coltivazione, il primo tentativo ha avuto esito negativo, su 5 semi autofiorenti femminilizzati, 1 solo è arrivato quasi alla fine, ma la resina prodotta era poca e non ha fatto in tempo a maturare bene perchè la pianta si è bloccata nella crescita ed è seccata. Ora stò provando con 10 semi naturali, sfida ancora più difficile, tutto sempre indoor, con lampade led di cui 5 con luce rossa e blu, 1 piu grande multispettro usata come “sole” che faccio accendere solo 6 ore al giorno alla metà delle 18 ore di luce, sperando che questo simuli appunto il sole nelle ore centrali del giorno.
    sono a 15 giorni dalla nascita, oggi sono alte circa 10/12 cm e stanno spuntando su quasi tutte la 3a coppia di foglie seghettate. Spero che non si blocchi la crescita, vorrei poi usare dei fertilizzanti naturali, come fondi di caffè, bucce di banana, acqua di fagioli, ma ho paura che questi formino delle muffe, secondo voi devo mischiare nel terreno o appoggiarle sopra al terreno nei bordi del vaso, per non far toccare il busto della pianta ?
    grazie

    1. Mark - Sensi Seeds

      Buonasera Aldo,

      Grazie per il tuo commento,
      Sembra che le tue piante stiano benissimo!

      In generale, le sostanze solide funzionano meglio se tagliate in piccoli pezzi e scavate nel terreno umido prima di piantare piantine o durante il rinvaso in contenitori più grandi. I liquidi possono essere diluiti con acqua e utilizzati come fertilizzante liquido. Fortunatamente, abbiamo anche un articolo sulla guida per principianti ai fertilizzanti fatti in casa qui! (Scusate, è solo in inglese!)

      Godetevi il blog!

      Mark

  5. raymond joseph getz 111

    Seems like every grower has their own ideas so much so that I find my ownb pattern which changes with every grow with every plant

  6. I’ve got my Automatic Hindu Kush seedlings in an open fish tank next to a window (uk) .. they Initially grew about 7 cm with one pair of leaves.. but for the past 2 weeks haven’t grown at all.. I’ve now put LED lights on to give more light.. but still no change. This also happened to me last year-they just stopped growing and I had to throw them?! What am I doing wrong? I feel like I’m wasting my money every time I try?

    1. Mark - Sensi Seeds

      Good afternoon A,

      Unfortunately, legal restrictions mean we can’t answer grow-related questions or give grow advice on this blog. However, other readers of this blog will often answer questions like yours.

      Please check out the following articles on our blog which I think you may find of use, and I hope you continue to enjoy the blog!

      The life cycle of cannabis: From seed to harvest

      When can seedlings be put under lights or in the sun
      With best wishes,

      Mark

    2. Check out Growdiaries if you want a few tips. I was where you are about a year ago, so I completely understand the frustration!! The end goal is totally worth it though so don’t give up!! Think of all those big juicy buds that are far better quality and potency when home growing.

    3. cover half your tank up, put a heating mat inside with a bowl of water and use blue light for the first couple weeks, you need some humidity

  7. 24 degrees celcius …….RH =85……moving air by oscilating fan.

    are these conditions ok for seed started plants in seedling stage

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    Sensi Seeds

    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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