Growing Marijuana Seedlings: getting your cannabis crop off to a great start

Germination Station


see bottom of the page for Campo’s Choice Picks to help you get your seedlings off to a great start!



Whilst buying good Cannabis Seeds provides you with the genetic capabilities of your cannabis plant, growing marijuana seedlings well enables these traits to be realized. Limiting factors at this crucial time in development will have repercussions for the mature plant – or more likely, kill it off whilst it is at a delicate phase. The seedling stage lasts from 1-4 weeks, so you have only this amount of time to get it right!
It’s important to give marijuana seedlings  a constant atmosphere with moderate humidity, warmth (21ºC) and reasonable light quality – therefore even if you intend to grow outdoors, it is worthwhile investing in a (heated) propagator and even a small CFL light to enable a healthy start to your plants.



A humid atmosphere enables the seedling to absorb water through the leaves whilst its root system is as yet immature, thus speeding up the growth rate of the marijuana seedling (85-90% rH).


Remember that these plants would naturally be growing in spring time in warm climates. 21ºC simulates that fact.


Not enough light and obviously the marijuana seedling will struggle to photosynthesize and thus grow. (see stretching): too much light however will overpower the delicate root system as the plantlet fails to absorb enough water and nutrients to maintain the transpiration rate. A natural indirect light is preferred for the first few sets of leaves, after which CFL lighting is very useful. These fluorescents can be hung 5 cm from plant tips to avoid stretching.

Seedlings should receive a minimum of 18 hours light a day, but can cope with 24 hours.


Always use a “soft” water (e.g. distilled or mineral) with a pH OF 5.5 – 6.5. If you have to use tap water, allow it to stand for 24 hours to allow the chlorine to evaporate. Whichever you use, let the water come to room temperature to avoid shocking the marijuana seedling.

Do NOT over water – this is a common cause of “damping off”.


Especially with marijuana seedlings it is vital that the growing medium allows oxygen to get through to the fledgling root system. Soil grown seedlings need to be in either a specialist seedling mixture with plenty of perlite or sand to allow good drainage. Rockwool must not be allowed to be sodden as this too will prevent oxygen reaching the roots and “drown” the plant.

Note also that any soil used should not be rich in nutrients as these will cause nutrient burn and inhibit the rooting process.


Marijuana seedlings are vulnerable to over fertilization (nutrient burn), whilst at the same time the nutrient free nature of most seedling mediums necessitates their addition before long. To avoid over fertilization, use either a specialist seedling feed or dilute a cannabis feed to ¼ strength.

Whilst not a fertilizer, it can be a good idea to give a diluted amount of RootBoost or similar to enable good root development in your young plants.

Marijuana seedling problems

Given these conditions, the cannabis seedlings have every chance of starting life well and growing into healthy mature marijuana plants. However, these are common problems with seedlings:

Damping off.

Fungal disease whereby stem becomes floppy and marijuana seedling dies. Avoid by making sure growing conditions are neither too wet nor too humid.

Over fertilization

See nutrients above


Whilst some strains (especially those with a sativa base) are more likely to stretch – or appear to – stretching should be avoided in marijuana seedlings as it leads to lanky growth with weak stems in mature plants. It also has the effect of a prevalence of small buds as opposed to good dense ones.

Stretching can be caused by a number of reasons:

  • Poor light: seedlings will stretch towards any available light source if they are not receiving enough.
  • Overheating

Campo’s Choice Picks to help you make sure your grow room gets off to a great start in 2013!


Hydrofarm CK64050 Germination Station with Heat Mat

Another great product from Hydrofarm (these guys really do seem to get the best customer reviews), this “germination station” or heated propagator will help get your seedlings off to the best start.

  • Germination station with electric heat, humidity control and 72-cell seedling inserts
  • Provides gentle warmth to the planting bed; controls humidity under 2-inch dome
  • Humidity dome with dual vent points for optimal airflow
  • Includes seed starting booster, instructions with growing tips
  • Watertight base tray measures 11 by 22 inches

Its reduced from $34.99 down to $24.88 on and that sounds like a fair price to us!

Use withHydrofarm MTPRTC Digital Thermostat For Heat Mats which is $25.29  and will enable you to provide a specific controlled temperature for your cannabis seedlings.

What we’d really recommend though – unless you are growing with LEDs is that you invest in a decent T5 system. This will prevent over stretching which occurs when young plants seek out light from an insufficient source and also heat burn from too hot a light source (eg too close a HID growlight).

We are going to recommend the Hydrofarm JSV4 4-Foot Jump Start T5 Grow Light System ($63.99) as if you buy all 3 items together, you get them for $114.16. The T5 system is described as a great light for the money, and one which will keep you producing healthy marijuana seedlings for years to come. There are a few quibbles about flimsiness, but this version is the more sturdy of the ones produced and sold by Hydrofarm, and it features on Amazon’s Best sellers list, so it cant be all wrong.

More to help your seedlings ...

10 thoughts on “Growing marijuana seedlings

  1. Midowo says:

    For the watering, I was under the impression that tap water is more useful for seedlings as it has some tiny amounts of nutrients but distilled water is too pure without any chemicals.

    • John says:

      I think tap water is fine for germination. The Ph should be 5.5 for seedlings. Because the seed contains a small amount of nutrients tap water is fine, but quarter strength nutrient solution and the 5.5 Ph should be used for seedlings. I usually start fertilizing when the first true leaves come in or the baby leaves start to yellow, whichever comes first.

      • weed widow says:

        I’ll be honest, we tend to use tap water too – and start fertilizing (quarter strength) around the second – third sets of leaves … but some people’s tap water can be very acid or very alkaline, so we suggest the pH water as a guide line ..:)

  2. Matt says:

    When do I start giving the seeds light? Right after germination? When they come out of the soil? Or have a couple leaves grown? Also how much wattage and lumens per plant? And how far away to keep the lights when using cfl, T5, or MH?

    • weed widow says:

      putting them under a gentle light (or windowsill) before they come out of the soil helps the young seedling know which way to grow .. its best to use cfl lighting (or leds) for young seedlings – the heat and intensity of light from a metal halide bulb (use MH for veg growth, HPS for flowering, or a dual spectrum bulb will do both) – means that you risk either burning delicate seedlings if HID lights are too close or stretching if HID lights are too far

  3. redfish74 says:

    I live in south Florida. Started kinda late in season growing outside. My girls just started flowering. It starts to cool off around November. Dont know if im going to finish the cycle by then. Is there something I can do to speed up process before it might get cold down here. I know cold in Florida right

  4. weed widow says:

    Hi Redfish! How cold does it get? Cos unless you have some super sativa strain as long as they have started flowering now, they should be finished from mid-October.

  5. Grower 4 sure says:

    Indoor growing is da bomb, allowing for complete vontroll. Also allowing to start your outdoor plant’s much earlier making for true pound plus monsters outside.

    • weed widow says:

      In the 1st 2 weeks they shouldnt need any nutrients at all – then around week 2 start off on 1/4 feed and build up alternate feeds. Its easier to add more nutrients than it is to take them away!

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