Getting a good root system is half the battle when growing photo-period cannabis plants in soil, and a lot of old hands swear by the potting up method to develop one. Its also called transplanting, potting-on , re-potting and no doubt a few other things but they're all the same.
So I've put together a quick guide to potting up to help growers out a bit.
The problem is, that potting up, if not done carefully, can often damage the plant, its called "transplant shock" and can set plants back weeks. Not good with a fast grower like cannabis. And nine times out of ten, its due to roots getting damaged, often compounded by not having a long enough recover/settling in period.
Here I've got a 5 week old plant that's been growing in a 3l pot for a few weeks. It had plenty of roots showing at the bottom and was ready to move onto a bigger pot, In this case a 15l. The plant was watered 24 hours previously to ensure that it did not need to 'feed' itself for a few days and to ensure the roots were pliable enough to withstand the transplant.
1) Start by filling the bottom of the bigger pot with some fresh soil.
Use the smaller pot as a guide for just how deep its filled. You want the stem-base of the plant to be about 2-3" from the top of the newer one
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2) Position the smaller pot centrally and put fresh soil all around it until it reaches the lip of the small pot.
Gently pack down the soil around the small pot and top up as needed.
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3) Lift out the smaller pot leaving a perfectly fitting hole for transplant.
This is a good time to sprinkle any beneficial mycho's around the hole so the new roots will be 'inoculated' straight away.
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4) Carefully and gently, squeeze around the small pot to loosen it, make like Spock with your fingers around the stem to support the plant and soil then flip it over
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5) Lift the small pot away from the roots/soil, put your hand on the top (bottom of the plant) and flip over again.
It should come away in one whole with very little falling away. It is at this point that the damage occurs so be careful not to let big chunk of soil come away and damage the roots. Watering the day before helps to keep it together.
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6) Carefully slide the plant into the pre-prepared hole, sprinkle a little fresh soil over the top and gently but firmly press it down to 'set' the plant in place.
You should be able to hold the stem at the very base between your fingers to lower it in. If the soil all falls away, then you're doing it too soon and there is not a good enough root system developed. But we live and learn.
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That's it. All we have to do is water the plant in and leave her be. Go easy on the water, there's no new roots to suck it up yet. In a week, she'll be settled, have put some roots down in the fresh soil mix and, for this plant, it will be ready for changing light cycles to flowering 12/12 It's very important to try and give the plant time to settle in before changing light cycles, and there should be no need to feed her. This will help encourage roots to grow in to the fresh soil and get over any stress.