This guy makes a case for not topping, saying it just subtracts from the plants without any benefit:
http://www.gardenscure.com/420/training-scrog-sog-supercropping-etc/63090-low-stress-training-lst-more-walk.html
http://www.gardenscure.com/420/training-scrog-sog-supercropping-etc/63090-low-stress-training-lst-more-walk.html
Topping: There is this whole belief that doing this increases your yield. Someone says "Well, how can it not, there are twice as many tops now"
Are there really? NO. In fact, if you actually counted the amount of tops, you just LOST one! YEA, imagine that, YOU cut it off.
Here is where the misinformation comes in. When everybody talks bout topping, they make it sound as if two NEW tops magically sprout from the cut you just made. Now, as nice as that would be, thats not what happens. What does happen is that the branches below the newly cut top become the new main tops. Now, people do this for a couple reasons. First as I said before, they think they have just doubled their tops. And secondly, because they want to promote a bushier plant.
It baffles my mind how what they just did actually achieved the exact opposite of BOTH of their objectives. Those branches below the cut were there before, and if they werent, they should have been. You shouldnt have to induce them by cutting the main top off. Branches happen REALLY early, if they dont, its a lack of light, good health or some other such reason thats really irrelevant. The point is, that the branches should have (and in most cases) are ALREADY there.
So when you cut that main top off, you are LOSING a top, SUBTRACTING. All that changes is that the prior secondary tops are no longer secondary.
You have LESS plant, LESS bush, LESS everything; please dont do this.
This creates undue stress. You just lost a finger.
The plant has to heal, takes time to close the wound; slows down your grow.
NOTHING BENEFICIAL
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Wait, so you are saying topping serves no purpose what so ever?
Gonna make me work for this one eh? Fine, I take it back for now. Let me explain.
As far as topping for the reasons most do it, i'm holding firm to my position. The exception comes with cloning. You obviously need a cutting, so you are going to have to top the plant somewhere (rarely the main top).
Let me just say this though. When taking a clone, its important that you take the clone during the day. The plant has a growth hormone called "auxins" that are in the growth tips during the day, and in the roots at night.
This is the same reason its advised to NOT top a plant during the day with no intentions to take a clone (all the wrong reasons). It will take the plant that much longer to recover if it loses its valuable auxins because you cup off the top during the day.
Needless to say, if you intend to clone, and root the cutting you just made, take the clone during the day, as it will root faster since all the auxins are there (as it was taken during the day).
So what did we learn? IMO dont top unless you are taking a cutting for a future clone. If you are going to top anyways, and dont intend to take a clone, do so at night (toxins in the roots, so none are lost, and you get a quicker recovery time).