These are going to yield so poorly, sorry to be the bearer of bad news, but that's some big facts.
I wouldn't even defoliate those. Auto's NEED 18hrs of light MINIMUM to be productive enough to produce anything other than leafy, airy bullshit rat turd nugs. It needs all the leaves it grows. As stated previously, topping it will 100% stunt growth for several days, and these autos run on a time clock. You're veg period is pretty much over, far too late to top. Not sure why so many people are saying to top a 4wk old AUTOFLOWER thats under like 13hrs max of indirect sunlight. Must be amature hour lolol
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