I topped this plant at day 21.
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4 days later, you can see it already recovered nicely
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This picture is one day after the second pic. You can see it has recovered wonderfully and is growing like crazy
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So you can FIM or top when very young. I like to top or FIM around the time the plant is the size in the first picture, which gives it plenty of time to recover so i can top again.
And even at day 26 its small for its age because it was stunted as a tiny seedling