Bongkhananswered grow question 4 years ago Hey mate ; you could start it as soon as it reaches 4 to 5inches in height and the main stem is still soft. Do not wait longer as they tend to get harder once they enter vegitative stage from seedling. Also if you notice that the main stem is too soft at 5 inches ;just tie a support at the bottom of main stem in opposite direction of your preferred LST to give it supoort and strength.