Vegetable seedling 'legginess' happens for one reason and one reason only... not enough hours of intense light. A plant becomes leggy when it germinates and breaks the surface of the starting mix and it isn't met by intense light.. If the light is bright enough or intense enough, it will stop growing its stem and start producing leaves. If the light isn't intense enough it will continue to grow its stem until it reaches a light level that triggers it to stop. It is trying to get to the right place to start leaf production and normal growth.
When you are growing transplants indoors treat the surface of the starting mix as if that is a plant leaf. You want the light bulbs to be 1 to 2 inches above the starting mix so that when the vegetable plant breaks the surface it is met by bright intense light within 12 hours. If not... the plant will continue to grow rapid stem growth to reach for the light. That creates the leggy seedling. You have to let the plant know it broke the surface (that it is out of the earth) and the right light is available. So... 1 to 2 inches of space between the starting mix and light bulbs.
This video will show you how I created 'leggy' beet seedlings. I accidentally put the seed tray in my closet level that had bulbs 4-6 inches above the starting mix. As I say in the video, getting old stinks. I should have put it in my seed germination level where the bulbs are 1-2 inches above the starting mix.
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