Under a 600W HPS, I haven’t ever seen a main cola that’s much longer than 2 feet even if light is getting down almost to the floor. So, I’m not sure how much benefit you would get by switching to 12/12 after the plant is 2 feet tall. These plants were switched at around 20″ tall under a 600W.
Although it’s true you want to flip to the flowering stage when your plant is about half the final desired height (since it will about double in size after the flip to the flowering stage), here are some general guidelines that have worked well for me:
Examples with Common Grow Lights
Here’s another example of a plant that was a good height at the end. You can see there isn’t a whole lot of extra growth at the bottom with small buds. However, you can see the buds end where the thick layer of leaves begins. This grower could have used some light defoliation to expose more bud sites lower down, and produced buds further down into the plant. In that case, their grow light could possibly have supported a plant that was even taller!
“Tall” vs “Short” Strains
In this example, the plant was “lollipopped” (the lower part of the plant was stripped of leaves and bud sites before being switched to the flowering stage). However, on the parts of the plant that weren’t stripped, you can see significant buds going a lot further down. This plant likely would have yielded even more if it’d been allowed to keep more bud sites. The grower could have still stripped all the leaves, but they may have gotten better results if they’d left bud sites for at least a few more inches if not all the way to the bottom.
While most, if not all, medical and recreational farms cultivate cannabis from seeds, guaranteeing that their plants are free from viruses, most home growers, even those with experience, typically begin with clones — essentially trimmed pieces of female cannabis plants that have been rooted in separate pots. “When most people think of cannabis — you know, what you smoke — they’re thinking about the flowers of female plants,” Lipton said. “When you grow from seeds, half of them will be males. If you’re only going to do one or two plants, you don’t want to waste your time with that stuff.” A clone sourced from a dispensary or a knowledgeable friend guarantees that the plant is female and will eventually produce bud pending proper care. “You can get up to four ounces off the right plant — if you know what you’re doing.”
Know the law.
To understand the flowering cycle, it’s important to remember that cannabis is a plant. And, like most plants, it follows the seasons. To trigger flowering — which will take 55 to 60 days to complete — growers reduce the time plants spend exposed to the light source from 18 hours to 12. “You’re basically telling your plant it’s mid-September,” Lipton said.
Trigger the flowering cycle.
Even in our most progressive states, however, the law is far from simple. “In Colorado, it’s now county-specific,” Lipton said. “When the amendment first passed, they said you could grow six plants per person. But now, certain counties and municipalities have come out and said it’s just six per house — there’s no combining plant counts. That means you can have three vegetating and three flowering at any given time.”