Indoor marijuana growers have long assumed that photoperiod marijuana plants have a built-in “bloom phase stretch” that starts when light hours are shortened from 18 to 12 hours per day and ends about 2-3 weeks into bloom phase.

This trait was assumed to be genetic, baked in to each strain. You’d expect your plants to at least double in height, it not more, in the first weeks of bloom phase.

Anticipating bloom phase stretch influences major decisions such as how tall to let plants get before starting bloom phase, as growers calculate how tall plants will be after bloom phase stretch stops, worrying about grow room, grow tent, and grow light vertical height dynamics.

But in the past ten years, cannabis light researchers such as Dr. Bruce Bugbee have unlocked many former mysteries about how light intensity and wavelengths affect plants. Read about his discoveries here, but the most important thing to know is that bloom phase stretch is likely NOT a genetic trait, and can be prevented or manipulated by you.

My discovery of this came when Dr. Bugbee explained two crucial concepts every indoor marijuana grower must understand: daily light integral (DLI), and photosynthetic photon flux density (PPFD).

PPFD is the total amount of photosynthetically active photons that actually reach your plant canopy. These are the only photons that do any good for your plants. DLI is the total PPFD per 24 hour day. This article gives more details on PPFD and DLI.

Using this information, I did experiments that revealed the following:

  • Indoor marijuana plants stretch at start of bloom phase because the number of light hours per day has dropped by six hours, reducing DLI by one third.
  • Plants need a certain amount of DLI per day to manifest robust growth—the loss of six hours of light robs them of needed light.
  • The plants start stretching to get closer to the grow lights to make up for DLI loss caused by reduction from 18 to 12 hours of light.
  • When I increase my grow light PPFD to ensure my plants get as much DLI in 12 hours as they’d been getting with 18 hours, bloom phase stretch drops by more than half and in some cases barely happens at all.

These results were stunning, but only the beginning of my research. For one thing, my early experiments produced plants that barely stretched in bloom phase. Internodes were extremely short and branches were very sturdy. Buds developed from the very bottom of each branch near the soil line and filled in thick all the way to the top.

But overall yield was lower than usual, because the plants stayed pretty much the same height as they started bloom phase with. They were like dwarfs or bonsais.

I experimented with NOT boosting PPFD as much at the start of bloom, and gradually increasing it, and found I could tailor the amount of stretch and internode length. Gradually, I’ve learned to use PPFD and DLI to sculpt plants to give me branch stretch and internode length that increases plant height about 55% from its terminal grow phase height.

This gives me longer branches for longer buds, keeps the plants short enough to fit almost any grow tent or other vertical space, and maximizes branch bud density. Internodes fill in so well that I’ve had branches of solid buds that are 24-41 inches long!

Fact is, excessive bloom phase stretch is not a genetic trait, except to the extent that genetics influence how tall plants get, how much light they want, and internode length by interacting with how much PPFD and DLI you give them.

Sativa-dominant varieties do tend to stretch more and grow taller than Indica-dom marijuana strains, regardless of DLI and PPFD. In a worst case scenario, for example, if you have Sativa-dom genetics and cut your light hours by six hours per day starting bloom phase without increasing PPFD to make up the lost DLI, those Sativa plants are going to stretch wildly trying to get enough light. They may end up like vines!

Once you fully understand how to use PPFD and DLI, you can max out plant morphology to give you shorter Sativas with more bud, or taller Indicas with more bud.

Another input that greatly impacts bloom phase stretch and internode length is the quality of the light spectrum you’re delivering. Most indoor grow lights overdeliver some photosynthetically active wavelengths while underdelivering others. They may have too much, or not enough, red, green, or blue.

This is true for almost all grow light brands, and yes, we’ve tested all of them. Only one LED grow lights company has done professional, scientific, ORIGINAL grow light spectrum and diode research on cannabis and properly designed their spectrum to create maximal marijuana growth rate, beneficial morphology, heaviest harvests, most potent buds.

That company is called SANlight, and you can read about them here.

Experimenting with DLI and PPFD to decrease bloom phase stretch and ensure each branch has the most buds possible with the least amount of wasted vertical growth requires careful observation of your plants using the right tools.

At minimum, you need an LED-specific PPFD meter from Apogee, the amazingly innovative and professional horticultural technology company founded by Dr. Bugbee. You also need to carefully monitor and record plant performance, PPFD, and DLI and do comparative analysis over time to discover the ideal light diet for strains you grow.

It’s wonderful that marijuana growing myths we all assumed were totally based in fact are being debunked by latest research. If you’ve ever been dismayed by plants that start bloom phase 25 inches tall but end up 65 inches tall at end of bloom phase, now you know what to do!