Grow Myths vs The Science

Does cranking potassium (PK boosters) in flower grow bigger buds?

Verdict: Not supported. This is the big one, because nearly everyone does it. The story goes that flower is the "fruiting" stage, so you load up on P and K — the "PK boost" — and the buds swell. It borrowed its logic from tomato fertiliser, where fruiting potassium demand genuinely is higher. Cannabis isn't a tomato.

When Bevan, Jones and Zheng varied nitrogen, phosphorus and potassium independently, with controls, across a roughly sixfold range of potassium (60 to 340 mg/L), potassium had no statistically significant effect on yield. Nitrogen and phosphorus moved the needle; potassium was along for the ride. The most-repeated nutrient advice in the hobby, and the controlled data said it does nothing within the normal range. The lesson isn't "K is useless" — the plant needs it. It's that adding more of it past adequacy doesn't buy you bud, and a "bloom maximiser" that's mostly potassium is selling you a problem you don't have.

Bevan, Jones & Zheng (2021), Frontiers in Plant Science 12:764103. doi.org/10.3389/fpls.2021.764103. CC-BY.

Does adding a UV-B light boost THC?

Verdict: Not supported. The reasoning sounds airtight: plants at altitude get more UV and make more protective resin, so bolt on a UV bar and the plant cranks up THC as sunscreen. Plausible. Someone measured it.

Rodriguez-Morrison, Llewellyn and Zheng ran supplemental UV-B under controlled conditions and found it did not reliably increase inflorescence (bud) yield or cannabinoid concentration. There was a catch that keeps the myth on life support: cannabinoid levels rose in the small sugar leaves — but not in the harvested flower you actually keep. A real effect, in the wrong place. Meanwhile UV is a stressor that damaged plant tissue, and it's genuinely hazardous to your eyes and skin. So you'd be paying money and risking your retinas to enrich the bits you trim off. Potency is mostly genetics plus enough light and a stable room. There's no UV shortcut around the strain you chose.

Rodriguez-Morrison, Llewellyn & Zheng (2021), Frontiers in Plant Science 12:725078. doi.org/10.3389/fpls.2021.725078. CC-BY.

Do you have to run exactly 12/12 to flower?

Verdict: Not supported (as a hard rule). 12/12 works, so it never gets questioned. But "you must run 12/12 and it's optimal" is a different, bolder claim — and that one didn't survive testing. Flowering is triggered by the long, unbroken night (around 12 hours of dark for most strains), not by the magic of twelve hours of light. So twelve hours of darkness is a safe threshold. It is not a yield ceiling.

Peterswald and colleagues compared flowering photoperiods head to head and found 12/12 was not optimal for every line. A longer 14-hour light period in flower more than doubled target-cannabinoid yield in a high-CBD line (Cannatonic), raised it about 50% in one high-THC line (Northern Lights), and made no significant difference in a third (Hindu Kush) — while the shortest, 10-hour schedules trended to the lowest yields. The takeaway for a beginner: 12/12 is a fine, reliable default, so use it. But it's a default, not a law, and the real, non-negotiable rule is that the dark period stays genuinely dark and unbroken. A light leak at night does more damage than your exact schedule ever will.

Peterswald, Mieog, Azman Halimi, Magner, Trebilco, Kretzschmar & Purdy (2023), Plants 12(5):1061. doi.org/10.3390/plants12051061. CC-BY.

Do you need a special "bloom" or "blurple" spectrum for good flower?

Verdict: Not supported (in practice). Spectrum does affect the plant — that's the next section, and it's true. But the marketing claim, that you need a special red-heavy "bloom" spectrum or a blue/red "blurple" panel to grow proper flower, is where it falls apart. What actually drives bud is intensity — how much light reaches the canopy (PPFD) — and a modern full-spectrum white LED already carries the wavelengths for both veg and flower from one fixture. The old metal-halide-for-veg, HPS-for-flower routine died when white LEDs covered the whole range. A perfect spectrum at starvation intensity grows airy bud; a good white at the right intensity grows dense bud. Spectrum is the fine-tuning; intensity is the engine. Most "bloom switches" are really just dimming half the diodes — changing intensity while you think you're changing colour.

Rodriguez-Morrison, Llewellyn & Zheng (2021), Frontiers in Plant Science 12:646020, doi.org/10.3389/fpls.2021.646020; Eichhorn Bilodeau et al. (2019), Frontiers in Plant Science 10:296, doi.org/10.3389/fpls.2019.00296. CC-BY.

Does more light mean more bud?

Verdict: Supported, up to a point. This is the lever people under-use while chasing the ones that don't work. Rodriguez-Morrison and colleagues ran cannabis under canopy light levels from 120 all the way to 1,800 µmol·m⁻²·s⁻¹ and found dry flower yield increased roughly linearly with light intensity right up to 1,800 — far past where a single leaf's photosynthesis saturates. Potency (THC%) barely moved across the range; the gain was in how much bud, not how strong. Two honest caveats. First, it's PPFD at the canopy that matters — the actual photons hitting the plant — not the wattage number on the box, which tells you what the light draws from the wall, not what it delivers. Second, more light means more heat, more water, and more everything else to manage; you can't crank PPFD and ignore the rest of the room.

Rodriguez-Morrison, Llewellyn & Zheng (2021), Frontiers in Plant Science 12:646020. doi.org/10.3389/fpls.2021.646020. CC-BY.

Does packing plants tighter just waste them?

Verdict: Supported — and it's the opposite of what people fear. Beginners worry that crowding plants chokes the yield. Danziger and Bernstein tested planting density directly and found higher density increased yield per unit area. The catch worth knowing: it also reduced cannabinoid uniformity — the buds across a crowded plant varied more in strength, because the lower, shaded, inner sites lagged behind. So for total weight off a given floor area, denser can win; for an even, consistent crop bud-to-bud, give them room. Either way, "more plants = less yield" isn't what the controlled data shows.

Danziger & Bernstein (2022), Frontiers in Plant Science 13:713481. doi.org/10.3389/fpls.2022.713481. CC-BY.

Is flavour really down to genetics, not your feeding?

Verdict: Supported. Growers love to credit their flavour to a feeding trick or a secret additive. The aroma palette is mostly written before you open a single bottle. Allen and colleagues mapped the complete terpene synthase (TPS) gene family in Cannabis sativa — the genes that build the terpenes you smell — and the profile a plant can produce is genetically encoded by cultivar. Your environment and how you dry and cure decide how much of that potential you keep; the genetics decide what the menu was in the first place. You can ruin good genetics with a bad cure. You can't feed cheap genetics into a gourmet terpene profile they were never coded for.

Allen et al. (2019), PLOS ONE 14(9):e0222363. doi.org/10.1371/journal.pone.0222363. CC-BY.

Can you actually diagnose a deficiency by where it shows up?

Verdict: Supported. This one's good news — the rule of thumb is real. Llewellyn and colleagues ran single-element deficiencies under controlled conditions and confirmed the pattern: mobile nutrients (like nitrogen, phosphorus, potassium, magnesium) show their deficiency first in the older, lower leaves, because the plant robs its old growth to feed the new; immobile nutrients show up in the new growth at the top, because the plant can't move them once they're placed. Nitrogen and phosphorus deficiencies cut yield the most; some, like iron and manganese, hit the leaves' looks more than the harvest. Symptoms developed over roughly a 7-to-28-day window, so where the yellowing appears, and how fast, is a legitimate diagnostic — not a guess.

Llewellyn, Golem, Jones & Zheng (2023), Plants 12(3):422. doi.org/10.3390/plants12030422. CC-BY.

Does light spectrum actually change the plant?

Verdict: Supported. Spectrum is a real variable, not pure marketing. Magagnini, Grassi and Kotiranta grew cannabis under different light spectra and found that plant morphology can be manipulated by spectrum, and cannabinoid accumulation can be affected too. So the underlying physics is sound — the colour mix the plant receives does change how it grows and, to a degree, what it makes. The reason this sits in "supported" while the "buy a bloom spectrum" claim above sits in "not supported" is the gap between spectrum matters and this product's spectrum gimmick is what's holding back your grow. The first is true. The second is usually intensity wearing a costume.

Magagnini, Grassi & Kotiranta (2018), Medical Cannabis and Cannabinoids 1(1):19–27. doi.org/10.1159/000489030. Open access.

Does drought-stressing the plant boost potency?

Verdict: Mixed. The idea that a controlled dry-out late in the grow pushes the plant to make more resin has some support — and a big asterisk. A 2025 review of water-deficit stress in cannabis found that some controlled studies report modest rises in cannabinoid and terpene concentration under drought, while others find no effect, and yield commonly falls. Here's the trap the headline hides: concentration going up is not the same as harvesting more. If the percentage rises 10% but the plant makes 20% less bud, you've gone backwards in total cannabinoid and stressed the plant to do it. "Potency up" and "more product" are different claims. The evidence supports the first, sometimes; it does not support a free lunch.

Sharma et al. (2025), Horticulturae 11(6):646. doi.org/10.3390/horticulturae11060646. CC-BY.

Does defoliation (stripping fan leaves) increase yield?

Verdict: Mixed. Few topics start more forum fights. The fan leaves are the plant's solar panels, so every one you pull is photosynthesis you gave up; the bet is that the light and airflow you free up to lower bud sites pays you back. When defoliation was run as a controlled architecture treatment, the effects on cannabinoids varied by location on the plant rather than delivering a clean, repeatable yield bump. Translation: it can help a healthy, bushy plant where lower sites are genuinely shaded, and it can cost you on a stressed or already-open plant. It's a situational technique with a real downside, not a guaranteed gain — which is exactly why honest growers argue about it forever.

Danziger & Bernstein (2022), Frontiers in Plant Science 13:713481. doi.org/10.3389/fpls.2022.713481. CC-BY.

Does starving the plant late in flower make better bud?

Verdict: Mixed — and it's a trade-off, not a win. The belief is that easing nutrients off near the end "ripens" the plant into stronger flower. Massuela and colleagues ran nutrient stress in the flowering stage and found something more interesting than yes or no: biomass dropped, but CBD concentration rose — netting about 95% of the CBD yield while using a third less fertiliser. So you don't get more cannabinoid by starving the plant; you get roughly the same total from a smaller, more concentrated harvest, with a real saving on inputs. That's a legitimate efficiency argument. It is not "starve it and the bud gets stronger." It's the concentration-versus-yield trade again, which is the quiet theme running through half this report.

Massuela et al. (2023), Frontiers in Plant Science 14:1233232. doi.org/10.3389/fpls.2023.1233232. CC-BY.

Does flushing before harvest make bud smoother, tastier, or stronger?

Verdict: Insufficient evidence (under our open-access bar) — but the research that does exist leans firmly toward "no benefit." Flushing — plain water for the last week or two instead of feeding — is treated as a sacred quality step. No open-access, peer-reviewed study supports a taste, smell or potency benefit from flushing alone, so by our own rule we don't bank a verdict. What's striking is that everything we can find points the same way, and nothing contradicts it: a controlled study across several cultivars (Saloner, Sade & Bernstein, 2024) found no quality benefit; a separate controlled study on high-CBD cannabis found no gain from a normal flush and a loss of cannabinoid content from a month-long one; a University of Guelph trial found a two-week flush changed neither cannabinoid nor nutrient concentration; and an industry research-coalition series reached the same "no horticultural merit" conclusion.

We don't count those at the same level as open-access peer-reviewed work — two are paywalled, one is an unpublished thesis, one is industry-run — so the formal verdict stays unproven. But four independent investigations converging on "no benefit," with none pointing the other way, is worth saying plainly: on the current evidence we lean toward flushing offering no quality benefit, and we're calling it unproven rather than confirmed only because the qualifying, free-to-read study doesn't exist yet. What flushing reliably does do is stop salts stacking up at the very end after heavy synthetic feeding — a sensible finishing tweak sized to how hard you fed. The thing that actually makes smoke smooth isn't the flush. It's a slow dry and a proper cure. The flush is a footnote; the cure is the chapter.

Sources (not banked — see "how we weigh evidence below the bar" above): Saloner, Sade & Bernstein (2024), Industrial Crops and Products 220:119157, doi.org/10.1016/j.indcrop.2024.119157 [paywalled]; "Exploring the Legacy Practice of Flushing in Controlled-environment Production of High-CBD Cannabis" (2024) [paywalled]; Stemeroff (2017), University of Guelph [MSc thesis, not peer-reviewed]; Cannabis Research Coalition (2023) [industry, not peer-reviewed].

Does feeding molasses or sugar make buds bigger or sweeter?

Verdict: Insufficient evidence. The theory is that molasses feeds the soil microbes, which feed the plant, and the sugars sweeten the bud. There's no controlled cannabis study behind any of it. The plant makes its own sugars from light; it doesn't drink yours through its roots and pipe it into the flower as flavour. Could a carbon source do something useful for soil biology? Maybe. As a bud-size or sweetness booster, it's untested lore.

Does cold-shocking or ice water late in flower make buds purple and stronger?

Verdict: Insufficient evidence (and the mechanism is wrong). Purple buds come from anthocyanin pigments, which some cultivars express more strongly as temperatures drop late in flower. That's genetics plus a cold finish making a colour — it has nothing to do with potency. No controlled cannabis study links cold-shocking to higher cannabinoids. Chasing purple by chilling a strain that isn't coded for it mostly just stresses the plant for a colour it was never going to make.

Are silica supplements essential?

Verdict: Insufficient evidence (for cannabis). Silicon has a respectable research record in other crops, where it can stiffen cell walls and help under heat, drought or pest pressure. The leap people make is "therefore cannabis needs a silica bottle." There's no controlled, open-access cannabis study establishing that silica is necessary or that it lifts yield or quality. It may help at the margins under stress. "Essential" is a marketing word the evidence hasn't earned.

Does a lunar or biodynamic planting calendar improve the grow?

Verdict: Insufficient evidence. Planting and harvesting by the moon is old, and it's sincerely held. There's no cannabis study supporting it, and the broader agronomy literature has repeatedly failed to find a reliable lunar-cycle effect on plant growth. It costs nothing to follow if you enjoy the ritual. It just isn't doing what it's credited with.

Does talking, singing, or playing music to plants help them grow?

Verdict: Insufficient evidence. Charming, harmless, and unproven. There's no controlled cannabis study, and the general plant-acoustics research is weak and contested. Your plants don't need encouragement; they need stable VPD. Talk to them all you like — it's better for you than it is for them.

Does the "entourage effect" mean terpenes clearly change your experience?

Verdict: Insufficient evidence (and we make no medical claims). The entourage effect — the idea that terpenes and cannabinoids work together to shape the effect — is biologically plausible and shows up in some preclinical lab work (Chacon et al. characterise the synthesis and proposed synergy). What's missing is robust human evidence. So it's a promising hypothesis worth understanding, not a settled fact to build claims on. We're flagging it as unproven rather than banking it, exactly as we'd want anyone else to.

Chacon et al. (2022), Biomedicines 10(12):3142. doi.org/10.3390/biomedicines10123142. CC-BY.

Does your choice of medium (soil vs coco vs hydro) dramatically change yield?

Verdict: Insufficient evidence (for a clear winner). People will tell you, with total certainty, that their medium is the one that grows the biggest. A review by Nemati and colleagues characterises the properties of the common media — water-holding capacity, the cation-exchange capacity of coir, and so on — which genuinely differ. But that's not the same as a controlled head-to-head crowning a yield champion, and no open-access study does that cleanly. The honest read: media differ in how they behave and how forgiving they are, and the best one is the one you can run consistently. The medium is a tool, not a cheat code.

Nemati et al. (2021), Agronomy 11(7):1366. doi.org/10.3390/agronomy11071366. CC-BY.