# "Bloom boosters" for cannabis: what's real and what's marketing

> Most bloom boosters are concentrated phosphorus and potassium. Four independent controlled cannabis trials found no increase in flower yield or cannabinoids from raising phosphorus above roughly 25–60 mg/L, and one found no yield response to potassium anywhere from 60 to 340 mg/L. A complete, balanced flower feed is what matters; extra P and K mostly end up in the runoff and can induce magnesium, calcium, zinc and iron problems. Some boosters have been reported to contain undeclared plant growth regulators, which have no place in a consumable crop.

- Canonical: https://genetixbay.com/knowledge/nutrients/bloom-boosters.html
- Category: Nutrients · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Flower · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Do bloom boosters work on cannabis?** Bloom boosters mostly do not work on cannabis beyond what a balanced flower feed already provides. Four independent controlled trials found no gain in flower yield or [cannabinoids](https://genetixbay.com/knowledge/glossary/cannabinoid.md) once solution phosphorus passed roughly 25-60 mg/L, and one trial found no yield response to potassium from 60 to 340 mg/L. The extra phosphorus and potassium largely end up in the runoff.

**Should I use a PK booster in week 4 or 5 of flower?** A PK booster in week 4 or 5 of flower is generally not needed if the cannabis base feed is complete. Research puts the useful flower range at about 40-60 mg/L phosphorus and 200-280 mg/L potassium. Stacking a booster on a full bloom base can push P above 100 mg/L and K above 400 mg/L, adding no yield and risking magnesium and calcium problems.

**Does high phosphorus increase bud size?** High phosphorus does not increase cannabis bud size once the plant has enough. Controlled trials testing 5-90 mg/L phosphorus found yield plateaued around 25-60 mg/L, and in one study leachate phosphorus rose about twelvefold for a threefold increase in input. Flowers took up more than 1% phosphorus with no benefit, a pattern called luxury uptake.

**Are bloom boosters a waste of money?** For growers already running a complete flower feed, bloom boosters are largely a waste of money, according to controlled cannabis research. The money is better spent on light intensity, environment and root-zone control, since cannabis yield rises with light. Anyone who wants to test one should split a room of identical plants of one cultivar, then compare dry flower weight and lab potency.

**What is in a bloom booster?** A typical cannabis bloom booster is mostly concentrated phosphorus and potassium, often sold as a 'PK' product. The idea stretches a real fact, that phosphorus and potassium are needed for flowering, past its limit, because need has a ceiling. Some bloom or hardening additives have also been reported to contain synthetic plant growth regulators, sometimes undeclared, so full ingredient disclosure matters.

**Are there growth regulators in some cannabis additives?** Some cannabis bloom or hardening additives have been reported to contain synthetic plant growth regulators such as paclobutrazol or daminozide, sometimes undeclared on the label. These chemicals shorten [internodes](https://genetixbay.com/knowledge/glossary/internode.md) and densify flowers. Daminozide is classed as a probable human carcinogen and is not approved for food crops in many jurisdictions. Use only additives with full ingredient disclosure and check local rules.

**Short answer.** Most bloom boosters are concentrated phosphorus and potassium ("PK"). Four independent controlled cannabis trials found no increase in flower yield or cannabinoids from raising P above roughly 25–60 mg/L [1][2][3][4], and one found no yield response to K anywhere from 60 to 340 mg/L [4]. What matters is a complete, balanced flower feed. Extra P and K mostly end up in the runoff [2] and can induce Mg, Ca, Zn and Fe problems. Some boosters have been reported to contain undeclared plant growth regulators, which have no place in a crop people smoke or eat.

## What's happening

The bloom-booster idea comes from a real fact stretched too far: P and K are needed for flowering. But "needed" has a ceiling. Once the plant has enough, more does not make more flower.

The cannabis evidence:

| Study | What was tested | Result |
|---|---|---|
| Shiponi and Bernstein 2021 [1] | P 5–90 mg/L, two medical [genotypes](https://genetixbay.com/knowledge/glossary/genotype.md) | Biomass highest at 30–90 mg/L; [THCA](https://genetixbay.com/knowledge/glossary/thca.md)/[CBDA](https://genetixbay.com/knowledge/glossary/cbda.md) concentration highest below 30 and down up to 25% at higher P (mostly dilution) |
| Westmoreland and Bugbee 2022 [2] | P 25, 50, 75 mg/L, high-CBD cultivar | No effect on yield or cannabinoids; leachate P up ~12× for 3× input; flower P >1% (luxury uptake) |
| Hershkowitz, Westmoreland and Bugbee 2025 [3] | P 15–90 mg/L and 2 vs 4 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md), deep-flow hydro | Neither excess P nor doubled strength improved yield or quality |
| Bevan, Jones and Zheng 2021 [4] | N, P, K response surface, [DWC](https://genetixbay.com/knowledge/glossary/dwc.md) flower | Yield optimum ~59 mg/L P; no response to K across 60–340 mg/L |
| Velechovsky et al. 2024 [6] | Augmented P, K and Fe | Augmented regime did not substantially increase biomass |
| Bernstein et al. 2019 [5] | Enhanced P, enhanced [NPK](https://genetixbay.com/knowledge/glossary/npk-ratio.md), [humic acid](https://genetixbay.com/knowledge/glossary/humic-acid.md) | Enhanced NPK raised [CBG](https://genetixbay.com/knowledge/glossary/cbg.md) 71%, lowered [CBN](https://genetixbay.com/knowledge/glossary/cbn.md); no large THC gains reported. |

Meanwhile, high K competes with Ca and Mg at the root, which was seen in cannabis [7], and high P is associated with Zn and Fe problems in many crops (see [nutrient antagonism](https://genetixbay.com/knowledge/nutrients/nutrient-antagonism.md)).

### Growth regulators

Some "bloom" or "hardening" additives have been reported to contain synthetic plant growth regulators such as paclobutrazol or daminozide, which shorten internodes and densify flowers, sometimes undeclared. Daminozide is classed as a probable human carcinogen and is not approved for food crops in many jurisdictions. Use only additives with full ingredient disclosure, and check what your jurisdiction allows on cannabis.

## Numbers that matter

| Parameter | Practical target in flower | Notes |
|---|---|---|
| Solution P | 40–60 mg/L | No gain above this range [1][2][3][4] |
| Solution K | 200–280 mg/L | No yield response found 60–340 [4] |
| Total EC | 1.6–2.2 mS/cm | 4 mS/cm no better than 2 [3] |
| Leaf P when overfed | >1% in flowers | Luxury uptake, no benefit [2] |

## What to do instead

1. Run a complete flower feed at sensible strength (see [building a feed by stage](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)).
2. Keep N adequate through mid-flower; many "boosters" are paired with cutting N too early.
3. Spend the money on light, environment and root-zone control. Yield in cannabis rises with light intensity; see the environment articles.
4. If you want to test a booster, split the room: identical plants, same cultivar, half with, half without, weigh dry flower and send both for [potency testing](https://genetixbay.com/knowledge/glossary/potency-testing.md).

## Common mistakes

- Stacking a booster on a full bloom base, pushing P above 100 mg/L and K above 400 mg/L.
- Reading bigger runoff EC as "the plant is eating more".
- Trusting before-and-after photos with no side-by-side control.

## Evidence

- High P does not increase cannabis yield or cannabinoids: *Established* (four independent controlled trials in different labs and systems [1][2][3][4]). House position agrees: there's a plateau, and past moderate P more doesn't help.
- High K does not increase flowering yield: *Supported* (one trial [4]).
- "More P = bigger buds": *Myth* beyond sufficiency.
- PGR contamination of some additives: *Supported* by grower and media reports

## Sources

1. Shiponi S, Bernstein N. 2021. The highs and lows of P supply in medical cannabis. Frontiers in Plant Science 12:657323. https://doi.org/10.3389/fpls.2021.657323
2. Westmoreland FM, Bugbee B. 2022. Sustainable cannabis nutrition: elevated root-zone phosphorus significantly increases leachate P and does not improve yield or quality. Frontiers in Plant Science 13:1015652. https://doi.org/10.3389/fpls.2022.1015652
3. Hershkowitz JA, Westmoreland FM, Bugbee B. 2025. Elevated root-zone P and nutrient concentration do not increase yield or cannabinoids in medical cannabis. Frontiers in Plant Science 16:1433985. https://doi.org/10.3389/fpls.2025.1433985
4. Bevan L, Jones M, Zheng Y. 2021. Frontiers in Plant Science 12:764103. https://doi.org/10.3389/fpls.2021.764103
5. Bernstein N, Gorelick J, Zerahia R, Koch S. 2019. Impact of N, P, K, and humic acid supplementation on the chemical profile of medical cannabis. Frontiers in Plant Science 10:736. https://doi.org/10.3389/fpls.2019.00736
6. Velechovsky J, Malik M, Senkyrik JB, Tlustos P. 2024. Frontiers in Plant Science 15:1322824. https://doi.org/10.3389/fpls.2024.1322824
7. Saloner A, Sacks MM, Bernstein N. 2019. Frontiers in Plant Science 10:1369. https://doi.org/10.3389/fpls.2019.01369