Will my buds still get bigger, and do bloom boosters help?

By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed 2026-10-14

The short answer

• Yes, they are probably still growing. In an indoor study that harvested the same cultivar at four points between week five and week eleven of flower, flower dry weight kept rising the whole way. Week six is not the end of the swell.
• Light is what builds it. In a trial at three light levels, the plants under the most light made 1.6 times the flower dry weight of those under the least. If there is a lever left in week six, it is usually light at the canopy, not what goes in the water.
• Extra phosphorus did not add yield. Four controlled studies raised phosphorus above what the plants needed. The flowers took up more of it. They did not get heavier, and in one study the extra flower mass came with a lower cannabinoid concentration.
• Extra potassium did not add yield either. In a flowering-stage trial, yield did not respond to potassium anywhere from 60 to 340 mg per litre.
• A PK booster is those two — whatever the bottle calls itself. The same product is sold as a bloom booster, a PK booster, a flower hardener or a finisher, and those names describe marketing rather than four different chemistries: phosphorus and potassium, on top of a feed that already contains both.
• And the trials say the same thing about all of them. We found no published controlled trial of a booster product added to a complete feed that showed bigger cannabis flowers. The claim lives on the label and in the forum, not in the literature.
• If the plant stalls after a booster, stop it and go back to the base feed. Then check the runoff. More salt in the root zone is the likeliest thing a booster changed.

What the trials actually did

The best evidence comes from growing the same cultivar side by side and changing one number. Here is what happened when the number was phosphorus, potassium or overall strength.

• Phosphorus at 25, 50 and 75 mg per litre (Utah State, a high-CBD cultivar, continuous liquid feed): no effect on flower yield or cannabinoid concentration. Phosphorus in the flowers rose past one percent. What did change was the runoff: tripling the input raised phosphorus in the leachate twelvefold.
• Phosphorus at 15 to 90 mg per litre, and feed strength doubled from 2 to 4 mS/cm (Utah State, closed hydroponics): leaf phosphorus doubled and flower phosphorus rose 70 percent, with no difference in yield or quality. Doubling the whole feed did not significantly increase yield or quality either. Notably, the strong solutions did not burn the leaves; the plants tolerated them and simply did not use them.
• Phosphorus at 5 to 90 mg per litre, two medical cultivars (Volcani Institute): 15 or less was too little, and growth suffered. 30 mg per litre gave 80 percent of maximum yield, and 30 to 90 was the optimal range. More phosphorus meant more flower mass but a lower THCA and CBDA concentration in it, by up to 25 percent: more weight, diluted.
• Nitrogen, phosphorus and potassium varied together in flower (University of Guelph, a high-THC cultivar, deep-water culture): yield followed a curve for phosphorus, with the best predicted near 59 mg per litre and less above it. Yield did not respond to potassium at all across the range tested.
• Potassium in the vegetative stage, two cultivars (Volcani Institute): 15 mg per litre was deficient, 240 was damaging to one cultivar and helpful to the other, and potassium competed with calcium and magnesium for uptake. That last part matters when a high-potassium booster goes into a plant already short on either.

The limits are real and worth saying. These were soilless and hydroponic systems, one or two cultivars each, fed from start to finish rather than switched to a booster late. None tested a named product. So the finding is narrow and sturdy: once phosphorus and potassium are adequate, more of them is not what makes flowers bigger. A plant that is genuinely short of phosphorus is a different case, and it would respond.

IG-027What the last weeks respond to

Same cultivar side by side, one number changed at a time.

Flower weight, most light vs least 1.6× at 1,000 against 600 µmol·m⁻²·s⁻¹

Weight kept rising to week 11 harvested at four points from week five; week six is not the end of the swell

Extra phosphorus, four studies 0yield gain the flowers took up more of it and did not get heavier

Phosphorus in the runoff 12× when the input was tripled

THCA and CBDA with more P −25% at most: more flower mass, diluted

Potassium, 60 – 340 mg/L noresponse in the flowering-stage trial

Soilless and hydroponic trials, one or two cultivars each, fed start to finish; none tested a named product, and none was in living soil.

So what does make the last weeks count

• Light at the canopy. Flower yield rose in step with light intensity in the Guelph trial, and in an earlier trial from the same lab yield kept rising in a straight line all the way to 1,800 µmol m⁻² s⁻¹, far above what most hobby lights put on a canopy. A tent under a light run at half power, or a canopy that has stretched away from the fixture, is where week-six weight is lost. How much light it needs has the numbers.
• Time. Weight kept accumulating to week eleven in the harvest-timing study. Taking the plant a week early gives up that week of weight, and nothing in the feed has been shown to make it back. Trichome colour is the harvest call, not the calendar alone.
• A plant that keeps its leaves. Buds are built from what the leaves make. Some yellowing at the bottom late is normal; a plant dropping its canopy in week six is a different conversation. Fade or deficiency tells the two apart.
• A root zone that is not fighting it. Steady moisture, a sane pH and a feed strength the medium is not accumulating.

How to check this yourself

• Photograph the same bud every three or four days. Same distance, same angle, a coin or a ruler beside it, light on. Buds that are still filling out show it within a week. That settles “will they get bigger” without guessing.
• Look for new calyxes and white hairs at the tips. New ones stacking on the colas mean the plant is still building. All hairs turned and nothing new appearing means it is ripening, not growing.
• Measure the light at the top of the canopy. A PAR meter if you have one; a phone app as a rough check. If the canopy sits well below what your light can deliver, closing that gap is the booster.
• If a booster went in and the plant stalled, check the runoff. Runoff EC well above what you are feeding means salt is building in the root zone. Go back to the base feed, water to a little runoff, and watch the new growth over four or five days. pH or EC covers reading those numbers.
• If you want to know whether a booster works in your grow, run the only test that answers it. Two plants of the same cultivar, same light, same pot. Give one the booster, give the other the base feed. Weigh both dry. One pair is not proof, but it is better evidence than the label.

What would show that we have this wrong

If a side-by-side of the same cultivar, under the same light and in the same medium, finishes with clearly heavier dry flower on the boosted plant, that is a result the controlled trials did not find, and we would want to know the medium and the product. Soil is the obvious gap: none of these trials was in living soil, where phosphorus availability behaves differently. And a plant that was actually phosphorus-deficient before the booster would be expected to respond, so an improvement after a booster on a plant that was showing deficiency is not a contradiction.

Where our device fits, stated narrowly

Croplock does not weigh buds or measure what is in the water. What it keeps is a dated record: a feeding entry when you log one, the tent’s temperature and humidity around it, and a frame of the canopy from its camera pod every hour. So “did anything change after I started the booster?” becomes a before-and-after you can look at, with dates, instead of a memory.

Sources

• Crispim Massuela, D., Hartung, J., Munz, S., Erpenbach, F. & Graeff-Hönninger, S. (2022), Impact of Harvest Time and Pruning Technique on Total CBD Concentration and Yield of Medicinal Cannabis, Plants 11(1):140. DOI 10.3390/plants11010140 — inflorescence dry weight increased continuously across harvests from week 5 to week 11 of flowering; one chemotype III genotype, indoors.
• Llewellyn, D., Golem, S., Foley, E., Dinka, S., Jones, A.M.P. & Zheng, Y. (2022), Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content, Frontiers in Plant Science 13:974018. DOI 10.3389/fpls.2022.974018 — inflorescence dry weight 1.6 times higher at 1,000 than at 600 µmol m⁻² s⁻¹.
• Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021), Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment, Frontiers in Plant Science 12:646020. DOI 10.3389/fpls.2021.646020 — dry inflorescence yield increased linearly with canopy PPFD from 120 up to 1,800 µmol m⁻² s⁻¹.
• Westmoreland, F.M. & 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. DOI 10.3389/fpls.2022.1015652 — 25, 50 and 75 mg P per litre; no effect on yield or cannabinoids; leachate P up twelvefold.
• Hershkowitz, J.A., Westmoreland, F.M. & 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. DOI 10.3389/fpls.2025.1433985 — 15 to 90 mg P per litre and 2 versus 4 mS/cm; no difference in yield or quality; no leaf necrosis at the high concentrations.
• Shiponi, S. & Bernstein, N. (2021), The Highs and Lows of P Supply in Medical Cannabis: Effects on Cannabinoids, the Ionome, and Morpho-Physiology, Frontiers in Plant Science 12:657323. DOI 10.3389/fpls.2021.657323 — 30 mg P per litre gave 80% of maximum yield; 30–90 optimal; higher P reduced THCA and CBDA concentration by up to 25% through yield dilution.
• Bevan, L., Jones, M. & Zheng, Y. (2021), Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis, Frontiers in Plant Science 12:764103. DOI 10.3389/fpls.2021.764103 — yield quadratic in P with a predicted optimum near 59 mg per litre; no yield response to K from 60 to 340 mg per litre.
• Saloner, A., Sacks, M.M. & Bernstein, N. (2019), Response of Medical Cannabis (Cannabis sativa L.) Genotypes to K Supply Under Long Photoperiod, Frontiers in Plant Science 10:1369. DOI 10.3389/fpls.2019.01369 — vegetative stage; 15 ppm K deficient, 240 ppm damaging to one genotype; K competed with Ca and Mg uptake.
• The photo-a-bud test, the new-calyx check, the runoff check, the two-plant side-by-side and the “stalled after a booster” advice are first-party practice and grower knowledge, not citations. We found no controlled trial of a commercial bloom-booster product on cannabis, and none of the trials above used living soil.

More of these at Answers. The device is at croplock.com.

Related: How much light does it need? · Is it cal-mag? · Flushing before harvest