Cannabis Flowering Nutrients: The Short Answer
During flowering, cannabis still needs nitrogen, but the plant wants less of it than it did in veg. Shift the feed toward phosphorus and potassium, hold calcium, magnesium, and sulfur steady, and keep the full micronutrient package in the mix. Most growers run a bloom base with roughly half the nitrogen of their veg formula, add a phosphorus and potassium booster from week 3 through week 6, then taper strength over the final 10 to 14 days. Exact numbers depend on your medium, water source, and cultivar, so the schedule matters less than what the plants show you.
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What Each Nutrient Does in Flower
- Nitrogen (N): supports leaf health and enzyme function. Too little turns lower fan leaves yellow before the plant is done; too much delays flower set and stretches internodes.
- Phosphorus (P): drives energy transfer, root activity, and bud site development. Excess builds up in soilless media and locks out zinc and iron.
- Potassium (K): regulates water movement and sugar transport into flowers. Heavy K loads late in the run can slow ripening and leave a mineral taste if you do not taper.
- Calcium and magnesium: keep cell walls and chlorophyll intact. Coco and reverse osmosis water almost always need a cal-mag supplement through week 6.
- Sulfur and micronutrients: iron, manganese, zinc, boron, copper, and molybdenum. Small amounts, large effects on terpene expression and overall plant health.
Prerequisites
- Calibrated pH meter and EC or PPM meter.
- Known starting water: a tap report or a zero-EC source.
- A base nutrient line with a matching bloom formula, plus cal-mag for coco or hydro.
- A written record of your veg feed strength, so the switch is a controlled change.
Week-by-Week Feeding Steps
- Confirm the stretch has ended, about 10 to 14 days after the light flip, and switch from the veg base to the bloom base at the same EC.
- Mix the bloom base at 1.2 to 1.6 EC for weeks 1 through 3 in coco or hydro, keeping nitrogen low and phosphorus and potassium moderate.
- Add a PK booster at weeks 3 through 6, following the label rate and holding total EC at or below 2.0 to 2.2.
- Keep cal-mag at 2 to 5 ml per gallon through week 6, then cut back if leaf tips show burn.
- Hold mid-flower EC steady from week 5 through week 7 while buds swell, and water to runoff in soilless systems to limit salt buildup.
- Reduce EC by 20 to 30 percent starting week 7, dropping nitrogen first while keeping potassium available for ripening.
- Finish with plain pH-adjusted water for the last 7 to 14 days, with the length based on your medium and how dark the leaf tissue still is.
Read the Plant, Not Just the Label
Dark, waxy, clawing leaves point to nitrogen excess. Raise your water volume and cut the nitrogen fraction. Lower leaves that yellow from the bottom up while buds stall point to nitrogen shortage or a pH lockout. Rust spots on upper leaves with curled edges suggest calcium or magnesium trouble, often caused by low pH in the root zone. Red or purple stems mean little on their own, since many cultivars show them regardless of feed.
Bud density, resin production, and aroma are the real scorecard. If flowers look thin and leafy at week 5, the plant is getting too much nitrogen relative to phosphorus and potassium. If tips burn and growth stalls, your EC is too high for the cultivar and the light intensity.
Common Mistakes
- Switching to bloom nutrients the day of the light flip instead of waiting for stretch to end.
- Pushing PK boosters past week 7, which stalls ripening.
- Dropping cal-mag too early in coco, which shows up as brown spots in mid-flower.
- Chasing a label schedule while ignoring runoff EC and plant response.
- Running plain water for three weeks, which starves the plant before harvest.
Medium Adjustments
Soil buffers nutrients and needs less frequent feeding, so a top dress or a bloom feed every other watering often works. Coco and hydro give you instant control, which means you can respond to a deficiency within days, but salt buildup happens fast if you skip runoff. Organic growers can lean on compost, bone meal, and kelp, though those inputs release slower and are harder to correct mid-flower.