What to Feed Cannabis in Flowering: NPK & Feeding Chart

Feed flowering cannabis a bloom-stage nutrient mix that is high in phosphorus and potassium, low in nitrogen, and includes calcium, magnesium, and micronutrients. Switch to the bloom formula when you flip to 12/12 or see the first pistils, raise the dose from week 1 through week 5, then taper by week 7 and finish with plain pH-corrected water before harvest.

Cannabis Flowering Time by Strain Guide

What nutrients does cannabis need during flowering?

Flowering plants still need nitrogen to build green tissue, but demand shifts toward phosphorus for bud sites and potassium for resin, starch transport, and water regulation. Calcium and magnesium stay critical because rapid cell division and chlorophyll production drain both. Iron, zinc, manganese, boron, and copper matter in small amounts and are covered by a quality base nutrient.

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What is the best NPK ratio for flowering cannabis?

Most growers run an NPK near 1-3-2 to 2-6-4 in mid-flower, compared with a veg ratio closer to 3-1-2. Boosters add extra P and K on top of the base instead of replacing it. Ratios matter less than total strength and balance, so keep the base feed steady and try any booster at half the label rate first.

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How much should you feed during flowering?

Target a rising EC of about 1.2 to 2.0, roughly 600 to 1000 PPM on the 500 scale, for most photoperiod strains in soil or coco. Increase in small steps, watch leaf tips, and back off if tips burn or leaves claw. Water to 10 to 20 percent runoff in containers to limit salt buildup.

Week-by-week flowering feed plan

1. Week 1 to 2: switch to bloom base, keep some nitrogen, hold EC near 1.2 to 1.4.
2. Week 3 to 4: raise P and K, add a PK booster at half strength, keep calcium and magnesium in the mix.
3. Week 5 to 6: peak feed at EC 1.6 to 2.0, add bloom booster, support heavy buds.
4. Week 7 to 8: reduce nitrogen and total strength, watch trichomes and pistils.
5. Final 7 to 14 days: plain pH-corrected water only, no nutrients.

Organic versus synthetic flowering feeds

Organic choices include bat guano, bone meal, kelp meal, worm castings, and molasses, which feed soil microbes that release P and K over time. Synthetic salts act fast and give precise control of EC and pH, which helps in hydro and coco. Many growers blend both, using organic amendments for flavor and mineral nutrients for measurable control.

What pH should flowering cannabis water be?

Keep nutrient solution at pH 5.8 to 6.2 in coco, rockwool, and hydro, and 6.0 to 6.8 in soil. Phosphorus and potassium lock out fast outside that window, so correct pH does more than extra booster. Calibrate your meter each week, because a drifting probe causes the same symptoms as a deficiency.

Common flowering feeding mistakes

• Pushing bloom boosters too early, which stalls stretch and stresses young pistils.
• Cutting nitrogen to zero, which yellows leaves before buds finish.
• Ignoring runoff EC, which climbs as salts accumulate in the medium.
• Flushing too long, which starves plants during the final swell.

FAQ

Can you overfeed cannabis in flowering?

Yes. Excess salts cause tip burn, clawing, dark waxy leaves, and stalled bud growth. Flush with pH-corrected water and resume at a lower EC.

Do bloom boosters really work?

They help only when the base feed is balanced and the plant is healthy. Extra P and K without enough calcium, magnesium, or light gives no yield gain.

Should you flush before harvest?

A 7 to 14 day water-only finish lowers salt in the medium and is standard practice, though research on final taste remains mixed.