Cannabis Flowering Stage: The 12/12 Flip

What is the Cannabis Flowering Stage?

The flowering stage is when your cannabis plant produces the resinous buds you've been waiting for. After weeks of vegetative growth, changing the light cycle signals to the plant that autumn is approaching—time to reproduce before winter.

During flowering, the plant undergoes dramatic changes:

• Initial Stretch: Rapid vertical growth for 2-3 weeks
• Bud Site Development: White pistils emerge at nodes
• Flower Formation: Calyxes stack to form buds
• Trichome Production: Resin glands develop, producing cannabinoids and terpenes
• Ripening: Pistils darken, trichomes mature

For photoperiod strains, flowering is triggered by the 12/12 light cycle. For autoflowering strains, flowering begins automatically based on age (typically week 3-4), regardless of light schedule.

The 12/12 Light Cycle Explained

To trigger flowering in photoperiod cannabis, switch your timer to 12 hours of light and 12 hours of uninterrupted darkness. This mimics the shorter days of autumn.

Critical rules for the dark period:

• Complete darkness required: Even brief light exposure (a flashlight, indicator LED, light leak through a vent) can interrupt flowering hormones.
• Check for leaks: Turn off lights, close the tent, and wait 10 minutes for your eyes to adjust. Any visible light is too much.
• Use green light if needed: Plants can't "see" green light well, so a green headlamp is safest for inspections during dark periods.

The Flowering Stretch (Weeks 1-3)

After you flip to 12/12, your plant doesn't immediately start making buds. Instead, it enters a rapid vertical growth phase called the "flowering stretch." Plants can double—or even triple—in height during these first 2-3 weeks.

Planning for the stretch:

• Indica strains: Typically stretch 50-100% (double in height)
• Sativa strains: Can stretch 100-200%+ (triple or more)
• Hybrids: Varies based on dominant genetics

This is why we recommend flipping when your plant is at 50% of your maximum desired height. A 2-foot plant at flip could become a 4-foot plant by week 3 of flower.

What to Do During the Stretch

• Continue veg nutrients for the first 1-2 weeks—the plant is still growing vegetatively.
• Final training: You can still do light LST or supercropping to manage height, but avoid topping/FIM.
• Support tall plants: Stakes or trellis netting prevent branches from snapping under bud weight later.

Identifying Male vs Female Plants

Within 1-2 weeks of flipping to 12/12, plants reveal their sex. Since only female plants produce the buds we want, identifying and removing males is critical (unless breeding).

Female Signs

• White hair-like structures called pistils emerge from nodes (where branches meet stem)
• Pistils grow from a small tear-drop shaped structure called the calyx
• Over time, calyxes stack together to form buds

Male Signs

• Small round pollen sacs appear at nodes, looking like clusters of tiny balls or grapes
• No pistils or hairs
• Pollen sacs eventually open and release pollen

Remove males immediately to prevent them from pollinating females. A single male can pollinate an entire grow room, filling your buds with seeds.

Feminized seeds eliminate this guesswork with 99.9% female rates—highly recommended for beginners.

Bloom Nutrients: Phosphorus & Potassium

As the stretch ends and bud formation begins (around week 3), switch from Grow nutrients to Bloom nutrients. The key change is less nitrogen (N) and more phosphorus (P) and potassium (K).

• Phosphorus (P): Critical for flower formation and energy transfer
• Potassium (K): Supports resin production, enzyme activation, and disease resistance
• Nitrogen (N): Still needed but in lower amounts; excess causes leafy, airy buds

Look for bloom formulas with NPK ratios like 1-3-2, 0-3-3, or 2-4-4.

Bloom Boosters

Many growers use "bloom booster" supplements (pure P-K additives) during weeks 4-6 to maximize bud density. Popular examples include:

• General Hydroponics Liquid KoolBloom
• Advanced Nutrients Big Bud
• Fox Farm Tiger Bloom

Use conservatively—more isn't better. Nutrient burn during flowering can stunt bud development.

Week-by-Week Flowering Timeline

Environment: Temperature & Humidity

Environmental control becomes even more critical during flowering. Dense buds are highly susceptible to mold and rot if conditions aren't managed properly.

Temperature

• Day: 65-80°F (18-26°C). Slightly cooler than veg is fine.
• Night: 60-70°F. A 10°F drop from day temps can enhance purple coloring in some strains.
• Avoid: Temps above 85°F cause foxtailing (buds elongate abnormally) and terpene loss.

Humidity

• Early flower: 40-50% RH
• Late flower: 30-40% RH (critical for mold prevention)

Dense buds trap moisture inside, creating perfect conditions for bud rot (Botrytis). Invest in a quality dehumidifier if you're growing in a humid climate.

Airflow

Strong, consistent airflow prevents moisture pockets from forming inside the canopy. Use oscillating fans to keep air moving around all bud sites. CO2 supplementation can boost yields if other factors (light, nutrients) are already optimized.