1. Understanding Poinsettia Pigmentation
Poinsettias (Euphorbia pulcherrima) turn red due to the production of pigments called anthocyanins in their bracts (modified leaves). This process is triggered by specific environmental cues, primarily photoperiodism (day length) and temperature. The plant requires long, uninterrupted dark periods to initiate color change, typically 12–14 hours of darkness daily for 6–8 weeks. Without this, the bracts remain green.
2. Light Control for Color Induction
To ensure your poinsettia turns red again, replicate its natural light cycle:
- Short-Day Treatment: Starting in early October, provide 14 hours of complete darkness nightly. Even brief light exposure (e.g., streetlights) can disrupt the process.
- Daylight Exposure: During the day, place the plant in bright, indirect sunlight (6–8 hours). Insufficient light weakens pigment production.
- Duration: Maintain this regimen for 6–8 weeks until bracts begin reddening.
3. Temperature Management
Poinsettias thrive at 60–70°F (15–21°C) during the day and slightly cooler at night (55–60°F/13–16°C). Avoid:
- Extreme Heat: Temperatures above 75°F (24°C) delay or prevent color change.
- Cold Drafts: Below 50°F (10°C) causes leaf drop and stunts growth.
Consistent temperatures mimic their native Mexican habitat, optimizing anthocyanin synthesis.
4. Nutrient and Water Requirements
Proper nutrition supports vibrant bracts:
- Fertilization: Use a balanced fertilizer (e.g., 20-20-20) monthly until color induction begins. Then reduce nitrogen to avoid excessive foliage.
- Watering: Keep soil evenly moist but not waterlogged. Overwatering causes root rot, while drought stresses the plant, fading color.
5. Post-Blooming Care for Reblooming
After the holidays, prepare the plant for next year:
- Pruning: Cut stems back to 6–8 inches in March to encourage bushiness.
- Repotting: Refresh soil annually and ensure proper drainage.
- Summer Growth: Move outdoors to partial sun, resume fertilizing, and pinch back tips until late summer to promote branching.
6. Avoiding Common Stressors
Stress inhibits color development:
- Light Pollution: Shield from artificial light during dark periods.
- Pests: Whiteflies and spider mites weaken plants. Treat infestations promptly.
- Transplant Shock: Avoid repotting during the color-induction phase.
