Growing Geraniums in a Greenhouse: Temperature, Light, and Care

Geraniums: Greenhouse Color All Year

Geraniums (Pelargoniums) are greenhouse classics—prolific bloomers, diverse forms, and responsive to controlled culture. With a few environmental dials tuned correctly, they flower continuously from winter through summer.

Ideal Climate Parameters

Ventilation: Critical for Geranium Health

• Roof vents + side louvers = convection flow
• Automatic openers prevent human-error overheating
• Circulating fans (HAF) eliminate stagnant pockets—reduces fungal pressure dramatically

Watering & Feeding Regimen

• Water: Morning, at base; allow media surface to dry between. Never overhead late day.
• Misting system: 2–3× daily in heat maintains turgidity without waterlogging
• Fertilizer: Balanced soluble (20-20-20 or 15-15-15) at ½ strength weekly
• Clear water flush: Once weekly prevents salt buildup
• Epsom salts: 1 tbsp/gal monthly prevents Mg deficiency (yellowing between veins)

Pest & Disease Management

Greenhouse exclusion reduces but doesn't eliminate:

• Aphids/whitefly: Yellow sticky traps + insecticidal soap/neem at first sight
• Spider mites: High heat/low humidity trigger; misting + predatory mites (Phytoseiulus)
• Botrytis (gray mold): Deadhead religiously; space plants; airflow; remove infected tissue instantly
• Rust: Resistant varieties; avoid wet foliage; copper spray if needed

Year-Round Bloom Cycle

1. Winter: Maintain 60°F min; supplemental light for bud initiation
2. Spring: Increase feed/water as growth accelerates; take cuttings
3. Summer: Vent aggressively; shade midday; heavy bloom
4. Fall: Reduce feed; propagate for next cycle; clean house thoroughly

Geraniums reward attention with relentless color. A well-run greenhouse makes them almost effortless—stable conditions do 80% of the work.