Year-Round Greenhouse Growing: Extend Your Season Forever

Thermal mass works by absorbing heat when temperatures rise above its own temperature, and releasing it when the air cools below its temperature. The key is sufficient quantity — too little thermal mass and your greenhouse overheats then freezes.

• Water drums — the most effective thermal mass. 200L drums of water painted black can store enormous amounts of heat. Place them along the north wall where they'll get maximum sun. A row of 10 x 200L drums can significantly buffer a medium-sized greenhouse.
• Masonry/stone raised beds — stone or brick beds store heat in the root zone of your plants. Slow but steady effect.
• Dark aggregate in concrete floors — dark-coloured concrete absorbs and re-radiates heat from the floor throughout the night.
• Rule of thumb: You need approximately 20-30 litres of water thermal mass per cubic metre of greenhouse volume for moderate climates, 50-75L for cold climates.

• South-facing glazing at correct angle — in southern Australia, the optimal angle for winter sun penetration is equal to your latitude (~30-38°). This angle maximises winter sun entry while minimising summer overheating. Adjustable angle is ideal.
• Thermal curtains — a simple fabric curtain pulled over the greenhouse at night can reduce heat loss by 50-70%. Use a heavyweight thermal fabric (fleece or reflective bubble wrap). Manual pull is fine; automatic systems use a wax actuator that needs no electricity.
• Compost heating — a compost bin inside or immediately adjacent to the greenhouse provides consistent bottom heat. Hot compost can keep a small greenhouse 5-10°C above ambient. Layer greens and browns and keep it moist.
• Wood chip mulch on floor — 15cm of wood chip mulch acts as insulation over soil while slowly decomposing to add gentle warmth and CO₂ for plants.
• Hoop mulching — for minimal setups, row cover fabric draped over wire hoops creates a micro-climate 2-3°C warmer than outside. Combined with a cold frame, this extends the season by months.

• Shade cloth (30-50% density) — the single most effective summer cooling tool. White or silver shade cloth reflects heat. Install on the outside of the greenhouse, not inside. In southern Australia, 30-40% shade is sufficient for summer; remove it for winter.
• Cross-ventilation — hot air rises and escapes through high vents. Every greenhouse needs ridge vents or high side vents plus low intake vents to create convection airflow. This is the most effective passive cooling mechanism.
• Evaporative cooling (bamboo chiller) — wetting a porous surface (bamboo lattice, hessian) at the intake vent creates evaporative cooling. Effective in hot, dry climates. Requires periodic manual watering.
• Plant deciduous vines on the north wall — a grape vine or kiwi on the north-facing wall provides natural summer shade as leaves grow, then drops them in autumn to let winter sun in.
• Automatic vent openers — wax-piston actuators open vents when temperature rises above ~24°C and close them when it drops. No batteries or electricity needed.

• Succession planting — plant seedlings every 3-4 weeks in autumn for continuous harvest through winter. In a mild greenhouse, you can harvest fresh salad greens continuously.
• Vertical growing — use trellises for climbing beans, peas, and cucumbers to double your productive area. Hanging baskets for herbs and strawberries.
• Heirloom varieties perform better — modern F1 hybrids are bred for controlled-climate commercial growing. Heirloom varieties are generally more cold-tolerant and adapted to variable conditions.
• Wicking beds inside greenhouse — self-watering raised beds reduce daily watering chores and provide consistent moisture. Essential if you're not there every day.