Operating a greenhouse means managing a controlled environment where temperature, humidity, light, airflow, and irrigation work together to keep plants healthy year-round. The basics aren’t complicated, but each system affects the others, so understanding how they interact is what separates a productive greenhouse from a frustrating one.
Choosing the Right Glazing
Your greenhouse covering determines how much light reaches your plants and how well heat stays trapped inside. Single-layer glass transmits 88% to 94% of light and lasts over 25 years, making it the gold standard for permanent structures. Single-wall polycarbonate panels match glass at about 94% transmission, while the more common double-wall polycarbonate panels drop to around 83% but offer far better insulation. These panels resist UV degradation for at least 10 years and typically last 20 to 25 years.
Polyethylene film is the budget option. A single layer transmits 85% to 87% of light, and a double layer drops to 74% to 77%. Standard polyethylene breaks down in two to three years. UV-treated versions stretch that to three to five years, but you’re still replacing film regularly. If you’re starting out or running a seasonal hoop house, polyethylene keeps costs low. For a year-round operation, polycarbonate or glass pays for itself in durability and light consistency.
Temperature Control: Heating and Cooling
Most greenhouse crops thrive between 65°F and 80°F during the day and 55°F to 65°F at night, though this varies by species. Keeping temperatures in that range requires active cooling in summer and heating in winter.
The most common cooling setup is the pad-and-fan system: water trickles over cellulose pads on one end of the greenhouse while exhaust fans on the opposite end pull air through. New pads operate at roughly 71% cooling efficiency, reducing air temperature by 8°C to 17°C (about 14°F to 31°F) depending on outside humidity. In dry climates, this works extremely well. In humid regions, the effect is smaller because evaporation slows when the air already holds moisture. Shade cloth (30% to 50% shade rating for most crops) can supplement evaporative cooling on the hottest days.
For heating, natural gas or propane unit heaters are the most common choice. Radiant heat systems warm plants and soil directly rather than heating the air, which can be more efficient in larger houses. Whatever your heat source, the real savings come from insulation, and thermal curtains are the single most effective upgrade. Closing a thermal curtain at night reduces heat loss by about 21% on average, with savings ranging from 14% to 26% depending on conditions. That translates directly to a 21% reduction in fuel cost and CO₂ emissions.
Ventilation and Airflow
Plants need fresh air for CO₂ replenishment, and stagnant air breeds fungal diseases. Greenhouses use two types of ventilation: natural (roof vents and side vents that open to let hot air escape) and mechanical (exhaust fans paired with intake shutters). Most operations use both.
A good target is at least five full air exchanges per hour during active growing periods, though in summer you may need considerably more to manage heat. Horizontal air flow fans, the small circular fans mounted overhead, keep air moving between plants even when the exhaust fans aren’t running. This prevents cold or humid pockets from forming and strengthens stems by providing gentle, consistent air movement. Position them so they create a circular airflow pattern throughout the house.
Getting Light Levels Right
Light is measured in daily light integral, or DLI: the total amount of usable light a plant receives over 24 hours. Shade-tolerant plants like ferns and some bedding plants do well at 5 to 10 mol/m²/day. Most full-sun crops, including tomatoes, peppers, and flowering annuals, need 15 to 25 mol/m²/day for high-quality growth.
The challenge is that greenhouses cut incoming light significantly. In winter, a structure that transmits only 40% of outdoor light (common with older or double-layer glazing) can leave crops far below their minimum. In a location like Seattle in January, where outdoor DLI is already just 5 to 10 mol/m²/day, a 40% transmission greenhouse delivers only 2 to 4 mol/m²/day. That’s below the survival threshold for most crops, let alone their optimum. Supplemental lighting, either high-pressure sodium or LED fixtures, bridges the gap. LEDs have become the standard for new installations because they run cooler, use less electricity, and can deliver up to 40 to 50 mol/m²/day in fully indoor setups. For most greenhouse growers, the goal is more modest: adding 1 to 5 mol/m²/day during winter months to keep plants growing steadily.
Managing Humidity and Preventing Disease
Humidity that’s too high invites mold, botrytis, and powdery mildew. Too low, and plants close their pores and stop growing. The most useful way to think about greenhouse humidity is vapor pressure deficit (VPD), which accounts for both temperature and moisture content. For rooting cuttings and newly propagated plants, aim for a low VPD around 0.3 kPa, which means warm, humid conditions. Once plants establish roots, a range of 0.3 to 0.7 kPa works well for young growth. Mature vegetable crops generally perform best at 0.8 to 1.2 kPa.
When VPD climbs too high (air too dry relative to temperature), plants lose water through their leaves faster than roots can replace it. Stomata close, photosynthesis stalls, and growth stops. When VPD drops too low (air nearly saturated), moisture sits on leaf surfaces and fungal problems explode. Your two main tools for raising VPD are ventilation and heat, both of which reduce relative humidity. To lower VPD, you can mist or fog, though this must be done carefully to avoid wetting foliage.
Irrigation Systems
The two most popular greenhouse irrigation methods are drip irrigation and ebb-and-flow (flood and drain), and each fits different situations.
- Drip irrigation delivers water directly to each plant’s root zone through individual emitters. Water waste is minimal because very little hits the floor or evaporates. The tradeoff is labor: every pot needs a dripper inserted and removed manually, and the system requires a good pump and filtration setup to prevent clogging. Drip works well for large containers, hanging baskets, or crops spaced irregularly.
- Ebb-and-flow floods a bench or floor with a shallow layer of water, lets plants absorb it from below for about 20 minutes, then drains it for reuse. It requires more infrastructure upfront (waterproof benches or specially graded floors with collection channels) but almost no ongoing labor. Automation is straightforward since you’re not connecting individual drippers. Some water evaporates during the flood cycle, but the loss is small because exposure time is brief.
Whichever system you choose, consistency matters more than volume. Overwatering is the most common mistake new greenhouse growers make. Water when the top inch of media feels dry, or use a moisture meter until you develop a feel for your crops’ needs.
Growing Media and Fertility
Most greenhouse growers use soilless media (peat-based mixes, coco coir, perlite blends) rather than field soil, which compacts in containers, drains poorly, and often carries pathogens. Soilless mixes are lightweight, drain freely, and give you precise control over nutrition.
Two numbers tell you whether your growing media is in good shape: pH and electrical conductivity (EC). For most vegetables and ornamentals, keep pH between 5.5 and 6.5. Outside that range, certain nutrients become locked up and unavailable to roots even when they’re present in the mix. EC measures the total dissolved salts, essentially how much fertilizer is available. A desirable EC range for general crops is 750 to 3,500 micromhos per centimeter using a saturated media extract test. Low EC means plants are hungry. High EC means salts are accumulating and can burn roots.
Test your media every two to four weeks by collecting samples from several pots, mixing them, and either running a pour-through test yourself or sending them to a lab. Adjusting pH is easier to prevent than to fix: choose a fertilizer with the right acid or base reaction for your water source, and monitor regularly rather than correcting after problems appear.
Pest and Disease Scouting
Greenhouses are warm, humid, and full of tender growth, which is paradise for pests. The key to keeping them under control is catching problems early through weekly scouting. Walk through the greenhouse and inspect plants systematically, paying extra attention to “indicator plants” (species that tend to attract pests first, like petunias for thrips or yellow chrysanthemums for aphids). Flip leaves over. Look at stems near the soil line. Record what you find, including the pest’s life stage (eggs, larvae, adults), because that determines your response.
Yellow and blue sticky cards placed throughout the greenhouse are your early warning system. Yellow cards attract whiteflies, fungus gnats, and winged aphids. Blue cards catch thrips. Check and replace them weekly, and keep a log so you can spot population trends before you see damage on plants.
When pests do appear, biological control often works better than spraying in an enclosed environment. Parasitic wasps in the genus Aphidius target aphids. Minute pirate bugs feed on thrips. Beneficial nematodes applied as a soil drench kill fungus gnat larvae. These natural enemies can be purchased from commercial insectaries and released directly into the greenhouse. One important detail: remove yellow sticky cards before releasing parasitoids, since the cards will trap your beneficial insects. You can put them back a week after the release.
For situations that require stronger intervention, plant-derived oils like neem oil and products containing garlic, clove, or cottonseed oil are greenhouse-compatible options that break down quickly and pose less risk to beneficial insects than synthetic pesticides.
Winter Operations
Winter is when operating costs spike and management gets trickiest. Shorter days mean less light and less passive solar heating, so your heating system runs harder while your plants grow slower.
Thermal curtains, deployed at sunset and retracted at sunrise, are the single best investment for winter efficiency. Beyond the 21% average energy savings, they also reduce temperature swings at plant level, which lowers stress on cold-sensitive crops. If your greenhouse lacks curtains, bubble wrap or twin-wall polycarbonate panels fitted to the interior frame as a temporary liner provide modest insulation, though they block more light than a proper curtain system.
Reduce watering frequency in winter because plants transpire less and media stays wet longer. Overwatering in cold conditions is a fast track to root rot. Ventilate briefly even on cold days, cracking vents for 15 to 30 minutes during the warmest part of the afternoon to exchange humid air for drier outside air. This brief burst of ventilation costs less heat than you might expect and significantly reduces condensation and disease pressure overnight.