Jump to a problem
Greenhouses make growing easier, but they also magnify small problems. Before changing three things at once, check the actual temperature, humidity, light, airflow, and root conditions your plants are experiencing.
These are beginner-safe starting points, not a replacement for a crop-specific diagnosis. Make one change, watch the plant for several days, and keep a simple note of what changed.
Common greenhouse problems
Measure the actual minimum at plant height with a min/max thermometer rather than relying on the outdoor forecast. Match each crop to its temperature tolerance, then reduce heat loss with sealed gaps, insulating bubble wrap or a thermal curtain, and protection around the north wall. If warm-season crops need more heat, use a properly sized heater controlled by a thermostat — never leave a heater running unattended without temperature control.
Ventilate in the morning so overnight moisture can leave, and water early enough that leaves and the soil surface are not wet at night. Add gentle air movement with a small fan, and check the real conditions with a hygrometer. Avoid routine misting for established plants; it adds moisture without solving the cause. Keep plants spaced so air can move through the canopy, and vent again whenever the greenhouse warms up.
Leggy seedlings usually mean insufficient light, excess warmth, or both. Move them closer to a bright window or add a supplemental grow light, keep the light close enough to be useful without heating the leaves, and use cooler conditions at night. Give seedlings enough space so they do not shade each other, and add gentle airflow from a fan to encourage sturdier stems. Turn trays regularly so growth stays even.
Isolate affected plants and inspect leaf undersides, new growth, and nearby plants. Rinse pests off with a firm stream of water, or use an appropriate insecticidal soap exactly as its label directs; test a small area first. Hang yellow sticky traps to monitor adult whiteflies, repeat treatment as directed because eggs and new hatchlings remain, and remove heavily infested leaves. Better daily scouting, spacing, and ventilation will make the next outbreak easier to catch early.
Check watering first: saturated roots lack oxygen, while a dry root ball also causes yellowing. Look for root damage, pests, and signs of poor drainage, then check soil pH and nutrition before adding fertilizer. Older lower leaves can naturally yellow as a fruiting plant ages, especially when new growth is healthy. Correct the growing conditions and observe the newest leaves before feeding; extra fertilizer can make the real problem worse.
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