Day-Night Temperature Swings Cause Leggy Seedlings — and Can Be Fixed

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Leslie Halleck explained a phenomenon called DIF (the differential between daytime and nighttime greenhouse temperature) that many home growers encounter without recognizing it. A large drop in temperature at night — say, 50°F days and 25°F nights — can trigger excessive stem elongation in seedlings, making them tall, weak, and poorly branched. The fix is a technique called negative DIF: raising nighttime temperature slightly (or briefly dropping the morning temperature before sunrise) to narrow the gap, which signals plants to stay shorter and stockier.

Halleck emphasized that plants generally benefit from roughly a 10-degree overnight drop, but a much larger swing can produce the undesirable elongation effect. Host Joe Lamp'l confirmed he solved the problem in subsequent seasons by opening greenhouse doors before sunrise to bring temperatures closer together. The takeaway for anyone starting seedlings in a winter greenhouse: monitor and manage the day-to-night temperature swing, not just the daytime high.