How do you harden off leggy seedlings?
Written by
Julia Anderson
Reviewed by
Prof. Martin Thorne, Ph.D.Leggy seedlings should be given special attention during hardening off. This can be due to insufficient light, resulting in elongated, weak stems. These plants require special attention and should be gradually introduced to sunlight. Without wise treatment, the leggy plants may break too readily when transplanted outdoors. I employ specific techniques that I have developed over the course of ten years of experience in starting seedlings.
Stem Support Systems
- Insert bamboo stakes beside stems at initial transplant
- Use soft plant ties for gentle support without constriction
- Buried stems develop adventitious roots for stability
Light Intensity Management
- Begin with 50% shade cloth for first 5 days
- Position under dappled sunlight 2 hours daily
- Increase light exposure by 15 minutes each day
Wind Conditioning
- Use oscillating fans on low setting 30 minutes daily
- Gradually increase duration to 2 hours by day 10
- Avoid natural winds exceeding 3 mph initially
Before hardening them off outdoors, strengthen any leggy seedlings to produce healthy plants. To maintain plants, I use full-spectrum grow lights placed 4 inches above the plants. These plants grow for 5 days under lights that are illuminated for 16 hours each day. This toughens the stems and reduces their tendency to be frail. I initially use a gentle breeze from a fan to stimulate their cellulose production while they are indoors.
Be cautious when transitioning leggy houseplants outside. Begin by placing the plants under trees or under a shade cloth where they get filtered light. For the first three days, expose them to no direct sunlight. After that, you may expose them to morning sunlight, but do so gradually, in increments of about 15 minutes each day. Also, watch for bending of the stems indicating stress, and be sure to support them promptly.
Water management alleviates further stress. It is essential to keep the soil consistently moist, but not overly wet or soggy. Water at the soil level, avoiding the leaves. Dehydrated seedlings will need to be monitored and watered more frequently, as they are taller plants. I check the soil's moisture content twice a day during the hardening process.
Prevent leggy plants in the future with proper indoor care. Provide high-intensity light from the time of germination and keep the sources two to four inches above the plants. Rotate the trays daily to secure uniform exposure of the plant material. Keep the temperature cool, around 65 degrees, to prevent the stem from elongating. These practices tend to lessen the hardening problems.
Read the full article: The Complete Guide to Hardening Off Seedlings
