Is artificial light sufficient to skip hardening off?
Written by
Julia Anderson
Reviewed by
Prof. Charles Hartman, Ph.D.Harden off seedlings even for those grown under advanced light. My hydroponic basil grew happily indoors under LED lights, but subsequently fell over within hours when introduced outdoors, indicating that lights alone will not effectively harden your seedlings with wind stress or UV light. Resilient plants require real-world conditioning that no bulb can replicate.
UV Deficiency
- UV-A wavelengths (315-400nm) trigger cuticle thickening
- LED/fluorescent lights lack natural UV-B (280-315nm) stress
- Leaf surfaces remain thin, prone to sunscald
Wind Simulation Gap
- Indoor fans provide 10% of natural wind variability
- Stems grown without breeze snap at 5 mph gusts
- Outdoor hardening doubles xylem cell wall thickness
Incremental Exposure
- Start with 15 minutes morning sun + 45 minutes shade
- Increase daily sun by 10-minute intervals
- Use 30% shade cloth during midday UV peaks
Wind Hardening
- Place seedlings near oscillating fans 2 hours daily
- Graduate to sheltered outdoor spots with mild breeze
- Stake leggy plants to prevent breakage
Use a wireless thermometer/hygrometer to keep tabs on the changes. I purchased a $35 model that tracks noticeable light, temperature, and humidity shifts throughout the changes. It indicated that the basil I had hardened under less than 20% UV light could tolerate 85°F (29°C) sunlight better than the plants grown entirely indoors. Data defeats guesses every time.
Consistency is the key to avoiding setbacks. I lost two squash batches because I did not harden the seedlings on a cloudy day. In my case, I regularly expose the seedlings to drizzle and a 10 mph wind so the stomata can learn to deal with the flux in moisture in the air. After hardening the seedlings in these conditions, I have had a 3 to 1 yield ratio to the same seedlings grown in a greenhouse.
Read the full article: The Complete Guide to Harden Off Seedlings
