What is the purpose of hardening off seedlings?
Written by
Julia Anderson
Reviewed by
Prof. Charles Hartman, Ph.D.Harden off seedlings to protect the plants from shocks to the roots and the leaves from excessive sunlight. If the process is rushed, transplanting can lead to leaf scorch and weak stems, neither of which are good for our plants. I discovered this for myself when I rushed to transplant a zucchini plant and it completely collapsed (the leaves fully withered) in just a few days.
Timeline by Plant Type
- Tomatoes: 10-14 days with incremental sun exposure
- Peppers: 14 days minimum, avoid temps below 55°F
- Kale: 7 days due to cold tolerance down to 25°F
UV Light Acceleration
- 30-minute daily UV exposure thickens leaf cuticles
- Reduces outdoor hardening time by 40% in trials
- Use 385-400nm wavelength bulbs for safe intensity
Sunburn Repair
- Apply 50% shade cloth for 72 hours
- Mist leaves with kelp solution twice daily
- Prune >50% damaged foliage to redirect energy
Cold Shock Revival
- Use seedling heat mats set to 70°F
- Water with 75°F liquid to warm root zones
- Delay re-hardening for 7 days minimum
Monitor progress using a wireless soil thermometer. I spent $25 on a model that lets me know when the temperature drops below a dangerous level. Last spring, my one delayed hardened basil seedlings when they were hit with a late frost because of my thermometer information. I like to gauge my data with a visual check for curled leaves or pale stem color.
Sticking to a work pattern will differentiate success from failure. Be consistent in your morning hardening sessions, protect your plants from the UV peaks between 11 AM and 3 PM, and always be aware of the frost dates for your planting zone. The long, hard work pays off, I have already bundled and frozen 3 times the amount of peppers that I had grown in the greenhouse last year.
Read the full article: The Complete Guide to Harden Off Seedlings
