How do you prevent sun damage during hardening off?
Written by
Julia Anderson
Reviewed by
Prof. Martin Thorne, Ph.D.Sun exposure prevention during hardening off requires a strategy of timing and sheltering. Avoid direct exposure to the burning sun during the hours of 10 a.m. and 4 p.m. Especially in the first three critical days, the tender young seedlings are not only losing moisture through transpiration but are also subject to sun scorching. Morning sun has gentler ultraviolet exposure. Therefore, I place plants in east-facing sun locations while the day is still young and safe from harm.
Physical Barriers
- Use 30-40% shade cloth suspended over seedlings
- Position cold frames with adjustable ventilation
- Employ row covers for combined sun and wind protection
Timing Strategies
- Limit exposure to 2-hour morning sessions initially
- Avoid transplanting during high UV index days
- Resume hardening after rain when light intensity doubles
Environmental Monitoring
- Track UV index via weather apps daily
- Check leaves hourly for curling or bleaching
- Measure soil temperature before sun exposure
Don't let cloudy weather fool you. Eighty percent of the UV rays penetrate clouds and can burn tender seedlings that are unprotected from these rays. Always use shade cloth when overcast weather prevails. I found this out when all my tomato leaves burned badly on a cloudy day. Watch the weather reports for signs of unexpected UV rays.
Cautious exposure gradually strengthens the cuticle over time.SUSY time in sunlight should be increased by approximately 15-30 minutes a day beginning on the third day. Watch for leaf responses that signify stress, such as slight curling of the leaves; however. If leaves show damage, move plants back into the shade the next day. Trim the damaged leaves, but delay hardening off for two days.
Soil moisture greatly affects sun tolerance. Water the roots thoroughly before exposing them to the sun and keep the leaves dry. The moisture in the soil buffers the temperature fluctuations at the root level. I water at dawn before plants strike the outside air. Be sure to avoid wetting the foliage, as this can increase the risk of sunburn.
Read the full article: The Complete Guide to Hardening Off Seedlings
