You can prevent sun damage hardening off by starting your seedlings in full shade and adding sun time in small steps. Begin with zero direct sunlight for the first three days of the process. Then add just one to two hours of morning sun each day after that until your plants can handle a full day outside.
When I first started gardening, I learned about sun damage the hard way through losing plants. I set my tomato seedlings out in full afternoon sun on their first day outside. Within four hours the leaves had white bleached patches all over them. The tan and papery spots spread across most of the foliage by the next morning. Half of those plants never bounced back from that one mistake.
I tested different seedling sunburn prevention methods over the next few seasons to find what works best. The key is starting slower than you think you need to go with new plants. Even morning sun can burn tender seedlings that have never felt real sunlight before. Now I keep all my plants in complete shade for the first 72 hours minimum before any sun hits them.
Your indoor plants burn so fast because they lack the defenses that outdoor plants build over time. The waxy coating on their leaves is thin and cannot block UV rays at all yet. When strong sun hits them, their tiny pores called stomata close up to save water. This stops the plant from making food through normal processes and causes the cells to die from heat and light stress combined.
Michigan State research gives clear numbers for how fast you should increase sun time during hardening. Add just 1 to 2 hours of sun exposure each day during the first week of the process. Use dappled shade under trees instead of direct sun when you first move past full shade. Avoid putting your seedlings in midday sun at all during the first seven days no matter how mild the weather seems.
Physical barriers work well for protecting seedlings from sun when you lack good natural shade spots. Shade cloth at 30% to 50% density lets you control exactly how much light reaches your plants each day. Row cover fabric draped over a frame creates a shelter that blocks the harshest rays. Even a patio umbrella positioned at the right angle can protect your seedlings during peak sun hours in your yard.
Sometimes your plants will get sunburn despite your best efforts to prevent it from happening. Move any damaged plants back to complete shade right away when you spot the problem. Keep the soil moist but not soggy while the plants try to recover from the stress. Cut off any leaves that have turned more than half brown or papery since they drain energy the plant needs.
Slow your hardening schedule way down after any sun damage shows up on your plants. Add two or three extra days at each shade level before you increase sun time again. Your burned plants need extra time to rebuild their leaf coatings before they can handle more light exposure. Most seedlings will recover if you catch the damage early and give them the shade they need to heal.
Read the full article: The Complete Guide to Hardening Off Seedlings
