The answer to seedlings direct sunlight depends on where your plants are in the growing process. Indoor seedlings need 14-16 hours of grow lights placed close overhead. Plants going through hardening need gradual sun exposure. New transplants need shade protection for the first few days after planting.
I learned about sun damage the hard way with lettuce transplants one spring. Put them straight into full afternoon sun without any shade protection at all. Within hours those soft leaves turned papery and scorched white in patches. The protected batch I planted in a shadier spot came through just fine without any burn marks.
Indoor-grown seedlings lack the waxy coating that outdoor plants build on their leaves. They also miss the UV-blocking stuff that forms with regular sun contact. Without these shields, tender leaves burn fast in direct rays. It takes 1-2 weeks of hardening for this protection to form.
Maryland Extension says indoor seedlings need grow lights 1-2 inches above the leaves. Keep those lights on for 14-16 hours daily to mimic long summer days. This strong close light keeps plants compact and sturdy. Raise lights as seedlings grow taller to prevent weak stretchy stems.
During hardening, you increase seedling sun exposure step by step over 7-14 days. Start with an hour in shade on day one. Move plants into morning sun by day four or five. By the end of two weeks, they can handle full sun all day long without burning or wilting badly.
The sunlight for transplanted seedlings matters most in those first few days after planting. Even hardened plants face extra stress from root damage during the move. Roots can't pull enough water to replace what leaves lose in hot sun. A few days of shade lets roots settle before facing full exposure.
Simple shade methods work great for your light requirements after transplanting. Row cover fabric filters harsh rays while letting air and water through. A cardboard box with the bottom cut out gives each plant a shade tent. Plant in late afternoon so roots get a cool night to settle before morning sun arrives.
In my experience, cloudy days make the best transplant weather for most crops. You skip the sunburn worry and plants adjust without the added stress of intense heat. But if sun is what you have, use shade cloth or fabric to filter the light for 2-3 days after planting.
Watch your transplants for signs of sun stress like wilting or pale bleached spots on leaves. Move shade covers back a bit each day as plants adjust to their new home. By day four or five, most plants can handle full sun as long as you keep the soil moist through the hot parts of the day.
Read the full article: When to Transplant Seedlings: Ultimate Guide
