It’s the first really warm day of spring, and you decide to put your seedlings outside so they can soak up some sunlight. You check back in on your plant babies in late afternoon and they look…awful. Wilted. Bleached out. You wonder, what on earth is going on?
Sunburn, sometimes called sunscald or leaf scorch, is exactly how it sounds – when your plants get burned by too much sun. Any plants transitioning from inside to outside light can be affected, especially young transplants. But don’t worry – if sunburn is caught and treated early enough, your plants will certainly live through it.
What does plant sunburn look like?
Plant sunburn looks very similar to sunburn on human skin – except that leaves turn white, not red. The affected foliage will look bleached out and may develop blisters.
Usually, only the top leaves will be discolored – if bottom leaves turn yellow, the plant may be suffering from another deficiency. Lack of water, light or essential nutrients are other common transplant ailments that manifest as off-color foliage.
Brown, crispy leaves are a dead giveaway of plant sunburn. Sunburned plants may wilt and new growth will likely be stunted.
Why are my plants getting sunburned?
Plant sunburn is a sign of stress. Usually, young seedlings are most at risk to get sunburned when they are transplanted out in spring.
The transition from artificial indoor light to sunlight is difficult for seedlings. Even seedlings raised in a greenhouse need time to adapt from filtered UV light to full-spectrum sunlight. Plants raised in these conditions need to be introduced to sunlight in intervals.
No plant is immune to sunburn. Older, established plants can become scorched during a drought or summer heatwave. Even houseplants can get leaf scald, just from being moved from a room with little sunlight to a sunny windowsill.
You can easily avoid the problem of sunburn by being aware of weather conditions and taking a little extra effort to protect the most vulnerable plants in your garden.
5 ways to protect seedlings from sunburn
1. Research growing conditions before planting
You may think you know exactly what your plants need, but don’t make the mistake of forgoing valuable research before planning your garden. All plants need adequate sun, but excess sunlight can still be damaging–even for heat-loving annuals.
Observe where the sun falls during different hours of the day. Take note of which areas get the most sun, and which areas get morning or evening shade. Research which sun-to-shade ratio your different crops need and plant accordingly.
2. Harden off your seedlings before transplanting them outside
You may have heard about the necessity of hardening off seedlings before planting them outside. While an important purpose of this process is to acclimate seedlings to the colder outside temperatures, seedlings also need this time to build up a tolerance to sunlight.
Here is a sample hardening off schedule that I present in another of my articles on transplant shock:
Gradually increase the time they spend outside. Start with one hour in a shady spot, and increase by one or two hours each day over the course of a week. While hardening seedlings off, water them less often. This will provide “good” stress to your plants, enough to make them sturdy and resilient by the time they need to be transplanted.
By gradually introducing tender young plants to the outside temperatures and light levels they will have a much better chance of surviving.
3. Plant out on an overcast day
You may not always have control of when you can transplant, but if you do, pick an overcast day to plant. On a cloudy day with little direct sun, plants can focus on sending energy to their roots instead of sending energy to the leaves. It’s far more important for plants to develop strong roots first and foliage after.
You may also mulch the row with straw or woodchips to protect transplants’ fragile roots. Mulch locks in moisture around the base of your plants, and doubles as a weed barrier. Dark mulch will also absorb some sunlight and divert harmful rays away from your plants.
4. Use a shade cloth to deflect sunlight
Shade cloths are pieces of cloth that cast shade. Also called sunshades, these fabrics are generally made of polyester and differ by the percentage that they block out light.
A 70% shade cloth would block out 70% of sunlight, leaving 30% of the sun’s rays for the plants to photosynthesize.
Polypropylene shade cloths tend to come in larger pieces of material and are perfect to pull over greenhouse plastic. They can be expensive, but are definitely worth the investment!
Knitted shade cloths are significantly cheaper and are better suited for covering raised beds.
To build a sunshade over your plants, bend pieces of wire in hoops over the row. Throw the shade cloth or row cover over the hoops to make a tunnel above your plants. Pull the fabric off the plants in the morning but be sure to cover the plants back up by midday. This setup triples as a sunshade, an insulated barrier from cold nights, and as chemical-free pest control.
If you want a semi-permanent structure for your garden, use a bamboo screen to cast shade exactly where you want it. Install a few t-posts and tie the screen to the posts with zip ties. When you rotate your garden beds next season, pick up the screen and move it to the beds that need more shade.
5. Maintain watering and feeding schedule
Don’t neglect watering or feeding transplants for fear of sunburn. Be sure to deep water seedlings before transplanting out and again after planting. Fertilize with an organic, diluted fertilizer once a week.
When watering, take care to water plants in the morning. Watering midday will cause moist foliage to scald. Watering too late in the evening promotes the growth of fungal diseases.
For the same reasons, apply liquid fertilizer and/or spray for pests in the morning. Young plant foliage and tender new growth are particularly sensitive to the harsh rays of late spring and early summer.
By switching up your watering schedule to morning, you can easily avoid a myriad of issues.
How to revive sunburned plants
If you notice your plants already have sunburn, don’t despair! Sunburn will eventually kill plants, but if you catch it early enough, you can certainly revive them.
- Cover plants with a shade cloth. Protect plants from direct sunlight with some kind of cover for the middle part of the day as soon as you notice sunburn.
- Prune away dead foliage. Unfortunately, sunburned foliage will not recover. Cut off affected leaves so the plant will put its energy into new growth. Be sure to protect the new growth with a sunshade before pruning.
- Water deeply and fertilize with seaweed. Give injured plants a deep soak the morning after show scalding. Wait one week, allowing the plant to recover, then apply a liquid fertilizer containing seaweed to speed up the healing process.
Just like human skin, many plants are sensitive to sunlight. Prevent damage to young seedlings by hardening them off to reduce transplant shock. If you leave your plants to continually scorch, they may eventually die; however, if you catch sunburn early enough, you can protect new growth by using a cloth cover until the plants are acclimated to full sunlight.
Trust me, your seedlings will thank you for going the extra mile to ensure their survival. A bountiful summer harvest begins with healthy spring seedlings.