Like many gardeners in Minnesota, I've been discovering crispy, browning leaves on my plants—the new hydrangeas I'm testing, winterberries that have been in the ground for years, green beans. Are these scorched plants due to the heatwave we've had in June? Or are they symptoms of disease? And, what should a gardener do about them?
What Do Scorched Plants Look Like?
Plants that have had too much sun suffer from leaf scorch, a condition where the plant can't keep up with its hydration needs and the outer leaves dry out and are burned by the sun. Leaf scorch can also be caused by bacterial infections or by nutrient deficiencies.
Signs of scorched plants include leaf edges that are white or brown and crispy such as those on the hostas below. Other signs of plant stress, such as wilting or leaf curling on tomatoes, may also be present.
You can prevent or reduce the effects of scorched plants through a variety of watering strategies, including watering deeply, watering early in the day if possible, and watering at the plant base rather than on the leaves. Shade clothes and other protective measures to keep sensitive plants out of the sun may also help. If plants are in containers, you can also move them to shadier spots in your garden during hot spells. Check how warm your containers are—you don't want the roots to fry.
Once the damage has happened, it's OK to remove the affected leaves, which should mostly be on the outside of the plant. Or, you can let them fall off naturally (they may be protecting leaves on the inside of the plant!) Make sure you are not watering too much or too little—do the finger test. If the soil around the plant is relatively cool and moist at root level, no water is necessary. Over-watering can also prevent a plant from moving water into its leaves. If the plant is in the ground, consider adding a mulch layer to retain moisture and keep roots cool. Finally, do not fertilize or otherwise stress the plant.
While we still have a bit of warm weather ahead and Minnesota really could use some rain, temperatures are expected to moderate in the next 10 days or so.
What If It's Not Leaf Scorch?
Not all leaf problems can be blamed on the heat at this time of year. For instance, the leaves below have leaf miners—a group of insects that burrow through leaves in the larval stage. I just ignore this and expect that the larva will soon be beneficial insects in the garden. It's unsightly but not a big problem.
Other leaf problems may be powdery mildew or various blights. If you are not sure what is going on with your plant, check out the University of Minnesota "What's Wrong with My Plant" site. This will allow you to look for specific plants and problems. It's filled with pictures, too, so you can make a clear identification of what's going on.
—Mary Lahr Schier
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