When plant leaves turn yellow or white or get brown at the edges in summer, it’s likely a sign the amount of water they’re getting is not keeping up with the heat and sunlight they endure.
The condition is called leaf scorch. “It can happen in hot spells, which we’re certainly having this summer,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum. Leaves need to be full of water for plants to cool themselves in hot weather and avoid drying out in the sun.
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Plants can easily be scorched if the conditions have changed so they are getting much more sunlight than they can handle. For example, if you have a tree trimmed, the new gaps may suddenly admit too much sun for shade-tolerant species such as hostas.
Young trees, which don’t have enough of a root system to store much water, are easily scorched. A tree that lacks sufficient water will first show scorched leaves on the top and the sunny side.
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“The only preventive for leaf scorch is to make sure plants consistently have enough water,” Yiesla said. That means watering deeply, until water soaks several inches into the soil.
The best way to check if the soil is well moistened is to dig a small hole and touch the soil 2 to 3 inches down to see if it feels moist. The soil should feel just moist, not sopping wet. “Soil that is too wet can cause rot and actually prevent the plant from absorbing water,” Yiesla said.
Check the soil often to make sure the plants have a steady supply of moisture for their roots. “You see the damage in the leaves, but it’s the roots that need to absorb the water,” she said. A layer of mulch will prevent moisture from evaporating and keep the soil around the roots moist longer.
Generally, leaf scorch will affect the leaves on the top and outer edges of the plant. To improve the plant’s appearance, you can remove those leaves down at the base of the plant or at a main stem. “Just make sure you have watered deeply first, so that the newer green leaves you expose to sunlight have enough water to resist getting scorched themselves,” Yiesla said. Don’t fertilize an affected plant; that would stimulate new green growth that would be extra vulnerable to scorching.
Leaf scorch is not the only possible reason for browned leaves. It can also be due to bacterial or fungal infections. Herbicides and fungicide sprays drifting onto plants can cause leaves to brown. Too much fertilizer can dry out plants, since chemical fertilizers are basically salts.
Lawn grass that is not regularly watered usually gets a brown cast in July and August as individual leaves turn brown. “That’s a sign of dormancy, which is part of the grasses’ natural life cycle,” she said. “They only stay green if we prevent the natural dormancy by watering.” The grass will green up when cooler, wetter weather returns in fall.
If you’re worried about browning leaves, experts and resources such as the Plant Clinic can help you identify the cause. But first, water. “Whatever the problem is, a sufficient supply of water in the soil will help plants resist stress,” Yiesla said.
When a large area of plants is scorched, such as that bed of shade-loving plants exposed to sun by tree trimming, you may need to plan on a lot of transplanting this fall.
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“Every kind of plant has its own sun tolerance,” she said. “If you chose plants that could tolerate the shade of a tree and the shade is gone, they will need a new patch of shade.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
