“What’s wrong with my plants this year?”
— Amy Heifitz, Hoffman Estates
The plants in your photo appear to be getting too much sun, which is burning their leaves. Damage to hosta leaves is easy to see with visible whitening and burning of the leaves. Hostas planted in sites with too much sun may show more subtle symptoms with an overall color fading (such as a less vivid blue) and less contrast in the variety of leaves. Some cultivars of hosta have more resistance to the sun. Damage on Japanese forest grass is less pronounced, with browning visible only on some of the leaves. This helps confirm that too much sun is the cause.
Something has likely changed in the area surrounding the plants that increased the amount of sunlight enough to start damaging the plants compared to previous years. Removing a large shade tree can dramatically increase the amount of sunlight reaching a bed and have an immediate impact on plants. An adjacent shade tree that has been in slow decline for several years with declining foliage can also slowly increase the amount of sun in a planting bed. Plants respond more dramatically once the additional sun becomes too much. There have been some very sunny and hot days this year, which may have increased the damage.
Increased shade from growing shade trees over a period of years can also affect adjacent sun-loving plants. The impact is generally gradual as the amount of shade increases from year to year, so gardeners don’t always realize it until plant performance has been significantly affected. Sun loving plants in too much shade have less vigorous growth and decreased flowering. Some shade tree pruning can increase the amount of light, but it’s best not to over-prune a large shade tree.
Installing new plants that prefer more suitable light levels is a good strategy. In most gardens, it is not feasible to increase sunlight sufficiently under large shade trees for sun-loving plants. Replanting shade trees to increase the amount of shade for shade-loving plants works, but it takes time for the trees to get large enough. You may need to switch plants to ones that are more sun tolerant.
Provide supplemental water during dry spells to help reduce stress on plants.
For more plant tips, contact the Plant Information Service at the Chicago Botanic Garden at firstname.lastname@example.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.