The annuals in my containers seem to be struggling, some with the foliage of the plants taking on an off-green color and even looking a bit yellow. The containers are in full sun, with sun-loving annuals on my back patio. What can I do to help my plants and get my color display back for the rest of the summer?
― Alma Rodriguez, Lake Forest
I will assume that you used a high-quality growing medium for your containers and did not use garden soil. Garden soils will not drain properly, even if they are amended with compost when used in containers. A lightweight soilless growing medium purchased at a garden center needs to be used in containers to ensure proper drainage and aeration for the annuals to grow properly. You may find some bags labeled as potting soil that are quite heavy to lift, I avoid using those products for container gardening. Your containers also need drainage holes in the bottom or water can back up in the pots and cause your plants to stop growing with off-color foliage. Eventually, the plants will die from too much water in this situation.
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It is likely that your containers just need to be fertilized. The more frequent watering of containers with established annuals during the hot summer days will leach nutrients out of the medium resulting in “hungry” plants that will have reduced growth and off-color and yellowing foliage like you are describing. Use a liquid fertilizer to give your containers a boost. Fertilizing once every three to four weeks should be adequate to bring your plants around. Over-fertilizing can result in large green plants with reduced flowering. Fertilize when the growing medium is moist to avoid burning your plants. Try adding a granular slow-release fertilizer when you plant your containers next year. These types of fertilizers slowly release nutrients over time and can be beneficial for container gardening. On occasion, I will see a bed of annuals in a garden that is stunted and not flowering well. Overwatering and wet soils, as well as sun annuals planted in too much shade, can cause this too, but if these scenarios can be ruled out, then a liquid feed for annuals growing in a bed can be beneficial too.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.