Answer: The most common reasons for yellowing leaves are over watering, poorly draining soil and excessive heat. Impatiens like moist, cool soils and can stop blooming when temperatures get too warm. You can usually encourage healthy new growth by cutting the plants back a bit. I'd prune them back now with the hopes that they will develop healthy new foliage and a second flush of blooms. Petunias can respond the same way when temperatures are too warm or the soil remains too moist. Pinching them back should encourage healthy new growth. You didn't mention whether or not you've fed your plants recently. If not, you can use a foliar feed (such as Miracle Gro) to help jump start new growth. Best wishes with your annuals.
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