Answer: Ageratum needs full morning sun with afternoon shade to grow its best. In afternoon only sun the plants may suffer heat stress. The soil should be evenly moist yet well drained, meaning damp like a wrung out sponge. Ageratum is prone to root rots, especially in overly wet or poorly drained soil. Although usually fairly easy to grow, it is potentially subject to infection by several soil borne diseases especially in poorly drained soil -- including Pythium, Rhizoctonia, and Fusarium -- which will persist in the soil and infect new plants. I would suggest you remove and discard the plants (not in the compost pile). Clean your tools with a solution of one part household bleach to nine parts water. Add generous amounts of good quality compost to the soil in that area (to try to encourage beneficial soil life to outcompete the diseases), and rotate your plantings from year to year.
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