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Also question is, why is my ageratum turning brown?
Ageratum generally do not have insect or disease problems, although sometimes spider mites can affect the plants, especially in hot, dry weather. On most varieties the old flowerheads turn brown and remain on the plants (some are self-cleaning).
Regarding this, how do you prune a ageratum?
It flowers best in sun, in well-drained soil. Occasional deadheading of the spent blooms will prolong the flowering period. I also prune it back occasionally and pull up any stems that have wandered too far. Tip-pruning in spring is very beneficial in promoting a bushy shape; it can get a bit straggly otherwise.
Use warm water to irrigate the plant for a bounty of blue blooms. You should also deadhead spent blooms as needed to encourage more flowers. Growing and caring for ageratums is simple. Stick with the popular blue blooms of the ageratum, deadhead as needed and enjoy the simple blue flower in your garden this year.