I have tiger lilies and trumpet lilies in my garden. After they bloom, the stem begins to turn brown from the bottom of the plant up to about halfway up to stalk. What could be causing this? The are planted in fairly direct sun for most of the day.
Lily Stem Turning Brown
Certified GKH Gardening Expert
This could be just the natural aging of the plant after blooming. Allow the stems to die down naturally to feed the plant for winter and blooms next year.
If you feel the darkening stem is not this, it could be fungus due to wet or damp conditions. Treat with Neem Oil as this works as both an insecticide and fungicide.
Here are a few links for you.