Answer: Like most annuals, zinnias grow best in a rich soil that is evenly moist yet well drained, meaning not soggy. They also need full sun. These plants are susceptible to fungal problems, so allowing for good air circulation and avoiding wetting the leaves especially in the evening can be helpful. Certain varieties are also more resistant to mildew so if this is a problem in your area you may want to plant those. Underwatering can cause brown leaves, as can some fungal problems, so these would be possible causes. Insects may attack a plant that is otherwise unhealthy, too. I hope this helps you trouble shoot.
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