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I am able to grow terrific zinnias. However, the blooms are big and beautiful while the leaves turn brown and look terrible. I have started several inside this year and they are blooming. Again the leaves are dying off. This does not affect the blooms, but it looks bad. Any ideas?
Perhaps some disease or pest on the leaves?
Zinnias can get powdery mildew, spider mites and other things. While the early symptoms of these are not instant brown leaves, ultimately the plant will recycle the nutrients in those leaves that are badly affected, and so the leaves will turn brown.
My current plants are in my home and have never been outside. They have just started blooming and the leaves are already browning. I have over 400 hundred plants started and only the zinnias are browning. My plants I had outside last summer did the same thing. There are no mites or powdery mildew present.
Zinnia generally go though two phases, first is the vegetative growth, where it concentrates all its energy on producing leaves, then a blooming phase where it concentrates all its energy to flower. What the problem may be is that since your blooms are so large the plant is not absorbing enough nutrients from its roots to support them. So what the plant does to compensate is take energy from the lower leaves of the plant that do not get a lot of sunlight and uses it for flowering. You may just need larger pots. A good organic fertilizer may help as well.