Hello, and welcome. I am a licensed attorney and happy to assist.
The provision can have one of two meanings, depending on the context and other provisions in the document, specifically the definition of "native trees." The phrase means either:
1) Flowers can be planted where trees already exist; or
2) Flowers can be planted where specific trees, in existence prior to property development, exist.
If the term "native trees" is elsewhere defined, that definition would govern.
If the term is not otherwise defined, it is open for interpretation to either of the two definitions listed above, but most likely would be the first.
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