Answer: The first dash should be placed after "Native plants."
Explanation: a dash is is a mark of punctuation used to separate a group of words or a phrase after an independent clause. It sets off information that interrupts the flow of a sentence. In the given sentence, the independent clause is "Native plants have adapted to soil, moisture, and weather conditions for thousands of years" but, with the use of dashes, we can include the definition of native plants: "Native plants -plants that grow naturally in an area- have adapted to soil, moisture, and weather conditions for thousands of years."