Your poem is lovely — just a couple of tiny grammar touches to make it tidy. Here is a neat version:
During the dry months, the grasses turn yellow. But once the rains fall, the grasslands turn green.
Step-by-step (easy words):
- Start with a capital letter: "During" and "But" should start with big letters because they start sentences.
- Use commas after an opening phrase: Put a comma after "During the dry months" and after "But once the rains fall" because these phrases come before the main part of the sentence.
- End with a period: Each sentence should finish with a full stop (a little dot) so it feels finished.
Another poem-style way (keep each idea on its own line) is also fine:
During the dry months, the grasses turn yellow. But once the rains fall, the grasslands turn green.
Both versions are correct. Great job — your writing paints a clear picture!