Zinnias benefit from supplemental nutrients as seedlings and as they begin to flower, which can help produce more prolific and larger blooms. Coffee grounds are one example that often comes to mind. For zinnias, coffee grounds can help, but perhaps not in the ways you might think. After brewing, coffee grounds have a pH of about 6.5 to 6.8 — nearly neutral — not enough to support plants that do well in even slightly acidic soil like zinnias. If you want to use coffee grounds to support your zinnia, you will need to take a few steps first.