Yes, coffee grounds can be used as an organic fertilizer for Plumeria. Coffee grounds are a good source of nitrogen and some trace minerals. When composted or aged, coffee grounds add organic matter to the soil and can enhance soil structure and moisture retention. Incorporate coffee grounds into the soil or use them as a mulch around the base of the plant. However, it’s essential not to overuse coffee grounds, as they are acidic and may cause the soil pH to drop over time. Regular soil testing can help monitor pH levels and adjust the fertilizer accordingly.