Looking up Feverfew on the NIH website I found this in their conclusion
” Flowers and leaves and parthenolide showed significant analgesic, anti-inflammatory and antipyretic activities, which confirmed the folk use of feverfew herb for treatment of migraine headache, fever, common cold, and arthritis, and these effects are attributed to leaves and/or flowers mainly due to the presence of sesquiterpene lactones and flavonoids.”
So it seems like you’ll still get a benefit by just using the flowers