elaeagnus
Nine letters, four of them vowels, and three of those vowels bunched together in the middle. Elaeagnus is the hardest thing on this site to write down from speech, because the ear hears el-ee-AG-nus and has no way of knowing that the ee is written ae, or that a fourth vowel is hiding behind it.
E-L-A-E-A-G-N-U-S · Elaeagnus angustifolia
Linnaeus built the name out of two Greek words, elaia for the olive and agnos for the chaste tree, and the Greek ai came into Latin as ae. That digraph is one sound in English but two letters on the page, and it sits next to a third vowel, so the middle of the word carries three vowel letters for two vowel sounds.
The oleaster or Russian olive, a small thorny tree from western and central Asia with narrow silver leaves and small scented flowers. It is not an olive and not related to one; the silver leaf is the whole resemblance, and it is what earned the tree its Greek name.