Late Flower
Late Blooming
noun
A slow bloom is a bloom that happens late. It's a late flower or late blooming. Important to know if you're going to go do 花見.
The reading is 遅い and 咲く put together (the same reading you learned from the 咲 kanji). Watch out for the rendaku.
冬が長引いたため、今年はうちのお庭の桜は遅咲きでした。
The cherry trees in our yard experienced a late bloom this year due to the lingering winter.