Late Flower
Late Blooming
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.
おそざき
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.