6th Street, Park Slope. Genus Helleborus was named after the Greek words for “killing” and “food,” since members of the family are often toxic. Also known as Lenten Rose. Much hybridized, popular as a very early bloomer. 
Another batch on Sidney Place, the Block of Perpetual Renovation, in Brooklyn Heights.
The native wildflower False Hellebore, Veratrum viride, a.k.a. Indian Poke, doesn’t look anything like these.
Mine out here in Ohio are blooming also. Great photo.
The forsythia is blooming out here in Kansas City, and the ubiquitous ornamental pears, but that’s it for now. Another two weeks though and watch out!
My daughter lives on 6th Street in Park Slope.
So that’s what brings you to Prospect Park! These were between 4th and 5th Avenues, mid-blockish, north-side.