Tulip Library

I've also seen this called the Floral Library.

From www.nps.gov:

Nearly 100 varieties of tulips can be seen in one location called the tulip library near the north side of the Tidal Basin between the Washington Monument and the Jefferson Memorial. Since it was established in 1969, the tulip library has become a popular place to make comparisons of color and shape between different types of tulips planted in individually numbered beds.