Planted during the summer of 2019, the Hydrangea Garden was established by the Dubuque County Master Gardeners in appreciation of the Arboretum providing its gardens and property as a learning opportunity for the Master Gardener organization.

As of June 2025, there are 54 hydrangeas representing 6 species and 44 cultivars, all chosen to educate Arboretum guests about size, growing requirements, and care needs.

More are added yearly as new introductions are released.