Multiple research areas within HCI have examined opportunities for the design and evaluation of interactive technologies for the garden. In this paper we present a mapping review of HCI papers on the topic of gardening to map out themes that span across these research areas and to identify gaps and opportunities for future research. A review of 28 papers has highlighted the following themes: discovery in the garden, connecting through gardening, bringing the garden inside, coordinating gardens, sustainability through gardening and learning new gardening skills. Based on the review, we present opportunities for future research to 1) design for specific user groups, including older adults; 2) connect gardeners to the world from their gardens (i.e., technologies for gardeners to share their discoveries with the world); and to 3) explore activities beyond gardening, e.g., playing, relaxing, admiring nature and citizen science.