Here are some sources for funding for plant conservation work.
Riverbanks Zoo and Botanical Garden have a Conservation Fund that can be used for plant conservation projects.
The National Science Foundation (NSF) funds all types of projects large and small. Of particular interest are: (1) Dimensions in Biodiversity, which is looking for proposals that focus on the intersection of genetic, phylogenetic, and functional dimensions of biodiversity; (2) the Ecosystem Science Cluster, which supports research on natural, managed, and disturbed ecosystems; and (3) the Population and Community Ecology Cluster, which supports research that advances the conceptual or theoretical understanding of population ecology, species interactions and community dynamics in terrestrial, wetland and freshwater habitats.
Grants.gov handles information on a wide variety of grant types.
The Alfred P. Sloane Foundation is a not-for-profit grantmaking institution that supports basic scientific research.
The Stanley Smith Horticultural Trust offers funding to projects involving ornamental horticulture.
The American Public Garden Association (APGA) has a page of funding possibilities.
Science Magazine’s suggestions for funding sources.
The Institute of Museum and Library Services (IMLS) funds projects involving collections, which can include botanical garden and herbarium collections.