Level III-accredited arboreta have met the following enhanced levels of arboretum standards:
- Satisfy all criteria for Level I and Level II accreditation.
- Substantial arboretum collection with a minimum number of 500 kinds (species or varieties) of trees or woody plants that have been planted and are growing in accordance with the arboretum plan. Plants in the arboretum collection must be labeled in some way as to identify them taxonomically, including scientific name and cultivar if possible, and documented in some way so that information on their acquisition (source or origin, date of acquisition, etc.) is available for access.
- A dedicated curator, or curator-equivalent employee, who is focused on the care and development of the arboretum collection, in accordance with the arboretum plan and collections policy.
- Professional capability to collaborate in some way with other arboreta relevant organizations (e.g. public gardens, universities, local government, NGOs, student groups, etc.) preferably with evidence of existing collaboration. Examples of collaborations may include plant evaluations, research projects, in situ or ex situ conservation projects, educational programs, exhibits, public events, interpretation, collecting expeditions, plant exchanges, professional meetings and co-authoring scientific research papers.
- Sharing of plant collections data with networked collections databases, such as the BGCI Plant Search Database (bgci.org/worldwide/plant_upload).
- An active agenda related to tree science, strategic planting, or conservation. This agenda should include direct research or the facilitation of scientific activities beyond public educational activities, in which data are acquired to solve problems in tree science or tree conservation. Examples include conducting plant trials, habitat monitoring, detecting pests and diseases, hosting collections-based research projects, or conducting research in forest ecology, physiology, systematics, seed and tissue banking, horticulture, or tree care.
- Substantial program of education related to trees, conservation, and other related topics.