Oceanic Islands are host to high levels of species confined to specific regions, termed as endemic. This is true in the Hawaiian Islands, where over 90% of its 1389 flowering plants are endemic. Unfortunately, Oceanic Islands are also host to high levels of extinction as invasive species have infiltrated natural regions. In Hawaiʻi, 134 endemic plants are considered extinct or extinct in the wild. In Chapter I, preliminary IUCN Red List assessment of the native vascular plants of Hawaiʻi showed that 72% (753 taxa) are threatened. The flora of Hawaiʻi may therefore be one of the world's most threatened, highlighting the urgent need for increased and novel conservation measures in this diverse archipelago.

Of these endemic Hawaiian species, three Hawaiian Gardenia spp., known locally as Nānū or Nāʻū, are all designated as Critically Endangered with fewer than 100 mature trees of each species remaining in the wild and no recent observations of natural regeneration. Hawaiian Gardenia are recognized as having important cultural and ecological significance throughout the archipelago and stand as an important part of the biocultural heritage of this space. Remaining wild Gardenia trees continue to decline while wild habitat struggles to cope with degradation and herbivory by introduced invasive ungulates, rats and slugs. Competition by non native plants as well as damage by storms, landslides, rock falls, erosion, and climate change also threaten remaining habitat.

Understanding more about how to properly cultivate the species is paramount, as conservation facilities like botanical gardens ensure genetics are safeguarded ex situ. Chapter II provides a literature review of Gardenia ethnobotany in the Hawaiian Islands. This sets the stage for historical pretext and appreciating the plant’s heritage and significance to Hawaiʻi while also making recommendations to encourage co-management with lineal descendants and cultural practitioners in order to not just maintain the plants physically, but also reestablish long held cultural use of these taxa. This work echoes the need for plants to be available for not only reversing the tide of extinction, but continued cultural use, which supports more public effort to cultivate, understand, and appreciate these species.

Chapter III investigates the genetic structure of Gardenia remyi, the only remaining Hawaiian Gardenia species that occurs on multiple islands. Understanding genetic diversity and connectivity between wild populations are foundational to conservation management plans, however, little is known of the population structure of the species. To guide ex situ conservation, breeding strategies, and mitigating inbreeding depression, we utilized ddRADSeq on both historical herbarium specimens and samples from living ex situ collections. Phylogenetic and population structure analysis revealed a monophyletic ingroup and a clear division between G. remyi samples of the northern island of Kauaʻi and those from the more southeastern younger islands of Molokaʻi, Maui, and Hawaiʻi islands. The Kauaʻi samples were further split into a subpopulation from Southern Kauaʻi and subpopulations of Northern Kauaʻi.

Conservation horticulturalists have long attempted to collect, cultivate, and reintroduce Hawaiian Gardenia, with little success, particularly with Gardenia remyi and Gardenia mannii. In Chapter IV, we review restoration of G. remyi and other threatened plant species in Limahuli Valley on the Hawaiian Island of Kauaʻi in the framework of the Global Tree Assessment and find that only 8% of 581 outplanted individuals of G. remyi had survived, whereas other targeted species had much higher survival rates.

Horticulturalists have expressed that fresh Gardenia seeds germinate readily. However, they continue to struggle to cultivate Hawaiian Gardenia to a point that consistently reaches reproductive maturity. A potential aspect to maximizing plant health and resiliency are plantfungal relationships. It is estimated that 90% of native Hawaiian plants have some level of relationship with mycorrhizal fungi, though it is unknown in many cases to what extent it supports plant health. Therefore, in Chapter V, we investigate the efficacy of indigenous soil fungi from Kalalau valley on plant health of Gardenia remyi. Soil collections from the base of G. remyi in Kalalau Valley, Kauaʻi were used to initiate a trap culture process to increase fungal abundance prior to inoculating young plants. While the process to collect soil from the wild, quantify spore slurries, and inoculate them to plants was successful, the lack of quality seedlings proved to be a central challenge. Only one plant survived after two months, which was inoculated with mycorrhizal fungi. Fortunately, roots extracted from this plant allowed us to confirm the fungi had colonized them, confirming that G. remyi is able to resource nutrients through mycorrhizal symbiosis. The experiment will be expanded in 2024 as new seedlings have been sourced from wild plants, and additional soil collected. We expect that based on previous studies, and our initial findings, that mycorrhizal fungi will prove to be an important part of G. remyi health ex situ, as well as when planted back into habitat.

The Hawaiʻi Strategy for Plant Conservation, modeled after the Global Strategy for Plant Conservation highlights the need to build ex situ collections either as germplasm repositories for rare taxa, or as shorter-term storage until habitats are suitable for restoration and recovery efforts. To date only a few gardens hold small collections of Hawaiian Gardenia and wild seeds are hard to find. This raises the question about how long Gardenia seeds last in storage, if they experience dormancy, and the conditions to maximize their germinability. Therefore, in Chapter VI we investigated Gardenia seed physiology, with emphasis on rare Hawaiian species. Part of this effort was a look into what germination data for Gardenia taxa globally exists. Of the 128 recognized species of Gardenia J. Ellis, only 40% are reported in ex situ facilities, and 66% have not been assessed for the IUCN Red List, giving a window into the lack of cohesive data that surrounds these important plants. Very little published Gardenia germination data exists for the few species that do have representation, and therefore, we tested 19 accessions of fresh seeds stored for varying lengths of time in the NTBG Conservation Seed Bank and Laboratory. These species included five Pacific species, including two Hawaiian taxa G. remyi and G. brighamii. This study provides the first published data of G. remyi seed germination. Seeds were also found to propagate in the dark, pointing to the unlikely nature of a soil seed bank in habitat. Although all species tested were physiologically dormant, viable accessions were easily propagated from seed at warmer temperatures, giving some hope to the ability to maximize germination of any Gardenia seeds that can be harvested in the future. While in a single remarkable case G. brighamii seed collected in 2001 demonstrated 92% germinability after 22 years, the results of the study confirm the three Hawaiian Gardenia spp. are exceptional species as their seeds may be desiccation tolerant, but their viability will decline to 50% (p50) in <20 years.

In the concluding Chapter VII, we survey 116 botanical gardens and arboreta across 34 U.S. states asking how they define the field of horticultural research, what the current status of horticultural research is, and how it may better contribute to horticultural excellence and conservation horticulture in the future. As conservation needs, changing agricultural paradigms, and climate change impacts continue to increase, we find future research needs are increasingly holistic and focus on species and variety conservation, ecology, and mitigating pests and diseases. Horticultural knowledge is largely practical and shared informally and there is a need for recognition and means of sharing practical research as well as technical research. This study presents a new framework for horticultural research supporting the horticulture community with the conceptualization, communication, and implementation of horticultural research.

Using Hawaiian Gardenia as a model, insights have been gained toward the process of identifying and implementing conservation action and cultivation of rare species alongside the consideration of how horticultural research is conducted as a whole. We exemplify a more holistic approach to plant conservation that integrates multiple disciplines to simultaneously contribute toward its improved cultivation, restoration and cultural practice.