Codes:
International Code of Nomenclature for Cultivated Plants, Eighth Edition – ICNCP
International Cultivar Registration Authorities - ICRA: Search for ICRA by genus
Web based
Free access - F.
Paid member access only – P
ICRA
Non- ICRA, but helpful
ICRA by genus:
Begonia: The American Begonia Society (ABS)
Camelia: International Camellia Society (ICS)
Dahlia: Royal Horticultural Society (RHS)
F - RHS
F - Royal General Bulbgrowers' Association (KAVB, see other search engine below)
F – National Dahlia Society-UK(NDS) Classification
Heliconia: Heliconia Society International
Iris (excluding bulbous species): The American Iris Society
P - Check List
F - Species Iris Group of North America (SIGNA)
Bulbous species – KAVB
Brugmansia, Datura: International Brugmansia and Datura Society
Hosta: American Hosta Society
Multi-Genus ICRA Links:
Aroids ISHS-Genus covered International Aroid Society (IAS), of limited use.
Bromeliads ISHS-Genus covered Bromeliad Society International
F - Bromeliad Cultivar Register
F - Florida Council of Bromeliad Societies/ Bromeliad Databases
Carnivorous Plants ISHS-Genus covered International Carnivorous Plant Society
F - Carnivorous Plant Database
Gesneriads (except Saintpaulia, ISHS-Genus covered The Gesneriad Society
Bulbs, tulipa and many more ISHS-Genus covered) Royal General Bulbgrowers' Association (KAVB):
F - KAVB Search engine alphabetical
F - KAVB Search engine fill-in
Unassigned Woody Ornamentals ISHS-Genus covered: American Public Gardens Association (APGA), Brooklyn Botanical Garden
F - Cultivar Registration (limited)
Australian genera ISHS-Genus covered Australian Cultivar Registration Authority Inc. (ACRA)
F - List of Registered Cultivars derived from Australian native flora
Some additional helpful links:
International Society for Horticultural Science, ICRA Directory
American Public Gardens Association