It will be the first time the new "Welsh Rugby Daffodil" is displayed by the RHS. It has taken 15 years to develop the daffodil breed, and has been named 'Undeb Rygbi Cymru' by the RHS.
The flower has been bred by gold medal winning grower Ron Scamp, from Cornwall, and was selected by the RHS to present to the Welsh Rugby Union.
The RHS show director, Stephen Bennett said: "Ron Scamp is one of the world's leading daffodil growers and the 'Welsh Rugby Union' daffodil is a sturdy, stunning specimen with lots of stamina, just perfect for the phenomenal Welsh rugby team.
"We're passionate about plants and the RHS Spring Flower Show in Cardiff and there is huge passion for rugby in Wales so we're sure the flower will be a winning set piece for plant and rugby lovers alike."
Thousands of visitors are expected to flock to the three-day event hosted at Cardiff Castle. The show first returned to the city in 2005 after an 85-year break, making this its fourth visit in a row.
Other highlights include a garden built by inmates at Parc prison and young offenders institution in Bridgend, and an exhibition highlighting research by Cardiff University. The Parc garden has been named 'Choices' in what organisers said was a bid to reflect the steps offenders take along the path towards rehabilitation.
Martin Dykstra, the project's co-ordinator, added: "Gardening is a key fact in the rehabilitation and resettlement of offenders, giving prisoners an opportunity to learn many new and transferable skills."
Cardiff University's research garden is part of the celebrations marking 125 years of the college. Everything on show has a connection with the university, from chilli plants being used in cancer research to DNA helix sculptures representing research into genes and genetic disorders.
"We hope the garden delivers something for people of all ages," commented Andrew Emery from the university.