The plants are believed to be the only deciduous Kalmia cuneata in cultivation in the UK.
Seven came over from North America several years ago with four going to a garden in Scotland and three to Milford. The Scottish ones have since died, however Milford’s have survived and are now on display to the public.
Group plant buyer Tim Jacobs said: "We’ve opened the entrance to the gardens right up – on removing a lot of overgrowing Bamboo we came across the only three Kalmia cuneata in the country – possibly in the UK. It’s a very rare and unusual variety.
"We’re hopeful a local nursery in Hampshire will be able to propagate it for us to enable us to have plants for sale in the next few years."
The centre has also been planting some mature Cornus florida varieties – the other National Plant Collection on show at Milford alongside Kalmia latifolia.