The plant, Adam’s laburnum (+ Laburnocytisus 'Adamii'), is a deciduous tree and a graft-hybrid between laburnum and purple broom.
Janet Cubey, RHS chief of horticultural infomatics, said that the service had gone from strength to strength since it was relaunched for public use last year.
She said: "Now that we have more than 5,000 plants, gardeners have an even better choice and may choose to try plants that they hadn’t considered before."
"We'll continue to add to this diversity as we add more plants to Plant Selector every month."
The site sets out key search criteria including height, soil type, flower colour and plant type.
Users can also perform searches by plant name, and access advice from RHS experts, as well as supplier information from the RHS Plant Finder.