But, he added, customers often do not know the names of the plants they would prefer to buy.
Often, he said, if customers were given a blank piece of paper to list five plants they want to grow, "they would list five that they know rather than five they might buy if you put them in front of them in a garden centre".
Spencer said big breeders tended to be conservative and would only grow what they felt confident would sell. Smaller independent breeders often grow what they like and not what the market wants, he added.
The question is to "figure out who is going to buy a fascinating plant", he said.
Nursery trade show Plantarium in Holland on 23-27 August offers a chance to trial plants which are commercial, said Spencer.
He said the awards at the show were often given to more commercial plants than Chelsea Flower Show or the National Plant Show. Those shows have tight rules around newness of plants while Plantarium just wants plants that are not widely grown. This "relaxed" attitude means plants such as British breeder Peter Moore’s Antirrhinum majus (Pretty in Pink07) is an entry.
Spencer said the US-bred Lavendula Phenomenal was not new but he regarded it, and Syringa meyerii both as good plants.
He added: "Plantarium is not so much a beauty contest. The judging there is more thoughtful and not just things that look pretty on the day, but plants that could be a commercial success."
His Salvia ‘Love & Wishes’ and Euphorbia ‘Ascot rainbow’ won silvers at Plantarium and have been "huge successes".
Spencer said he never expected a mulberry to win Chelsea Flower Show, so judges can spring surprises. The Japanese-bred Morus Russe is available from Suttons and initial supplies quickly ran out thanks to the award.
Spencer will speak on plant breeding for popularity at the National Plant Show on 20 June with Farplants’ Brett Avery and Wyevale Nurseries’ Ben Gregory.