The class, at Capel Manor College in Regent's Park, London, was organised by RPG chairman Mike Fitt. In Bloom trustee Peter Holman spoke about assessing the contest, while RHS senior show judge Jim Buttress spoke about show judging. Green Flag award scheme manager Paul Todd gave an introduction to the awards.
The 20 apprentices, from Islington, Newham and the Royal Parks, worked on show judging and green space exercises.
St James's Park apprentice Rosemary Gray said: "RPG normally does good stuff. Its members have loads of experience.
"It's hard to imagine judging, because I'm too young, but the RPG says it's OK if you're with an experienced person."
Greenwich Park apprentice Zoe Basham added: "I hope to get some insight into something completely different to what we're used to. Hopefully we'll then try out judging with a senior judge."
Islington apprentice Lee Ellis said: "We do a lot of In Bloom and Green Flag in Islington. If we get an idea of what the judges are looking for, we can do even better."
Newham apprentice Sam Peters added: "I hope to become more knowledgeable about flowers because my main job is on bowling greens and sports marking."
Employers Veolia, Turfsoil, Enterprise, Connaught and Serco, caterer Company of Cooks and grower Merediths backed the day.
Fitt said: "We have had great support from employers. Nothing has been done like this before. I hope to get this enshrined in apprenticeship training."