1: Hicksii is part of a biological name. I think that its full name is Taxus x Media var. hicksii. It is a type of Yew.
2: The 'up' changes the meaning of 'prune'. 'These will prune nicely' means that they will be easy to prune. 'They will prune up nicely' means that if you prune them, they will be easy to train into a nice shape.
3: The whole idea is a realisation that the plants they are talking about are a certain cultivar of yew, and therefore they will be easy to prune/trim into a nice shape.