The Hampshire County Council-run 73ha garden, donated by Sir Harold Hillier, was created 14 years ago. The expansion is of an extra 4,000sq m - one-seventh of the current size.
Head gardener David Jewell said: "We're making it bigger, adding 40-50 different taxa of plants to encourage more people to come at the quieter time of the year. These include new-on-the-scene bamboo, for example, and Borinda from a specialist grower in Shropshire called Jungle Giants that is only now beginning to filter into the trade. Some of the Borinda have blue stems."
Jewell's team will also plant birch cultivars as well as a block of some 25 witch hazel that will be pruned each year to keep it "compact and floriferous".
He added: "The main focus is to have some big blocks of planting to get more impact - instead of planting four or five dogwoods we'll be planting groups of up to 30. We're working on the principle 'the more simple, the more bold the effect'."
The expansion has been helped by the storms that hit last winter, leading to gardeners having to remove an original shelter belt of Pinus radiata trees in the interests of health and safety.
Jewell said: "We lost a couple of one-offs, a Carya and a rare Rhamnus. We're doing our level best to repropagate them so that we don't lose any material from the collection. We've sent them to a specialist nursery. It was a bit smash and grab after the storms. Some of the propagation is done in house and at times it's sent out to specialist growers because they have the availability of the rootstocks. It's mix and match."
The garden has also secured a sponsorship deal with Rolawn, which is supplying 700sq m of medallion-grade turf for the grass vista running through the newly extended winter garden.