The NGS is holding the festival for the first time, kicking off Visit England's Year of the English Garden. More than 100 English and Welsh gardens are set to take part in the Snowdrop Festival through February and early March.
But the unseasonably warm weather - December was 4.1°C warmer than the long-term UK December average - means snowdrops have been blooming weeks or months earlier than expected.
However NGS chief executive George Plumptre said there are no plans to change the dates of the festival. Most gardens will open for one or two days, and given the long snowdrop flowering season the plants will still be in flower - albeit later in their cycle than normal.
"Our festival is only for the month of February, so while we will probably miss the early ones that will be out in January, we’ll get ones that would normally continue into early March," Plumptre said, adding the NGS is "confident" there will be a spell of cold weather in the next few weeks.
Other gardens have revised their timetables due to the mild weather. At Chelsea Physic Garden on the banks of the Thames, the annual Snowdrop Days were planned for 30 January - 7 February. But with a third of the snowdrops already blooming, the dates have been moved forward to 16-24 January. The National Trust's Anglesey Abbey in Cambridge has also brought forward the start of its Snowdrop Festival to 18 January, as many of its 300-plus varieties are already in flower.