People's Trust for Endangered Species, which launched Molewatch a year ago, will draw up a "distribution map" to show areas of high- and low-density mole populations.
The conservation charity based in south London recently extended the September deadline to January 2009, said promotions manager Hannah Stockwell.
"We want more volunteers to help us develop a more detailed picture of this elusive creature's distribution," she said.
"We need baseline figures, which have never been taken before, to make sure they are as common as people think they are. We are not saying they are endangered.
"And we are not trying to preach to greenkeepers or grounds staff. But moles are actually the gardener's friend, eating potentially harmful insect larvae."
They ate yummy pests like leatherjackets, cockchafers and carrot flies, said her charity, while their tunnels helped drain and aerate heavy soils.
"Grass and turf without molehills can be a worrying sign of something more sinister, such as a lack of worms or insects or a more underlying problem with the soil," said Stockwell.