Soldiers in the Philippines are using chain saws and machetes to clear thick mats of water lilies that are clogging the nation’s second longest river, causing floods that submerged more than 30 villages.

Officials said Monday they had cleared about eight hectares out of about 20 hectares of lilies that have caused severe backups in the 320-kilometer-long Rio Grande River on southern Mindanao Island. They said the action has reduced the number of flooded villages from more than 30 to about 15.

A regional military spokesman told the French news agency the growth was about 3 meters thick and so tightly packed in places that the soldiers could walk on top of it.

A government minister in Manila has recommended using explosives to clear the plants, but the spokesman said the military is reluctant to do that for fear of damaging the river’s ecosystem.