According to Catholic Canon Law, blessed holy objects are not allowed to be thrown into the trash, but must be treated with respect (canon 1171). During the Mass, those long leaves were blessed by the priest, making them become sacramentals, which are objects that help us celebrate the seven Sacraments more closely. Tossing those holy objects into the trash is to disregard their sacred nature and treat them like trash, which is extremely disrespectful to God.
So, "Leafs can't be used for anything. If they can't be thrown away, then what should we do?"
Can leaves that have been blessed be thrown away? | Catholic NewspaperDecoration
For generations, people have been using magical leaves to decorate their homes. You can simply hang them behind images of saints or crosses, or turn them into roses using coconut leaves. The benefit of using leaves as decoration is to help you remember Palm Sunday and the scene of the Lord's Passion that you celebrate during Holy Week. What a good way to always turn your heart to Christ, God the Savior, who saves you from eternal death.
Burn it or bury it
Most sacramental relics, such as sacred images, rosaries or blessed leaves, can be burned or buried for proper disposal. This method of destruction not only respects the sacredness of sacred objects, but also brings them back to earth with their dignity. Anyone can burn or bury leaves, but if you don't have the means to do so (because you don't have land or space to burn), simply bring them into the parish office and leave them there.
Normally, parish priests also remind parishioners to bring blessed leaves from the previous year to church before Ash Sunday so that the church can burn them as ashes for this holiday. That way, the liturgical year is coherently connected and nothing ends up in the trash.