When the garlic shoots grow to a certain extent, some of the garlic shoots can pierce the film to grow. And for the garlic shoots that cannot pierce the film, farmers can lay a waterlogged sack on top of the film, dragging it forward, and using the  friction to make garlic shoots pierce the film. 

         So about 90% of the garlic shoots can pierce the film. The remaining garlic shoots that still can't pierce the film can be pierced hand to hand.