[填空题]Over the last two decades，a new trend in American education has developed．

We are going to describe one way to build a rooftop garden that does not even require soil. Four things are needed for a small rooftop garden. One thing is a roof that can support the weight, another is grass cutting. The third thing is a sheet of plastic in which to spread the cut grass. And the last thing is a box about eight centimeters deep and made out of four pieces of wood.
Once you are sure the roof is good cut and collect some grass. Then lay down the plastic where the garden box will go. The four-sided box can be as long and as wide as needed. Place the box on top of the sheet of plastic. Then fill it with the cut grass. Next add water and walk on the cuttings to press them down.
After about three weeks, the rooftop garden is ready for planting. Put the seeds directly into the wet grass cuttings. This garden is a good place to grow peas, tomatoes, beans, onions and lettuce. If the box is deep enough, potatoes and carrots will