阅读理解 Four things are needed to build a small rooftop garden to grow vegetables．One thing is a roof that can support the weight．Another is grass cuttings．The third thing is a sheet of plastic on 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 also grow． It is important to keep the grass wet until the plants begin to grow．When the plants are growing, they will need watering every day, unless there is rain．And they will need some liquid fertilizer．If you can get chicken waste, you can make your own liquid fertilizer．Put the chicken manure in a cloth bag．Then, put the bag in a big container of water．After about one week, the water becomes a good liquid fertilizer． Rooftop gardens need a lot of water．Also, seeds and new plants must be protected from insects and birds． Rooftop gardens are increasingly popular, and not just to grow vegetables．They keep buildings cooler in the sun, so they save energy．They can also extend the useful life of a roof．Rooftop gardens also reduce the runoff of storm water and help clean the air．Plus they add beauty, and give birds and insects in the city a nice place to live． | 
