Flowers need watering every day.比较:We need to water flowers every day.sb need to do sth人需要做某事。主语sb是动作do的执行者sth need doing.某事需要(被)做主语sth是动作do的承受者,含被动意义,所以无需用被动语态。
花需要每天浇水The flowers need/require watering every day.sth needs/requires doing.某物需.....【此结构以主动形式表示被动含义】sth needs/requires doing.=sth needs/requires to be done.