if (field1.currentFrame == 1)
        {
            field1.nextFrame();
            infoText.text = "You've watered the crop. Let's wait and see how it turns out!";
            function plantStrawberry():void
            {
                field1.nextFrame();
                if (field1.currentFrame == 5)
                {
                    clearInterval(strawberryInterval);
                }
            }
            var strawberryInterval = setInterval(plantStrawberry,5000);
        }

package  {
    public class Crop {
        public function Crop() {
            // constructor code
        }
    }
}

package  {
    //imports should go here
    import flash.display.MovieClip;
    import flash.events.Event;
    import flash.events.TimerEvent;
    import flash.utils.Timer;

    //lets make this class extend MovieClip - that means it will be a MovieClip in addition to everything else you add below
    public class Crop extends MovieClip {

        //instead of setInterval, use a timer - it's easier to manage and cleanup
        //in class files, variables and functions have access modifiers, that's what the public and private words are about
        //private means only this class can ever use the var/function
        private var timer:Timer;

        public function Crop() {
            //initialize the timer - have it tick every 5 seconds, and repeat 4 times (to move you from frame 1 - 5)
            timer = new Timer(5000, 4);
            //listen for the TIMER event (which is the tick) and call the function 'grow' when the timer ticks
            timer.addEventListener(TimerEvent.TIMER, grow);
        }

        //a function that starts the timer ticking
        public function startGrowing():void {
            timer.start();
        }

        //this function is called every timer tick.
        private function grow(e:Event):void {
            this.nextFrame(); //go to the next frame of your crop
        }
    }
}

field1.startGrowing();  //assuming your instance `field1` is one of the crops that you assigned the base class `Crop` to