Is it true that only polyethylene film should be used to cover flats of cuttings?
No, any plastic film will do, and so will glass. But polyethylene is strong, easy to handle and can also transmit air but not water vapor. This means that cuttings under it are bathed in vapor yet are also exposed to a small amount of fresh air from the outside. Even with polyethylene you can get mildew on cuttings (though to a lesser extent than with other films). To prevent mildew, be sure to remove the film completely for about fifteen minutes every day. And it is advisable to spray cuttings with a fungicide every week.
We are sorry this answer was not as useful for you as it could have been.
Help us get better!
Will you tell us how we can improve this answer?