The lid has vents in the top that can be adjusted for good airflow and can be used initially when you sow your seeds if you have a large propagator, once the seedlings start gaining some height then you can swap it over with the high top lid that comes with the large propagator. If you have purchased a Height Extender Set with the intention of using it as an outdoors cloche, this Large Shallow Lid is designed to fit snuggly into place on the top, the ventilation is a welcome bonus, unlike the good ole sheet of glass we often use. Great safe and durable products that you will appreciate in no time at all. As with all of these quality propagation products, it is advisable to wash them in warm, soapy water and disinfect them at the end of each season, dismantle (if applicable) then store neatly to avoid warping in your shed or garage. When well-cared for these propagation items will continue to serve you from season to season.