A variety of plastic covers can be used to cover propagation frames or individual containers to maintain humidity.
Polythene bags can be placed over trays or pots or supported on three or four split bamboo canes or wire hoops inserted into the compost and kept in place with string or elastic bands.
Alternatively, cut the bottom off a 1-litre or 2-litre plastic bottle, which can then be placed over small pots; a 2-litre bottle fits perfectly over a 9cm (31/2in) pot.