In article ,
says...
In my greenhouse I have a "propagating "box = 6ft x 2ft x 6ins lined
with plastic sheet , filled with sand and containing a heating cable
laid 3ins deep.
I stand my seed trays/plant pots of seedlings on the top of the sand -
seems to work quite well.

The question : should I keep the sand damp? If so would it be worth
putting a plastic sheet on the sand in order to retain the dampness of
the sand?



Yes the sand should remain damp at all times or the heat is not able to
disburse and can cause damage to the heating cables
--
Charlie Pridham, Gardening in Cornwall
www.roselandhouse.co.uk
Holders of national collections of Clematis viticella cultivars and
Lapageria rosea