Posted: 12/10/2015 at 21:42
I use electric tubular heaters in my Robinson greenhouse. Mine is a 2 foot version and the greenhouse is only a small 6ft octagonal one. I only use the heater for keeping the frost off plants during the winter. It can be expensive to run electric heaters for long periods so I have a simple timer that plugs in to a 13amp socket in my shed nearby and set it so that the heater switches on and off for set periods during the nights and off during the daytime when temperatures are usually higher.
During spring when I'm sowing seeds in seed trays I have modified some metal biscuit and chocolate sweets tins by fitting a brass & ceramic bulb holder properly earthed and fitted with a 25watt bulb to each. I can keep seed trays on these which keeps the seedlings just warm enough to thrive. With these I have found there is a need to just crease one side of the tin body in below the lid to allow heat to escape or the filaments of the lamps tend to burn out. The lids need to stay on as seedlings are sensitive to light - particularly tomatoes.