Dec, Jan, Feb
If you're overwintering tender plants, choose a mild spell to empty your greenhouse completely. Cover plants or take them into the house, if necessary. Brush down framework and staging, then sweep the floor.
Once everything is outside promise yourself not to return anything that isn't absolutely essential.
Both the inside and outside of your greenhouse panes will need cleaning. Take a bucket and sponge and long handled sponge, thoroughly wiping over every pane with a solution of glass cleaner to soak and scrub off dirt. Use a stable platform or stepladder to reach the glass in high ridges.
Check your gutters for collected leaves and debris, which could filter down into water butts. Brush out the rubbish before it flows into the downpipes.
Empty out water butts and clean out any accumulated dirt in the bottom, then put them back into place.
Wipe down surfaces with scouring sponge and wash flower pots and seed trays with disinfectant before thoroughly rinsing. Return everything outside to its proper place in the greenhouse.