Over the course of a growing season, greenhouse glass gradually accumulates a film of dust, algae, and general grime, particularly on the outside where rain and wind carry debris onto the surface. This build-up happens so gradually that it's easy to miss, yet it can noticeably reduce the amount of light reaching plants inside, especially as the days grow shorter towards autumn. Plants positioned in the darker corners of the greenhouse tend to suffer the most from this gradual change.
A simple wash with warm water and a mild detergent is usually enough to shift most everyday grime, applied with a soft brush or sponge to avoid scratching the glass or the glazing bars that hold it in place. Stubborn algae or moss in shaded corners may need a little extra scrubbing to fully remove. Rinse the brush regularly, as algae can quickly clog the bristles and reduce how effective each pass is.
Working safely and thoroughly
Work from a stable position, using a proper ladder or platform for higher panes rather than stretching or balancing awkwardly, since greenhouse roofs and frames are not always designed to take much weight. Clean in sections, rinsing thoroughly as you go, so that dirty water and detergent don't dry into streaks on the glass.
Don't forget the inside of the glass as well as the outside, as condensation and plant debris can leave a surprising amount of residue on interior surfaces over the course of a season. Cleaning both sides properly makes a far bigger difference to overall light levels than tackling just one side alone. It's easy to forget this step entirely, since the inside is less obviously visible from outside.
Clean glass lets considerably more light reach the plants inside, which matters most precisely when daylight naturally begins to fade in early autumn. This is one of those unglamorous jobs that rarely gets noticed when done, but is immediately obvious in the poor growth that follows if it's neglected for too long.
