Quick answer: Clean greenhouse glass in autumn and late winter, working top-down: brush off debris dry, wash with warm water and a splash of white vinegar (kind to plants, kills algae), and use a plastic plant label — never a blade — to clear the green line where panes overlap. Empty staging away from the walls before you start the inside. For the roof, use a soft brush on a pole or a suction robot rather than leaning a ladder against the frame — greenhouse aluminium bends easily.
Gardeners obsess over feed and compost, then leave their plants behind glass that's lost a chunk of its light. In a British winter, that's the difference between seedlings that stretch and sulk and seedlings that get going.
Dirty glass is a light tax your plants pay daily
Horticultural guidance has long held that grimy greenhouse glass can cut light transmission by around 10% or more — precisely when winter light is already scarce. Algae films are the worst offenders because they build evenly and green, so you stop noticing them. The classic tell: a green tinge along the north-facing side and dark lines where panes overlap.
When to clean (the gardener's calendar)
The two cleans that matter are late autumn (maximise winter light) and late winter (before sowing season). A summer clean is cosmetic; the autumn one is the one your overwintering plants actually feel. Pick a mild, overcast day — glass in full sun dries too fast and streaks (here's the chemistry of why).
Outside: top-down, and mind the frame
Brush off leaves and cobwebs dry first. Then warm water with a good splash of white vinegar — it dissolves algae and mineral film and won't hurt the beds below like detergent runoff can. Work top-down so you're never re-dirtying finished panes.
Never lean a ladder against greenhouse glazing bars — aluminium frames aren't built for point loads. For the roof, use a soft brush on a telescopic pole from the ground, or put a suction robot on the panes: the CleanBot R1 grips glass at any angle and maps each pane edge to edge, which turns the sketchiest part of the job into a supervised one. One honest caveat: the robot needs panes larger than its own footprint — big modern panes and long roof runs are fine; Victorian greenhouses with small overlapping panes are pole territory.
The overlap line (the bit everyone skips)
Where panes overlap, a green algae stripe forms between the sheets by capillary action. Slide a flexible plastic plant label into the overlap and run it along — the algae comes out as a satisfying green ribbon. Follow with a squirt of vinegar solution. Never use a metal blade: one scratch inside the overlap and the stripe recolonises faster next year.
Inside: do it before spring crowds the benches
Empty what you can, drop the staging away from the walls, and wash inside faces with the same vinegar mix. Condensation feeds inside algae, so this is also your annual mould patrol — check the glazing clips and cills. Good airflow after (vents open on a dry day) finishes the job. If your greenhouse doubles as a conservatory-style lean-to, the conservatory roof guide covers the sloped-glass technique in more detail.
FAQ
Will vinegar harm my plants?
At washing dilution, splash and runoff are harmless once rinsed. Avoid spraying directly onto foliage, and rinse staging before standing seed trays back on it.
What about shading paint residue?
Coolglass-type shading washes off with a dry cloth once autumn humidity rises — do this before your wet clean, not during it.
Polycarbonate panels too?
Yes, but gently: polycarbonate scratches far more easily than glass. Soft cloth, plenty of water, no vinegar on the aged panels — and suction robots are for glass only.
If the greenhouse is one of several glass jobs you keep postponing — conservatory, skylights, upstairs windows — the maths on automating all of them at once is worth ten minutes of your time.