How and when should I clean a greenhouse?

Dirty glass can cut light transmission substantially, and the loss falls exactly in the months when light is scarce. An autumn clean also removes the overwintering sites of whitefly, red spider mite and fungal spores, which is why the same afternoon's work pays twice.

Safety

What you'll need

• Bucket, warm water and a little washing-up liquid or horticultural cleaner, true
• Soft brush and a long-handled brush or squeegee, true
• Thin plastic plant labels for the overlaps, true
• Stiff brush for the frame and floor
• Gloves and old clothes, true

Step by step

1. Do it in autumn, on a mild dry day.October is the classic month — the summer crops are finished, and the winter occupants have not gone in yet. Choose a day mild enough that anything you take outside is not damaged by a few hours in the open.
2. Empty it completely if you can.Everything out onto the lawn or a tarpaulin: plants, pots, staging, canes, trays. You cannot clean around things, and this is also the moment to sort and discard.
3. Brush everything down dry first.Sweep cobwebs, dead leaves and loose soil from the glazing bars, the ridge, the corners and the floor with a soft brush before introducing any water. Wet debris is far harder to shift.
4. Wash the inside glass.Warm water with a little detergent or a proprietary greenhouse cleaner, applied with a soft brush or sponge, then rinsed. Work from the top down so dirty water runs onto glass you have not done yet.
5. Clean the pane overlaps.On a glass greenhouse, the small gaps where panes overlap fill with green algae and block a surprising amount of light. Push a thin plastic plant label through the gap and draw it along, then flush with water. It is tedious and it is the step that makes the biggest visible difference.
6. Wash the outside.A long-handled soft brush from the ground for the roof, and a sponge for the sides. A pressure washer can be used with great care at low pressure and a wide fan from a distance, but it will lift glazing clips and force water into overlaps, so a brush is safer.
7. Clean the frame and the glazing clips.Algae and debris in the aluminium channels hold moisture and pests. A stiff brush and water. Check for missing or slipped clips while you are there and replace them.
8. Clear the gutters and the downpipe.Greenhouse gutters are small and clog easily with moss and leaves. A blocked gutter overflows down the glass and into the base, and if it feeds a water butt you get a butt full of leaf debris.
9. Finish with the floor and the staging.Sweep, then wash the path and staging with a garden disinfectant if there has been disease, or plain water if not. Let everything dry fully before putting plants back.

Tips

• Do a lighter mid-season clean of the outside glass in about February as well. It costs half an hour and gains you light at the point in the year when it matters most.
• Clean the max-min thermometer and reset it while you are at it, and check the vent hinges and the automatic opener at the same time.
• Bag up all the debris rather than composting the greenhouse sweepings — that is where overwintering pests and disease spores are.

Common mistakes

• Skipping the pane overlaps — They are the single biggest source of light loss on an old greenhouse and washing the flat glass does nothing for them.
• Using a pressure washer at full pressure — It lifts glazing clips, drives water into the overlaps where it freezes and cracks the glass, and can break the panes outright.
• Cleaning in spring instead of autumn — A spring clean means the greenhouse spent the darkest six months of the year at reduced light transmission, and the pests overwintered undisturbed.

Keeping on top of it

• Full clean — glass, overlaps, frame, gutters, floor (Annually, in autumn)
• Quick wash of the outside glass (Late winter)
• Clear gutters (Autumn and after heavy leaf fall)

Questions people ask