The end-of-season clear-out is the least appealing job in greenhouse growing and among the most valuable. It is not tidiness. It is the point at which you break the continuity that lets pests and diseases carry from one season into the next, and it is the point at which you get back the light a season of algae and dust has taken away.
Both of those matter more under cover than outdoors, and for the same underlying reason: nothing about a greenhouse resets on its own. There is no frost hard enough to clear the pests, no rain to wash the glazing, and no wind to blow the debris away.
The light you get back
Glazing accumulates a film of algae, dust, pollen and mineral deposits from splashing, and it does so gradually enough that nobody notices. Published figures for light loss from dirty greenhouse glazing commonly run around 10 percent and can be considerably more where algae has established in polycarbonate flutes or on a north-facing surface that never dries out.
Ten percent is a large number in context. In the shortest weeks of the year a greenhouse is already delivering less light than most crops want, so 10 percent off the top of a marginal figure is the difference between a crop holding and a crop growing. It is also free to recover, which almost nothing else in greenhouse growing is.
Twin-wall polycarbonate has a specific version of this problem. Where the flutes were left unsealed at the top or, worse, fitted horizontally, water gets inside the channels and grows algae there. That cannot be cleaned, which is why the fitting rules in the glazing guide matter so much.
What overwinters, and where
| Problem | Where it overwinters | What removes it |
|---|---|---|
| Botrytis, grey mould | Dead leaves and stems, spent flowers, old growing medium | Removing every scrap of plant debris from the house |
| Tomato leaf mould | Plant debris and crop residues in the bed | Clearing and disposing of foliage away from the greenhouse |
| Damping off organisms | Reused trays and unwashed pots | Washing trays and pots before storage |
| Whitefly and aphids | Any green plant left in the house, including weeds | A period with no susceptible host plants at all |
| Spider mite | Cracks in the frame, under bench edges, in debris | Physical cleaning of frame joints and bench undersides |
| Slugs and snails | Under trays, pots, benches and anything permanently damp | Removing standing objects from the floor over winter |
| Vine weevil larvae | Container medium left in pots | Emptying and washing containers rather than stacking them full |
| Fungus gnats | Wet medium and old trays | Letting media dry and cleaning trays |
The row worth dwelling on is host continuity. Whitefly and aphids persist in a greenhouse year round because there is always something green for them to live on, including weeds. A deliberate period with no susceptible plants in the house, even a few weeks, interrupts several life cycles at once and is more effective than anything sprayed. The wider context is in greenhouse pest management.
The order to do it in
Work top down, because everything you dislodge lands on what is below it. Doing the floor first means doing it twice.
- Empty the house as far as possible. Everything out onto a tarpaulin outside: plants, trays, pots, tools, bags. You cannot clean around objects and you will find pests under half of them.
- Remove all plant debris. Dead leaves, old stems, spent flowers, roots left in beds, anything on the floor. Take it away from the greenhouse rather than composting it beside the door, and be ruthless: this step does more than every other step combined.
- Brush down the frame and glazing dry. Cobwebs, dust and loose debris come off far more easily before water is involved, and they clog everything once wet.
- Wash the glazing inside and out. Warm water with a little detergent and a soft brush on a pole. Work from the ridge down. A cloth wrapped round a thin batten gets into the frame channels and the corners where algae and spider mite both live.
- Clean the frame joints. Overlaps, channels and the gaps behind glazing clips are where spider mite overwinter. A stiff brush and patience.
- Wash benches, staging and shelving. Undersides included, because that is where slugs and mites shelter.
- Wash trays and pots before storing them. Not after. A stack of dirty trays put away in autumn is a reservoir of damping off organisms waiting for the spring sowing.
- Do the floor last. Rake gravel over, sweep or wash a paved path, and remove anything that has been sitting in one damp spot all season.
- Check and service the hardware. Vent opener cylinders, door latches, fixings, glazing clips and anchor straps. This is the moment they are accessible and the moment before winter, which is when they are needed.
When to do it
Whenever the house is emptiest, which for most growers is between clearing the summer crop and starting winter greens, or in late winter before propagation begins.
Doing it before winter has the stronger argument. The debris left from a summer crop is the specific material that carries botrytis and leaf mould into the following season, and it is sitting in a cool humid structure for months if it stays. Cleaning the glazing before winter also recovers light in the season it matters most.
A late winter clean has one advantage: it puts sparkling glazing in place immediately before the propagation season, and it is a pleasant job on a mild day. The best answer is a thorough clear-out before winter and a quick glazing wash before spring.
The bed reset
Beds need their own version of this, and it is separate from cleaning the structure.
- Cut plants off at the base rather than pulling them, which leaves the bed structure undisturbed and the root channels open. Remove all the top growth from the house.
- Leach the bed deliberately. Under cover nothing is ever rained on, so salts from tap water and fertiliser accumulate. Applying a large volume of water, ideally rainwater, once or twice a year is the artificial substitute. The reasoning is in watering in a greenhouse.
- Top up with compost rather than replacing the fill. An inch or two worked into the surface maintains a bed indefinitely.
- Plan the rotation now rather than in spring. Even alternating a fruiting crop with a leafy one interrupts several soilborne problems, and the constraints are covered in raised beds inside a greenhouse.
- Empty and wash containers rather than stacking them full of old medium, which is where vine weevil larvae spend the winter.
The winter checks
Cleaning is also the natural moment to prepare the structure for the season that damages it. Check the anchor straps are tight, since ground settles and straps loosen. Check every glazing panel is retained, and add a washered screw at the top and bottom of any that rely only on clips. Confirm the door latches positively, because a door that blows open pressurises the house and lifts the roof, as covered in anchoring a greenhouse against wind. And cut a couple of props to length for the ridge in case of heavy snow.
Cleaning and reset equipment
A clean, empty, well-sealed greenhouse in autumn is the best possible starting condition for a winter greens crop, which is the next thing that should go into it. That is covered in winter greens in an unheated greenhouse.