Preparing Your Greenhouse For Autumn And Winter

Start with a thorough clean

September is the ideal month to give your greenhouse a proper going-over while the weather is still on your side. Algae, dust and the residue from summer shading all quietly steal light, and light is the one thing you simply cannot buy back in December. Wash the glass inside and out with warm soapy water and a soft sponge, then squeegee it dry — a splash of white vinegar in the bucket helps shift hard water marks. Don't forget the glazing bars, the gutters and the frame itself, because green film hides in every crevice.

Once the glass is sparkling, clear the floor of plant debris, fallen leaves and spent compost, and scrub your pots and trays. That debris is exactly where slugs, vine weevil larvae and the spores of grey mould like to sit out the winter.

• Replace any cracked or loose panes before the first gale.
• Check glazing clips or putty and tighten loose fixings.
• Clear leaves from gutters so heavy rain drains away.
• Lubricate vent mechanisms so they open freely on mild days.

Let in every scrap of light

Take down shading paint, netting or roller blinds as soon as the sun loses its sting, usually early to mid October across most of the UK. Dirty glass in October is a double loss, because autumn condensation mixes with dust to create a dull grey film that blocks a surprising amount of daylight.

In a small greenhouse, every trick counts. Lining the north-facing wall or the back of your staging with white board or foil bounces light back onto plants and makes a genuine difference to winter salad leaves and cuttings. Keep the roof area clear, too — resist the temptation to stack empty pots and compost bags up high where they shade the bench below.

Insulate, but not too early

The temptation is to wrap the whole thing in bubble wrap the moment the nights turn chilly. Hold off. Put insulation up too soon and you can lose up to 30 per cent of your available light, which matters far more than a couple of degrees of warmth. Mid to late October is about right for most gardens — before hard frosts, after the worst of the light loss.

Horticultural bubble wrap taped to the frame, bubbles facing inward, is the classic solution. Leave vents, doors and any automatic openers completely clear, and double up on the coldest, north-facing side. A sheet of fleece laid over plants on frosty nights, doubled if the forecast is severe, adds a useful degree or two.

Bubble wrap slows heat loss but does not create heat. For tender plants such as citrus, pelargoniums or fuchsias, a thermostatically controlled electric heater set to around 5°C will keep the frost off; set it nearer 10°C if you are overwintering genuinely tender specimens. Group pots together, raise them off the cold floor onto staging or a wooden pallet, and keep the compost on the dry side.

Water less, ventilate more

Growth slows to a crawl, so plants drink a fraction of what they needed in July. Overwatering in a cold greenhouse is the single most common way to lose overwintering plants — roots sit in cold, wet compost and rot quietly before you notice anything wrong.

• Water in the morning with tepid water, never straight from a freezing tap.
• Test the compost first: water only when the top inch feels dry.
• Empty saucers and trays so nothing stands in water.
• Open a vent for an hour or two on mild, still days, ideally mid-morning.
• Avoid wetting foliage and remove yellowing or mouldy leaves straight away.

Still, damp air is grey mould's best friend, so a little ventilation on every mild day does more good than any spray.

Choose your overwintering plants wisely

Bench space is precious, so be ruthless. The best value comes from winter salad leaves such as land cress, winter purslane, mizuna and rocket, plus hardy herbs like parsley and mint. Cuttings of rosemary, lavender, fuchsia, box and pelargonium will root happily in a sheltered greenhouse and give you free plants next spring.

Move tender potted plants under cover before the first frost is forecast, not after. Group them by how much water and warmth they need, so you are not fussing over a cactus while your bay tree sulks. Tiered staging, wall shelves and hanging baskets let a tiny 6x4 greenhouse hold far more than you would think — just leave enough gaps for air to move.

Keep watch through the winter

Check the structure after every gale and replace any glazing clips that have sprung. If snow settles on the roof, nudge it off gently from outside with a soft broom rather than pushing from within, which can pop panes. Keep an eye out for red spider mite and whitefly in any heated space, and inspect stored tubers and dormant pots for signs of rot every few weeks.

Finally, keep a small notebook of what survived and what didn't. Next autumn, that scrappy list will be the most useful thing in your greenhouse.