Greenhouse & Polytunnel Prep for September

As summer wanes, smart September prep in your greenhouse or polytunnel is the secret to productive autumn crops, protected winter greens, and a head start on spring sowings. With cooler nights and heavier dews on the way, it’s time to clear spent plants, upgrade your set-up, and line up new sowings. Here’s your practical, month-by-month guide to prepping undercover spaces for late and off-season success!

Why Prep Now?

• Crops shifting: Summer’s tomatoes, cucumbers, peppers, and melons are slowing down or ready to pull out.
• Fungal risks increase: Damp, cool air breeds botrytis, mildew, and other pests of cluttered, tired tunnels.
• September sowings: Hardy salads, spinach, Asian greens, and overwintering crops start now for a steady supply.
• Tools and supports need attention: Prevent next year’s problems by deep-cleaning and replacing tired kit now.

Step 1: Clear, Clean, and Disinfect

• Remove all finished plants: Pull or cut at the base, compost uninfected material, and bin anything diseased.
• Sweep thoroughly: Get rid of debris, old leaves, and fallen fruit from floors and bed surfaces.
• Wash glass or polythene inside and out: Use warm, soapy water and a sponge or soft mop. Clean frameworks, gutters, and doors.
• Disinfect benches, tools, pots, and any reusable ties: A mild garden-friendly disinfectant eliminates fungal spores and overwintering pests.
• Fix or replace torn polythene, broken panes, or leaky gutters.

Step 2: Soil Refresh and Bed Prep

• Replace or top up raised bed/bench soil: Remove surface crust/moss, add 5–10 cm (2–4 in) of new compost or manure.
• Remove and replace or thoroughly refresh grow bag compost.
• Add slow-release organic fertilizer if reusing beds for autumn plantings.

Step 3: Plan and Sow for Autumn/Winter Crops

Consider crops to sow for late harvests or overwintering:

• Salads: Mustard, mibuna, mizuna, claytonia, lamb’s lettuce, winter lettuce.
• Greens: Spinach, chard, rocket, pak choi, tatsoi, kale.
• Herbs: Parsley, coriander, dill.
• Early pea and broad bean starts in modules for winter planting.

Sow directly in beds, gutters, or trays, and thin as needed.

Step 4: Adjust Microclimate

• Shade removal: Most summer shading can come down now—let in more light for winter crops.
• Check vents and doors: Ensure they work well for good airflow on warm days (reduces fungal problems).
• Add insulation as needed: In late September, get bubble plastic or fleece handy for sudden cold snaps.

Step 5: Tools, Watering, Maintenance

• Check and clean all watering equipment; adjust for cooler, less frequent autumn watering.
• Store tools on clean hooks or racks.
• Restock on labels, seed trays, and compost while supplies last.

Bonus Tips

• Sow green manure (phacelia, vetch, mustard) in empty beds for a soil boost—cut down before flowering next spring.
• Keep surfaces dry, especially overnight, to deter slugs, snails, and botrytis.
• Keep an eye out for overwintering pests (aphids, whitefly) on tunnel doors and panes—treat early.

Wrapping Up

With a morning or two of September work, your greenhouse or tunnel can carry leafy crops, herbs, and even early flowers right through to spring. Clean spaces mean fewer pests and diseases, robust crops, and a rewarding season undercover.