October Polytunnel & Greenhouse Gardening: Winter Crop Planning

October arrives quietly here on our second terrace. The summer sun still lingers through the afternoons, but by dusk, the air turns sharp and restless — sliding downhill from the ridge, carrying the chill of the nearby river straight through the gaps of our greenhouse doors. Each autumn, I’m reminded that growing through winter isn’t just about plants. It’s about preparation, patience, and plugging the small leaks that let comfort escape.

This year, I’m sealing those gaps with hay bales — a simple barrier between my winter greens and the frost that loves to sneak in uninvited. For now, the doors and windows stay open 24 hours a day to keep air moving, but soon I’ll trade breeze for warmth. It’s the seasonal dance every Mediterranean grower knows: balancing humidity, light, and temperature in a space that’s both shelter and experiment.

Inside our greenhouse — late-season eggplants still thriving while we prepare for winter rotations.

If you’re standing in your own polytunnel or greenhouse wondering how to stretch the growing season just a little longer, this guide will walk you through it. From cleaning and insulation to crop rotation and ventilation, here’s how to make your October efforts pay off all winter long.

Don’t miss Herman’s Tough Kraut Fixes at the end — real-world troubleshooting for the most common winter greenhouse headaches.

Preparing for the Mediterranean Transition

October is a month of contradictions. The soil still holds warmth from summer, yet the air cools faster each evening. Days shorten, humidity rises, and the greenhouse becomes both sanctuary and testing ground. For growers in Mediterranean climates like ours in Central Portugal (Zone 8a), this is the time to shift from growth mode to maintenance and protection mode.

Inside a greenhouse or polytunnel, even small temperature swings can shape the success of your winter crops. Warm, sunny days quickly turn into damp, chilly nights. That mix breeds fungal issues, pest persistence, and condensation — all of which thrive when airflow drops and temperatures fluctuate. Getting ahead of these conditions now means fewer setbacks when the real cold settles in.

Our greenhouse sits below an ash tree on the second terrace, where cold air drifts down from the ridge — a microclimate challenge we manage each autumn.

So what’s the real difference between a greenhouse and a polytunnel?

A greenhouse is the sturdy, year-round workhorse — framed in glass or polycarbonate, anchored to the ground, and built to retain heat. It’s perfect for winter harvests and seed propagation. A polytunnel, meanwhile, is the seasonal sprinter — flexible, affordable, and easy to move, ideal for quick crops or protecting soil and plants from early frost. Both have their place in a Mediterranean setup: the greenhouse provides stability, while the polytunnel adds agility.

On our second terrace, the greenhouse faces an uphill battle against cold air flowing down from the ridge and creeping through every tiny gap. The nearby river adds an extra chill to nighttime air. That’s why October is my cue to start sealing those drafts with straw bales, check insulation, and plan rotations for winter greens.

By taking these steps now — cleaning, insulating, ventilating, and rotating — you’re not just keeping plants alive. You’re creating a controlled ecosystem that will carry your garden smoothly from autumn into spring.

Assess & Reset – Clean, Sanitize, and Rebuild Soil Health

Before new seedlings go in, October is your clean-slate moment. After a long, hot summer, your polytunnel and greenhouse have quietly collected a cocktail of dust, spores, and insect leftovers that can ambush tender winter crops. A few hours of cleaning now will save weeks of frustration later.

Start with the basics — wash down the inside surfaces with warm, soapy water or a mild vinegar solution. Don’t forget the corners, hinges, and door frames where condensation gathers and mold hides. Tools deserve the same care: pruners, dibbers, trays, and watering cans often carry invisible pathogens from plant to plant. A quick disinfecting dip in diluted hydrogen peroxide or white vinegar works wonders.

Even the outside needs attention — dirt and algae on the plastic film can cut light by up to 10%, making autumn crops work harder for growth.

If you’re working with soil beds, rake out any plant debris and check for signs of root rot or insect pupae. Then gently loosen the top layer to improve oxygen flow and reset soil biology. A light dressing of well-finished compost, worm castings, or leaf mold refreshes nutrients and supports beneficial microbes.

Think of this phase as your “reset ritual.” You’re not just scrubbing plastic and sweeping floors — you’re restoring balance to a miniature ecosystem that’s about to carry you through the cold months ahead.

Tough Tip: Don’t skip crop rotation — even inside a small polytunnel. Rotating crops helps prevent the buildup of soil-borne diseases and balances nutrient demands. If tomatoes or peppers grew in one bed all summer, switch to leafy greens or legumes this winter. It’s a small adjustment that can dramatically reduce disease pressure.

Insulate and Innovate – Keeping the Warmth In

By mid-October, cool nights start testing every gap and seam in your greenhouse. You can almost feel the cold air sliding downhill and sneaking through the cracks — especially if, like us, your greenhouse sits on a terrace near a river valley. It’s nature’s way of stress-testing your setup.

This is the moment to turn insulation into an art form. Start by sealing obvious drafts with straw or hay bales. They’re renewable, compostable, and — unlike foam or plastic — add a rustic layer of charm while blocking frost-prone air currents. Position them along door frames or at the base of leaky panels.

Next, check your internal insulation. If last year’s bubble wrap lining looks tired or dusty, replace or reposition it. The goal is to create a thermal buffer that traps radiant heat without shading plants too much. For delicate seedlings or greens like lettuce and mizuna, hang horticultural fleece loosely above growing beds — it forms a protective air pocket that keeps leaves from frosting on the coldest nights.

Here on our land, I’ll soon start sealing the door gaps with straw bales — one of those simple yet satisfying fixes that cost nothing and make a visible difference. When morning frost hits the terrace and the plants inside still stand tall, that’s proof enough that small actions build resilience.

Tough Tip: Reuse and rethink your materials. Old bubble wrap from packaging, clear shower curtains, or even cut-up polytunnel film can become perfectly serviceable insulation panels. Just make sure to leave space for airflow around vents and windows — warmth without ventilation quickly turns into a fungal paradise.

Ventilate Wisely – Fighting Humidity and Mold

Humidity is both friend and foe inside a Mediterranean greenhouse. It keeps young plants hydrated but, when trapped, turns into a breeding ground for mildew, botrytis, and rust. October brings cool nights and warm, sunny days — the perfect recipe for condensation on every surface. If left unchecked, those droplets can drip onto leaves, spread disease, and stunt growth.

Your best weapon is airflow. On our second terrace, I keep the greenhouse doors and windows open around the clock during this shoulder season. The steady breeze flowing downhill from the ridge keeps the air dry enough to prevent mold while temperatures still hover comfortably above frost risk. As soon as nights begin dipping close to freezing, I’ll start closing up in the evenings — but even then, a few small vent openings or a cracked window make all the difference.

Side ventilation keeps humidity in check during mild October days — essential for preventing mildew and leaf spot.

Remember, air movement isn’t wasted heat — it’s prevention. A few extra minutes of cross-ventilation each morning will save you from the heartbreak of losing crops to gray fuzz or leaf spot later in the season. Keep it simple, stay consistent, and let the air do half the work for you.

Tough Tip: Think of ventilation as your thermostat’s wiser cousin. It doesn’t just regulate temperature — it controls humidity, balances carbon dioxide, and strengthens plant stems. A small solar-powered fan or automatic vent opener can help if you’re not around daily, but even low-tech solutions like propping doors open with a brick can work wonders.

Plan Ahead – Winter Crop Success in Small Spaces

Once the greenhouse is clean, sealed, and breathing well, it’s time to think ahead. October isn’t just the wrap-up of summer — it’s the launchpad for your winter garden. The key to success is rotation and smart use of space.

Inside a polytunnel or greenhouse, soil fatigue creeps in fast. The same crop families pull the same nutrients, leaving gaps that pests and diseases are only too happy to fill. Rotate beds whenever possible: follow fruiting crops like tomatoes, peppers, and cucumbers with leafy greens, root vegetables, or legumes. Even a simple switch between beds helps rebalance soil biology.

This month is ideal for planting cold-hardy greens such as spinach, Swiss chard, kale, mizuna, and mustard, along with carrots, radishes, and broad beans for early spring harvests. In mild Mediterranean zones, you can still direct-sow many of these — just give them protection from cold snaps with fleece or low tunnels inside your main structure.

At Tough Kraut, I plan winter beds the same way I plan structures — with balance and longevity in mind. The goal isn’t to fill every square meter but to keep soil alive, roots active, and harvests continuous. A few resilient greens under cover now mean fresh leaves in January — proof that a little planning in October pays off when most gardens are sleeping.

Tough Tip: Treat your winter space like prime real estate. Mix quick growers (like lettuce or pak choi) with slower ones (like carrots or onions) to keep every patch productive. Interplant herbs like parsley, coriander, and chervil to add diversity and deter pests.

Small October Efforts, Big Winter Rewards

October greenhouse gardening isn’t about doing everything perfectly — it’s about doing the right few things consistently. Clean the space. Insulate the shell. Let the air flow. Rotate the crops. Each small action compounds into winter stability, turning what could be a season of dormancy into one of quiet productivity.

View of our from the food forest — a cornerstone of our winter resilience. Each October adjustment keeps this space thriving through the Mediterranean cold.

In a Mediterranean climate, we can’t fight nature — we adapt with it. The cold air will still roll down from the hills, the humidity will still rise with the morning dew, and condensation will still test your patience. But when you step into a well-prepared greenhouse on a frosty morning and see green leaves thriving against the odds, it’s worth every straw bale and scrub brush.

Success in winter gardening doesn’t come from gadgets or fancy systems — it comes from observation, timing, and a willingness to learn from last year’s mistakes.

If you’re ready to turn seasonal chores into year-round resilience, join the Kraut Crew — our growing community of practical dreamers building self-sufficiency one project at a time. Get free guides, member-only updates, and the motivation to keep your greenhouse (and your mindset) thriving all winter long.

Herman’s Tough Kraut Fixes: Common October Greenhouse Gardening Challenges

Even the best-prepared greenhouse faces surprises once autumn settles in. Temperature swings, condensation, and pests don’t take a season off — they just move indoors. Below you’ll find some of the most common October challenges and how to fix them, drawn straight from years of hands-on trial (and a few memorable errors) on our Portuguese terrace.

Q: My greenhouse walls are dripping every morning. Should I worry?

A: Condensation is normal but not harmless. Those droplets can carry fungal spores onto leaves. The fix: improve morning airflow. Open vents early, even for ten minutes, to exchange damp air for dry. Aim a fan gently at wet surfaces until they’re dry to the touch. Adding a layer of gravel under benches or pathways helps absorb excess humidity, too.

Q: Frost crept in and damaged my seedlings — now what?

A: It happens fast. Once temperatures dip below 2 °C (35 °F), tender seedlings suffer. Pile straw or hay bales along doors and baseboards to block drafts, and cover vulnerable trays with fleece at night. A few tea lights in a terracotta pot can raise temperatures just enough — but always use caution and supervision.

Q: Seeds are barely sprouting, even in the greenhouse.

October’s shorter days slow germination. Use a heat mat or seedling tray cover to maintain soil warmth around 18–22 °C (64–72 °F). Check moisture daily — overly wet soil can suffocate seeds just as quickly as dry soil can stall them.

Q: Fungus keeps returning no matter how much I clean.

A: That’s a signal of trapped humidity. Double-check insulation layers — sometimes old bubble wrap blocks airflow too tightly. Trim away lower leaves from dense crops to increase circulation. Rotate in quick, light feeders like spinach instead of heavy, thirsty plants.

Q: I thought pests died off in winter, but they’re back.

A: Unfortunately, aphids and whiteflies love the shelter of warm polytunnels. Use yellow sticky traps to monitor populations and introduce beneficial insects like ladybugs if available. Neem oil or mild soap sprays help, but always apply early morning to avoid leaf burn.

Recommended Books & Resources

Books

• The Winter Harvest Handbook by Eliot Coleman

  The classic on cold-season production with unheated structures. Clear, practical methods for succession timing, low tunnels, and microclimate stacking.

• The Year-Round Vegetable Gardener by Niki Jabbour

  A friendly, highly usable guide to stretching the season with cold frames, row covers, and smart crop picks for shoulder months.

• Greenhouse Gardener’s Companion (Revised & Expanded) by Shane Smith

  Deep dive on setup, ventilation, pest control, and day-to-day greenhouse routines. Great reference for troubleshooting humidity and light management.

• The Forest Garden Greenhouse by Jerome Osentowski

  A permaculture-forward perspective on creating resilient microclimates and diverse plant guilds inside protected spaces. Inspiring for design thinking.

Resources

• Seedling Heat Mat + Digital Thermostat Combo

  Keeps germination media at a steady 18–22 °C (64–72 °F) when nights cool off. Fewer failed trays, stronger starts.

• Govee WiFi Thermometer–Hygrometer (H5179)

  Real-time temp/humidity with phone alerts. Perfect for catching frost dips or humidity spikes before they bite.

• Tough Kraut Resources

  Our living library of field-tested greenhouse tools, winter gardening gear, and practical books we rely on to keep our tunnel thriving through Mediterranean winters.