S4ag Polytunnels

---
name: s4ag-polytunnels
description: "Get the most from a polytunnel — crop planning, rotation, ventilation, humidity, pest control, soil management under cover, and maximising season extension. Use when the user asks about polytunnels, tunnel crops, greenhouse growing, or indoor soil management."
allowed-tools: [Read]
---

# Polytunnels

A polytunnel is the most powerful season-extension tool on a small farm — but only if you manage the enclosed environment actively. The same cover that keeps frost off your crops also locks in humidity, limits airflow, concentrates pest populations, and stresses soil biology in ways open-field growing never does. Understanding those trade-offs, and managing around them deliberately, is what separates a productive tunnel from a disease factory. Your goal is continuous, high-value harvest across the longest possible season — and soil that improves year on year under cover.

**How this skill works:** Each sub-tool pauses at a **Checkpoint** to confirm the assumptions it is about to build on before producing output. A recommendation built on a wrong assumption wastes time and money — confirm the checkpoint before acting. Each sub-tool ends with **Next steps** — the skills worth running once you have acted on this one.

---

## Expert Lineage

*The thinkers whose frameworks underpin this skill — and what they specifically discovered that changes how you farm.*

**Eliot Coleman — Cold-Climate Season Extension**
Coleman developed the four-season harvest system at Four Season Farm in Maine, demonstrating that a single layer polytunnel provides approximately the equivalent of two USDA hardiness zones of protection. His most actionable finding: the critical factor is not the air temperature inside the tunnel but the soil temperature — soil above 7°C will support plant growth even when air outside is well below freezing. Practical implication: focus on soil thermal mass and ground temperature management, not just air temperature, when planning winter crops.

**Charles Dowding — No-Dig Under Cover**
Dowding's long-running no-dig trials in UK polytunnels demonstrated that a no-dig system with annual compost mulch builds soil biology faster under cover than any cultivated system, and dramatically outperforms conventional dug beds in both yield and pest/disease resistance. His specific contribution: the compost mulch provides a constantly replenished feeding layer for the food web, compensating for the lack of rainfall input that would normally sustain microbial populations. Practical implication: the single best investment for polytunnel soil management is a 5cm compost mulch at the start of each season.

**Joy Larkcom — Polytunnel Crop Planning**
Larkcom's exhaustive variety trials in UK conditions identified which crops genuinely benefit from covered growing versus those that perform adequately outdoors in temperate climates. Her finding: the highest-value tunnel crops are those requiring a longer season than the outdoor climate provides — tomatoes, cucumbers, aubergines, basil — not simply crops that can be grown outside but are brought under cover. Practical implication: prioritise tunnel space for crops that could not otherwise be grown in your climate, not simply for convenience.

**Elaine Ingham — Soil Food Web Under Cover**
Ingham's research identified polytunnel soils as among the most biologically stressed in any agricultural system. The covered environment removes the primary drivers of microbial diversity — rainfall variation, seasonal temperature fluctuation, and diverse root inputs from rotation — while concentrating soluble salts from repeated fertiliser applications. Her most critical finding for tunnel growers: the fungi-to-bacteria ratio collapses fastest in continuous covered cropping because fungal networks require undisturbed soil and diverse plant communities, neither of which continuous tunnel monoculture provides. Practical implication: compost additions and deliberate rotation are not optional extras in a tunnel — they are the minimum requirement to prevent biological collapse.

**Mary Louise Flint — Integrated Pest Management in Enclosed Spaces**
Flint's IPM framework applied to greenhouse environments established that the enclosed space changes the pest/predator dynamic fundamentally: pest populations build faster (no weather disruption, no dilution by outdoor populations), but biological control agents also establish more reliably because there is nowhere to escape. Her key finding: introducing biological control agents early — before pest populations peak — is dramatically more effective than waiting until damage appears. Practical implication: set up biological control as a preventive system from day one of each season, not as a response to infestation.

**Jean-Martin Fortier — High-Value Tunnel Cropping Economics**
Fortier's market garden model identifies the polytunnel as the highest-return-per-square-metre infrastructure investment on a small farm — but only when managed for continuous production. His specific contribution: the "tunnel bed formula" — six growing cycles per year of successional salads combined with a summer tomato/cucumber slot produces the returns that justify capital cost. Practical implication: plan for no fallow periods in the tunnel; every square metre that is not producing is a capital cost without return.

---

## Which tool fits

| You need to... | Tool |
|---|---|
| Plan what crops to grow, when, and in which beds | crop-planning |
| Manage ventilation, humidity, and temperature without expensive equipment | environmental-management |
| Maintain or rebuild soil health under cover | soil-management |
| Identify and control pests and disease in an enclosed environment | pest-disease |
| Maximise the shoulder-season and winter harvest window | season-extension |

## Routing Decision

- **Starting a new tunnel and need to plan what to grow** → crop-planning
- **Existing tunnel with humidity, grey mould, or leaf disease problems** → environmental-management
- **Soil under cover looks tired, compacted, or producing poorly** → soil-management
- **Pest outbreak or wanting to prevent one** → pest-disease
- **Wanting to harvest earlier in spring or later in autumn/winter** → season-extension
- **Unsure where to start** → crop-planning first; it surfaces the decisions that call the other sub-tools

---

## Crop Planning

*Plans what to grow in the tunnel, when to grow it, and how to organise beds for maximum output.*

The fundamental planning challenge in a polytunnel is that you have a finite, expensive space that needs to produce value year-round. The planning question is not "what can I grow in a tunnel" but "what should I grow in this specific tunnel given my climate, market, and rotation needs." Start with your highest-value crops that genuinely require tunnel protection, then fill the remaining time slots with succession salads and overwintering crops.

**Step 1: Assign the summer slot first.**
In temperate climates, the summer slot (May–September/October) belongs to heat-loving crops that cannot ripen reliably outdoors: tomatoes, cucumbers, aubergines, peppers, basil, melons. These are your highest-value tunnel crops. Do not sacrifice this slot for crops that can grow outside.

**Summer crop suitability by climate:**

| Crop | Min. night temp required | Can succeed outdoors in UK/Ireland/Northern Europe? |
|---|---|---|
| Tomatoes | 10°C | Only in best sites and years |
| Cucumbers | 12°C | Rarely reliably |
| Aubergines | 13°C | Almost never in UK/Ireland |
| Peppers | 12°C | Rarely reliably |
| Basil | 13°C | Not without cover |
| Melons | 14°C | Only in exceptional summers |
| Courgettes | 8°C | Yes — outside is fine |
| Salad leaves | 5°C | Yes — outside is fine in summer |

**Step 2: Plan the shoulder-season and winter slots.**
Outside the summer slot, the tunnel fills with: early spring salads and seedlings, overwintering salads (orientals, spinach, winter lettuce, claytonia, corn salad), and tunnel-specific perennials (established early strawberries, perennial herbs for early harvest).

**Typical temperate climate tunnel calendar:**

| Period | Slot | Crops |
|---|---|---|
| Jan–Feb | Deep winter | Hardy orientals, spinach, corn salad, claytonia |
| Mar–Apr | Early spring | Lettuce, spring onions, early carrots, peas, transplant raising |
| May–Oct | Summer | Tomatoes, cucumbers, aubergines, peppers, basil |
| Oct–Nov | Clearout and reset | One clear period for composting and bed preparation |
| Nov–Dec | Autumn/winter start | Oriental leaves, spinach, overwintering salads |

**Step 3: Design the rotation.**
Polytunnel rotation is harder than field rotation because space is limited and crops are concentrated, but it is not optional. Diseases — particularly soilborne pathogens like Fusarium and Sclerotinia — build up rapidly in continuously cropped tunnel beds. Minimum rotation principle: no crop family returns to the same bed within three years.

**Rotation by crop family:**

| Family | Examples | Key disease to break |
|---|---|---|
| Solanaceae | Tomatoes, peppers, aubergines | Fusarium wilt, Verticillium, TMV |
| Cucurbitaceae | Cucumbers, melons, courgettes | Powdery mildew, Pythium, Cucumber mosaic virus |
| Asteraceae | Lettuce, chicory, endive | Botrytis, Sclerotinia |
| Chenopodiaceae | Spinach, chard, beet | Downy mildew |
| Brassicaceae | Oriental leaves, rocket | Clubroot, downy mildew |
| Alliaceae | Spring onions, garlic | White rot |

**Step 4: Succession planning for salads.**
Continuous salad harvest requires a sowing every 3–4 weeks from late February through to late September for spring/autumn crops. Build a simple succession chart — sow date, transplant date, expected harvest start, expected clearout — and use it to ensure no week in season is without harvestable salad.

**Checkpoint — confirm before finalising:**
- How many beds does the tunnel have, and how long is each? This constrains how many rotation groups can genuinely rotate.
- Is the primary goal household production, direct market sales, or mixed? This determines whether Fortier's intensive formula or a more diverse planting plan is appropriate.
- What climate zone are you in — specifically, what is the average last frost date and the expected minimum winter temperature inside the tunnel (typically 5–8°C warmer than outside)?

A crop plan built on wrong assumptions about bed count or climate produces an impossible rotation or crops that fail in the first season.

**Output:**
```
POLYTUNNEL CROP PLAN

TUNNEL DETAILS
Size: [m x m], [number] beds, [bed length] m each
Climate zone: [region / last frost date]
Primary goal: [household / market / mixed]

SUMMER SLOT — [start month] to [end month]
Bed 1: [crop] — [variety] — [sow/transplant date] — [expected first harvest]
Bed 2: [crop] — [variety] — [sow/transplant date] — [expected first harvest]
[continue for each bed]

SHOULDER AND WINTER SLOTS
[Period]: [bed] — [crop] — [sow date] — [clearout date]
[Period]: [bed] — [crop] — [sow date] — [clearout date]

SUCCESSION SALAD SCHEDULE
Sow [date]: [species/mix] → transplant [date] → harvest [date range]
Sow [date + 3 weeks]: [species/mix] → transplant [date] → harvest [date range]
[continue sequence]

ROTATION TRACKING (3-year)
Year 1 — Bed 1: [family], Bed 2: [family], Bed 3: [family]
Year 2 — Bed 1: [family], Bed 2: [family], Bed 3: [family]
Year 3 — Bed 1: [family], Bed 2: [family], Bed 3: [family]

GAP ANALYSIS
[Any periods with empty beds — flag for succession filling]
```

**Next steps:**
- Run **season-extension** (within this skill) to push the harvest window further into winter and early spring.
- `/s4ag-seasons` — lock the tunnel calendar into the whole-farm production plan so outdoor and indoor schedules align.
- `/s4ag-vegetables` — variety selection and spacing detail for the crops identified in the plan.

---

## Environmental Management

*Manages ventilation, humidity, and temperature inside the tunnel without relying on expensive automated equipment.*

The enclosed polytunnel environment creates a fundamentally different growing climate from the field: higher temperatures in summer, higher humidity year-round, and significantly reduced air movement. Getting the environment right is not about reaching an ideal set-point — it is about managing the relationship between temperature, humidity, and airflow to prevent disease while maintaining crop performance.

**The core principle:** Most polytunnel disease — Botrytis, powdery mildew, damping off — is humidity-driven. The single most effective disease prevention measure is airflow management that keeps leaf surfaces dry and prevents condensation overnight.

**Temperature management:**

| Season | Challenge | Management action |
|---|---|---|
| Summer | Overheating above 35°C stresses crops and promotes mite populations | Open both end doors fully by 8am; roll up sides if possible; shade netting over the hottest section |
| Spring/Autumn | Large day/night temperature swings cause condensation | Ventilate from mid-morning to reduce humidity; close 1–2 hours before sunset to retain warmth |
| Winter | Frost risk below −8°C outside (tunnel interior typically 5–8°C warmer) | Use a secondary inner fleece layer over beds; thermal mass of moist soil helps buffer extremes |

**Ventilation decision tree:**
1. Is outside temperature above 20°C? → Open both ends and maximum side ventilation.
2. Is outside temperature 10–20°C? → Open one end door; use the door position to manage airflow direction (open the leeward end on windy days to avoid damaging crops).
3. Is outside temperature below 10°C? → Ventilate for 1–2 hours mid-day if humidity is high inside; keep closed otherwise.
4. Is there visible condensation on plastic or leaves overnight? → Increase ventilation window the following day; consider removing some condensation points by pruning lower leaves.

**Humidity benchmarks:**
- Above 85% RH: Botrytis risk elevated — increase ventilation immediately.
- 70–85% RH: Normal management range for active growing period.
- Below 60% RH: Fine for summer crops; increases spider mite risk in hot weather.

**Practical humidity management without instruments:**
- Watch for condensation on the plastic cover in the morning — its presence indicates overnight humidity was above 90%.
- Watch for leaf tip wetness at dawn — this is the direct Botrytis infection window; dry leaves before 8am.
- Remove all dead, dying, or touching leaves promptly — these are the primary Botrytis infection sites.
- Space plants to allow airflow between them: remove lower tomato laterals to knee height; keep cucumber plants trained to a single stem.

**Watering practice and humidity:**
- Water in the morning, not the evening — this allows leaf surfaces and soil surface to dry before overnight temperatures drop.
- Water at the base of plants, not overhead — wet foliage is the direct humidity problem.
- Drip irrigation or a soaker hose is significantly better for humidity management than overhead watering.

**Summer shading:**
- Above 30°C sustained inside the tunnel, photosynthesis slows and stress-related pest problems increase.
- Shade netting at 30–40% shade on the south-facing side (or roof) reduces peak temperature by 4–6°C.
- Temporary shading paint (diluted white emulsion applied to the outside plastic) is effective and washes off in autumn.

**Checkpoint — confirm before finalising:**
- What structure type is this — single-span polytunnel, multi-span, or greenhouse? This determines what ventilation options are available (roll-up sides, louvre windows, ridge vents vs. door ventilation only).
- What are the primary environmental problems currently presenting — overheating, condensation, disease, or all three?
- Is automation (thermostat-controlled vents, automatic louvres) in scope, or is manual management the constraint?

Building a ventilation plan around manual management for someone who has automated vents — or vice versa — produces recommendations they cannot implement.

**Output:**
```
ENVIRONMENTAL MANAGEMENT PLAN

TUNNEL TYPE: [single-span / multi-span / greenhouse]
Primary problem: [overheating / humidity / condensation / disease]
Management approach: [manual / semi-automated / automated]

DAILY VENTILATION SCHEDULE
Morning ([time]): [action — open doors/sides/vents]
Mid-day ([time]): [action — adjustment if needed]
Evening ([time]): [action — close to retain heat or maintain airflow]

HUMIDITY MANAGEMENT
Warning signs to watch: [condensation on plastic / leaf tip wetness / grey mould spots]
Response actions: [specific ventilation or pruning actions]

TEMPERATURE MANAGEMENT
Summer peak control: [shading action and threshold]
Winter frost risk: [inner fleece / thermal mass management / threshold for additional protection]

WATERING PROTOCOL
Method: [drip / soaker / hand — base-only]
Timing: [morning — specific target time]
Frequency: [crops and season-specific]

IMMEDIATE ACTIONS REQUIRED
[List any urgent changes based on current conditions]
```

**Next steps:**
- Run **pest-disease** (within this skill) — humidity problems and disease problems are closely linked; address both together.
- Run **soil-management** (within this skill) — soil surface management (mulching) affects tunnel humidity by reducing evaporation from bare soil.
- `/s4ag-pests` — if the humidity issue has already produced an active disease outbreak, get the full IPM ladder for the specific pathogen.

---

## Soil Management

*Maintains and rebuilds soil biology under the conditions that most damage it.*

Polytunnel soils fail faster than field soils for three interconnected reasons: they receive no rainfall (limiting the microbial diversity that rain-carried organisms would otherwise replenish); they are continuously cropped with a narrow range of species (limiting the root exudate diversity that sustains diverse soil communities); and they accumulate soluble salts from repeated fertiliser applications in a system that cannot flush them through leaching. The result, without active management, is a progressive collapse of soil biology and a rise in soilborne pathogen populations — the biological equivalent of a neglected aquarium.

**The diagnostic sequence:**

**1. Observe the surface.** Healthy tunnel soil: dark, friable, earthworm casts visible at the surface, a slight earthy smell. Problem indicators: white crystalline surface deposits (salt accumulation), grey or blue-grey colour at depth (anaerobic), soil that cracks and pulls away from bed edges when dry (poor aggregate structure), absence of earthworms.

**2. Count earthworms.** Dig a 20cm cube in the tunnel bed. Count all earthworms. Below 5 in a 20cm cube indicates poor biology; above 10 is good; above 20 is excellent for a covered system. Earthworm absence in a polytunnel is a serious indicator.

**3. Assess salt accumulation.** Mix a small soil sample with distilled water (1:1), let settle, taste the water. Obvious salty taste means salt accumulation is significant enough to affect osmotic uptake and suppress soil biology. Alternatively, an EC meter reading above 2.5 mS/cm in saturated paste extract indicates elevated salt.

**Managing salt accumulation:**
- The primary remedy is periodic deep irrigation — flooding the beds deliberately to flush salts downward. This works once, but must be paired with changing the fertility inputs that created the problem.
- Switching from soluble synthetic fertilisers to compost-based fertility is the long-term fix. Compost does not accumulate salt; soluble NPK does.
- If salt accumulation is severe, one complete fallow season with regular deep irrigation and a cover crop (winter rye, planted then incorporated) can reset the system.

**Compost-based fertility programme:**
- Apply 5cm of high-quality finished compost as a surface mulch at the start of each growing season — do not dig in.
- By mid-season, the compost will have partially incorporated through earthworm activity and microbial decomposition.
- This single practice — annual 5cm compost mulch — is Dowding's documented solution to maintaining soil biology under intensive tunnel cropping. His trial beds have been no-dig since 2003 with no decline in biology or productivity.

**Biological inoculants:**
- In severely depleted tunnel soils (earthworms absent, poor structure, salt issues), a biological inoculant — aerated compost tea, mycorrhizal inoculant at transplanting, or a commercial biological product — accelerates food web recovery.
- Apply to transplant holes at planting time and as a soil drench after the main summer clearout.
- The Ingham-method aerated compost tea (24-hour aeration with an air pump, molasses, and quality compost) can achieve bacterial populations of 10^9/ml — a meaningful inoculation event for a collapsed system.

**Irrigation and biology:**
- Drip irrigation or soaker hose is not just better for humidity — it is better for soil biology. Overhead watering disrupts the soil surface structure and the fungal networks near the surface. Base-only watering respects the biological architecture.
- Maintain consistent moisture, not wet-dry cycling. Extreme drying followed by heavy irrigation is biologically disruptive.

**The no-dig decision:**
Conventional cultivated tunnel beds (rotovated or dug each season) destroy the fungal networks that build soil structure. Transitioning to no-dig — removing spent crops at the base, leaving roots in the soil, adding compost as a top-dressing rather than incorporating it — produces measurable improvement in soil biology within 2–3 seasons. The transition is not complicated: simply stop digging, start mulching.

**Checkpoint — confirm before finalising:**
- Has the tunnel been cropped continuously for more than 3–4 years without a fallow period? If yes, salt accumulation and soilborne disease pressure are probable and need to be addressed explicitly.
- What fertility inputs are currently being used — soluble synthetic, organic granular, or compost-only? This determines whether salt accumulation is an active or potential problem.
- Is the grower open to no-dig, or is cultivation deeply embedded in the current system? This affects whether to recommend a full transition or a phased approach.

Recommending no-dig compost-only fertility to someone using soluble NPK on a heavily salted system without first addressing the salt problem will not work — the biology cannot recover until the salt burden is reduced.

**Output:**
```
TUNNEL SOIL ASSESSMENT AND PLAN

CURRENT STATUS
Earthworm count (20cm cube): [number]
Surface observation: [healthy / salt deposits / poor structure / anaerobic signs]
Salt assessment: [taste test result / EC reading if available]
Fertility inputs in current use: [soluble synthetic / organic granular / compost / mixed]
Years of continuous cropping without fallow: [number]

IMMEDIATE ACTIONS REQUIRED
[Priority 1 — e.g. salt flush if deposits present]
[Priority 2 — e.g. compost application]
[Priority 3 — e.g. transition fertility inputs]

ONGOING PROGRAMME
Annual compost mulch: [depth] applied [timing]
Biological inoculant: [product / compost tea / mycorrhizal] applied at [transplant / season-start / drench]
Irrigation method: [drip / soaker / hand-base-only]
Cultivation approach: [no-dig / transitioning to no-dig — timeline]

FALLOW PLAN (if needed)
Year: [year if fallow recommended]
Method: [flush + cover crop + biological reset]

RECHECK DATE: [12 months]
```

**Next steps:**
- `/s4ag-composting` — make the compost mulch on-farm from available materials rather than purchasing.
- `/s4ag-soil` — run a full mineral and biological soil assessment if the tunnel soil appears significantly degraded.
- Run **crop-planning** (within this skill) — a proper rotation plan is the single most important long-term soil management tool in a tunnel.

---

## Pest and Disease

*Identifies, prevents, and manages pest and disease pressure in an enclosed growing environment.*

The enclosed polytunnel changes the fundamental dynamics of pest and disease management. Pest populations build faster than outdoors — there is no weather disruption, no dilution by outdoor populations, and the physical barrier that protects crops from frost also prevents the beneficial predator insects that would naturally enter the system. Disease pressure — particularly fungal disease — is amplified by the humidity and limited airflow inherent to the environment. The response is a prevention-first, biological-control system established at the start of the season, not reactive spraying after populations have built.

**Most common polytunnel pests and immediate identification:**

| Pest | Signs | Crops affected |
|---|---|---|
| Whitefly | White triangular insects on leaf undersides; sticky honeydew; sooty mould | Tomatoes, cucumbers, peppers |
| Two-spotted spider mite | Fine webbing on stems; stippled, bronzing leaves; visible mites (0.5mm, red-orange) with hand lens | Tomatoes, cucumbers, aubergines, beans |
| Aphids | Colonies on growing tips; sticky honeydew; distorted new growth | Broad range — peppers, aubergines, salads |
| Thrips | Silver-scarred leaves; distorted fruit; tiny (1mm) insects visible on flowers | Cucumbers, peppers, strawberries |
| Vine weevil | Wilting plants; root ball eaten; c-shaped larvae in soil | Container-grown plants, strawberries |
| Fungus gnats | Adults flying from soil when disturbed; larvae in top 5cm soil | Seedlings, young transplants |

**Most common polytunnel diseases:**

| Disease | Signs | Conditions favouring |
|---|---|---|
| Botrytis (grey mould) | Grey fluffy mould; stem base rots; lesions on dying or damaged tissue | High humidity; dead tissue present; poor airflow |
| Powdery mildew | White powdery coating on leaf surfaces | Temperature swings; moderate humidity; stressed plants |
| Fusarium wilt | Wilting despite adequate water; brown vascular tissue when stem cut | Soilborne; builds with repeated solanaceous crops |
| Damping off | Seedlings collapsing at soil level; water-soaked stem base | Overwatering; cold, damp soil; poor airflow at soil level |
| Cucumber mosaic virus | Mottled, distorted leaves; cucumber and pepper crops | Aphid transmission — prevent aphids, prevent virus |
| Tomato blight (late) | Brown patches on leaves with pale halos; fruit rot | High humidity; overhead watering; warm nights |

**Biological control — the preventive system:**
The most effective polytunnel pest management is a biological control programme established early in the season before pest populations are visible. Biological controls in an enclosed tunnel work reliably because there is limited escape and stable conditions for predator establishment.

| Biological control agent | Target pest | Introduction timing |
|---|---|---|
| *Encarsia formosa* (parasitic wasp) | Whitefly | Introduce when first whitefly adult seen — do not wait |
| *Phytoseiulus persimilis* (predatory mite) | Two-spotted spider mite | Introduce preventively when temperatures reach 18°C consistently |
| *Amblyseius cucumeris* (predatory mite) | Thrips; young spider mite | Introduce at transplanting for susceptible crops |
| *Aphidius colemani* (parasitic wasp) | Aphids | Introduce as a preventive "banker plant" system |
| *Steinernema feltiae* (entomopathogenic nematode) | Fungus gnats, thrips pupae | Apply to soil monthly during growing season |
| *Bacillus thuringiensis* (Bt) | Caterpillars | Spray at first sign of caterpillar damage — organic-approved |

**The biological control rule:** Introduce agents early, at low pest pressure. Biological controls cannot rescue a tunnel from a population peak — they are a maintenance and prevention system. The question is not "should I spray or use biologicals" — it is "did I introduce biologicals early enough that spraying is unnecessary."

**When chemical intervention is needed:**
If a pest population has exceeded the threshold where biological control can recover it this season, targeted intervention is sometimes necessary. Prioritise in this order:
1. Physical controls first: yellow sticky traps for whitefly and fungus gnats; water jets for spider mite knockdown.
2. Approved biological insecticides: neem oil (aphids, mites, scale); insecticidal soap (aphids, mites, whitefly); *Beauveria bassiana* (thrips, whitefly).
3. Organic-approved chemicals: pyrethrin (fast knockdown, low residue; kills beneficial insects too — use only as last resort).
4. Conventional insecticides: copper-based fungicides for disease; specific systemic insecticides only if organic status is not a constraint and biological control has failed.

**Ingham lens on disease management:** Most polytunnel disease is plant stress disease. Stressed plants grown on impoverished tunnel soils with disrupted food webs have compromised cell wall integrity and reduced systemic resistance. Before reaching for any spray — biological or chemical — ask whether the underlying plant health is adequate. A Botrytis outbreak in a well-managed tunnel with strong soil biology is unusual; in a tunnel on depleted soil with salt accumulation, it is predictable. Fix the food web, and disease pressure often resolves.

**Checkpoint — confirm before finalising:**
- Is this an active outbreak requiring immediate intervention, or are you setting up a preventive system for a new season?
- Has biological control already been tried this season — and if so, which agents were introduced and when?
- Is organic status a constraint, or are conventional chemical interventions available if needed?

The intervention sequence is completely different depending on whether this is a current crisis or a seasonal planning question — confirm this before producing recommendations.

**Output:**
```
PEST/DISEASE ASSESSMENT AND PLAN

CURRENT STATUS
Active pests: [pest name] — [severity: low/moderate/high] — [crops affected]
Active diseases: [disease name] — [severity] — [conditions present]
Biological controls in place: [yes/no — which agents]
Season stage: [early / mid / late]

IMMEDIATE ACTIONS (if active outbreak)
Day 1: [action — e.g. yellow sticky trap deployment, water jet knockdown]
Day 2–3: [action — e.g. biological agent order/introduction if not already present]
Day 7: [assessment — check response]

PREVENTIVE BIOLOGICAL CONTROL PROGRAMME
[Month/stage]: Introduce [agent] for [target] — [source/supplier note]
[Month/stage]: Introduce [agent] for [target]
[Month/stage]: Soil drench [nematodes] for [fungus gnats/thrips pupae]

ENVIRONMENTAL ADJUSTMENTS TO SUPPORT CONTROL
[Ventilation changes to reduce humidity if disease is present]
[Leaf removal / spacing changes]

SPRAY PROGRAMME (if needed)
Product: [product] — [target] — [rate] — [timing] — [pre-harvest interval]
Note: [impact on beneficial insects / re-introduction timing if biologicals disrupted]

REVIEW DATE: [2 weeks from now]
```

**Next steps:**
- Run **environmental-management** (within this skill) — humidity reduction is the most effective disease prevention measure; address both together.
- `/s4ag-pests` — for the full IPM decision ladder if a specific disease or pest requires more detailed treatment protocol.
- Run **soil-management** (within this skill) — if disease pressure is recurring season after season, the soil biology is almost certainly the underlying problem.

---

## Season Extension

*Maximises the harvest window at both ends of the season — earlier in spring, later into autumn and winter.*

The primary value proposition of a polytunnel is season extension. In a temperate maritime climate (UK, Ireland, Pacific Northwest, similar), a well-managed single-span polytunnel advances spring by 4–6 weeks and extends autumn by 4–6 weeks — effectively adding 2–3 months to the productive season. In colder continental climates, the extension effect is even larger. Understanding how to maximise this window — without creating the humid, stagnant conditions that invite disease in shoulder seasons — is the final management skill.

**Understanding the thermal environment:**

A single-layer polytunnel provides roughly 4–5°C of frost protection on clear nights (radiative cooling is reduced by the cover). With a secondary inner fleece tunnel inside the main tunnel, effective protection extends to approximately −10°C outside (depending on thermal mass). Soil temperature is the critical variable for plant growth — root activity, germination, and microbial function all cease below 5°C soil temperature, and resume above 7°C.

**Spring extension strategy:**

The goal is to advance the soil temperature above 7°C as early in the year as possible, then use this to:
1. Start transplants in the tunnel 4–6 weeks earlier than field planting.
2. Direct sow cold-tolerant crops (salads, spinach, radish, spring onions, carrots) 6–8 weeks earlier than the outdoor sowing date.
3. Raise all farm transplants in the tunnel through late winter and early spring.

**Early spring sowing calendar for temperate maritime climates:**

| Crop | Outdoor sowing date | Tunnel sowing date | Harvest starts |
|---|---|---|---|
| Winter lettuce varieties | NA (outdoors) | Early January | March/April |
| Spring onions | Late March | Late January | May |
| Carrots (early varieties) | Late March | Early February | May |
| Peas | Late March | Late February | Late May |
| Broad beans | Feb–March | January | May–June |
| Tomatoes (transplants) | Under heat — April | Under heat — Late Feb/early March | July |
| Basil (transplants) | NA | Early March (with heat) | May |

**Autumn extension strategy:**

The goal is to transition smoothly from the summer slot — cleared by late September/early October — into an overwintering salad and leaf system that produces through the winter.

**Timing is critical:** Crops that will overwinter in the tunnel must be established before the days shorten below 10 hours of daylight (roughly late September/early October in UK/Ireland). After this date, plant growth essentially stops until late January. Plants established before the daylight threshold will hold their size through winter and resume growth in late January/early February — providing the earliest possible spring harvest. Plants sown after the threshold do not establish before the cold and produce poorly.

**Overwintering crop list for temperate tunnels:**

| Crop | Sow date for overwintering | Harvest period |
|---|---|---|
| Oriental leaves (mizuna, mibuna, pak choi, tatsoi) | Late August–early September | November through March |
| Winter spinach | Late August–September | November through March |
| Claytonia (miner's lettuce) | September | November through April |
| Corn salad (mâche) | September | November through April |
| Winter lettuces (Valdor, Arctic King, Winter Marvel) | August–September | November through March |
| Land cress | August–September | November through April |
| Kale (tender varieties) | August | November through April |
| Chard (for overwintering) | Late July–August | November through April |
| Spring onions | September | March–April |

**Inner fleece as the second protection layer:**

A secondary layer of fleece (30–40gsm) laid directly over crops inside the tunnel on the coldest nights extends the effective growing temperature range and protects against condensation drip damage in sharp frost. The fleece can be laid and lifted on a daily basis as temperatures warrant, or set up on a simple wire frame for permanent installation.

**The daylight threshold and supplementary lighting:**

Below 10 hours of daylight, most crops stop growing regardless of temperature. Supplementary lighting (LED grow lights) can extend the productive window in deep winter, but this is only economically justified if the enterprise produces high-value crops (microgreens, baby salads, herbs for high-margin markets). The skill does not recommend supplementary lighting as standard — the economics rarely support it in general vegetable production.

**Transition from summer to winter without a gap:**

The ideal tunnel management creates a seamless transition from summer crops to winter crops with no fallow period:
- August: Sow overwintering crops in modules while tomatoes/cucumbers are still in the beds.
- Late September/early October: Clear summer crops as soon as yield drops significantly.
- Immediately after clearing: Apply compost top-dressing; transplant or direct-sow winter crops.
- The goal is that winter crops are in the ground within one week of summer clearout.

**Checkpoint — confirm before finalising:**
- What climate zone are you in — specifically, how many days per year does the outdoor temperature drop below freezing, and what is the expected minimum outside temperature in your coldest month?
- Is the primary goal spring extension, winter production, or both? This determines whether to focus on early spring preparation or the autumn-to-winter transition.
- Is there any supplementary heating available in the tunnel, or is management passive (cover only)?

Season extension planning built around the wrong climate baseline produces sowing dates that either fail from cold or miss the early-harvest opportunity.

**Output:**
```
SEASON EXTENSION PLAN

CLIMATE BASELINE
Region: [region]
Last frost date (outside): [date]
First autumn frost (outside): [date]
Expected minimum winter temperature (outside): [°C]
Estimated tunnel interior minimum: [°C — approximately +4–5°C on clear nights]

SPRING EXTENSION
Earliest safe tunnel sowing date (with fleece): [date]
Earliest safe tunnel sowing date (without additional fleece): [date]
Spring crops and sow dates:
  [Crop]: sow [date] → transplant/thin [date] → first harvest [date]
  [Crop]: sow [date] → transplant/thin [date] → first harvest [date]

AUTUMN/WINTER TRANSITION
Summer clearout target date: [date]
Overwintering crop sow dates (in modules): [date — 4–6 weeks before clearout]
Transplanting date after summer clearout: [within 1 week of clearout]
Winter crops and expected harvest period:
  [Crop]: transplant [date] → harvest [date range]
  [Crop]: transplant [date] → harvest [date range]

INNER FLEECE PROTOCOL
When to use: [temperature threshold for inner fleece]
Method: [direct drape / wire frame installation]

DAYLIGHT THRESHOLD DATE: [date when days fall below 10 hours — typically late September]
NOTE: All overwintering crops must be established before this date.

EXPECTED EXTENDED SEASON SUMMARY
First harvest in spring: [date]
Last harvest in autumn/winter (without extension): [date — field crops]
Last harvest with tunnel extension: [date]
Season extended by: [weeks] at each end
```

**Next steps:**
- Run **crop-planning** (within this skill) to slot the season-extension crops into the full tunnel rotation.
- `/s4ag-vegetables` — detailed variety selection for the overwintering and early-spring crops identified here.
- `/s4ag-seasons` — integrate the extended tunnel calendar with the outdoor farm production calendar so no planting windows are missed.