Heatwave and strawberry crops: adapting your irrigation
Econome à LégumesWhen temperatures exceed 32°C for several consecutive days, strawberry plants do not suffer in silence. The signs are immediate: fruits turning white, flowers aborting, plants that stop fruiting altogether. For a professional strawberry grower, a heatwave is never an isolated incident — it is a compressed decision window where every hour of delay translates into lost yield or degraded quality.
Extreme heat events have multiplied and intensified across the main French and European production regions: 2019, 2022, 2025. This is no longer an exceptional risk — it is a structural constraint to integrate into crop management. The strawberry plant, with its shallow root system concentrated in the top 20 centimetres of soil, is among the most exposed crops: poorly managed water stress in July can compromise not only the current harvest, but also the floral induction of the following autumn.
The amateur gardener's response — "water more and cover with a fleece" — does not reflect the reality of a professional growing operation. Whether you are working in plastic-mulched open field, under a multi-span cold tunnel, or in coco substrate off-ground systems, the levers are different, the trigger thresholds differ, and the secondary risks — Botrytis, rising EC, root asphyxia — do not cancel each other out.
This article covers the mechanisms of heat stress in Fragaria × ananassa, practical irrigation adaptations by growing system, shading and thermal protection management, crop calendar decisions, and harvesting conditions during extreme heat.
Questions waiting for you in the field — do you have the answers?
Before going into the technical mechanisms, here are three concrete situations that the next heatwave will put to your operation:
- Your sensor reads 18 kPa during full fruit sizing, with a forecast of 36°C the next day — at what time do you trigger night irrigation, and at what volume?
- You have active Botrytis outbreaks under the tunnel and temperature is climbing to 30°C by late morning — do you activate misting, and until what time in the afternoon?
- Your everbearing varieties have been in their summer production dip for ten days — do you maintain irrigation levels or reduce them to encourage flowering resumption in August?
These decisions depend on your variety, your growing system and the exact phenological stage of your plants. Fraisibot advises you in real time on your strawberry crop — whatever the situation.
A heatwave is not just about water — what it does to the plant
Understanding the physiological mechanisms allows you to anticipate the right levers rather than react once the damage is visible.
The dual daytime and night-time threshold
Strawberry plants tolerate occasional high temperatures, but it is the duration and combination of thresholds that determine the extent of the damage. Above 30°C daytime temperature, the plant enters heat stress: photosynthesis slows, transpiration accelerates, and in both short-day and long-day varieties, runner production takes over from fruiting. This first threshold is not yet catastrophic if nights remain cool.
It is the night-time threshold that locks in the damage: once night temperatures exceed 20°C, the plant can no longer recover between two hot days. Night respiration burns carbohydrate reserves without replenishing them, and fruiting drops directly. A heatwave with nights at 22–24°C is far more destructive than a heat event where nights remain below 18°C.
Above 32°C over several consecutive days, consequences become systemic: flower abortions, pollen sterility (pollen grains lose viability beyond this threshold), fruits turning white instead of red (scald or thermal albinism), accelerated ripening with loss of firmness and sugar content. On a farm in full harvest, this can represent 20 to 30% of fruit downgraded in a single heatwave week.
The rooting problem: 80% of roots in the top 20 centimetres
This figure summarises the core vulnerability of strawberry plants in heat. Unlike a fruit tree that can seek moisture deeper in the soil profile, the strawberry plant depends entirely on the surface horizon — the most exposed to evaporation, the most sensitive to soil temperature fluctuations, the most affected by degraded or poorly laid mulch.
Once soil water tension exceeds 30 kPa, water stress symptoms become visible: leaves curling at midday, petioles wilting, fruits remaining small. At this point, part of the fruit sizing potential is already lost — it will not be recovered even if irrigation resumes.
The hidden impact on autumn floral induction
This is the least visible damage but often the most costly economically. For non-everbearing varieties, floral induction is triggered at the end of summer — late August to early September — when day length drops below 13 hours and temperatures fall below 18°C. The number of flower buds formed at that moment directly determines the yield of the following spring.
Heat and water stress left uncompensated in July–August weakens plants at the precise moment when they need to build reserves for induction. Plants exhausted by the heatwave enter autumn with less vigour, induce fewer flowers, and produce less in spring. A July heatwave can be paid for in April the following year.
Differences by growing system
| System | Risk amplification | Specific feature |
|---|---|---|
| Open field with plastic mulch | Moderate — soil thermal inertia under the film | Risk of root burn if black film is exposed to sun without organic mulch on top |
| Single-span cold tunnel | High — greenhouse effect amplified without ventilation | Temperature under cover can exceed outdoor temperature by 8 to 12°C if vents are insufficient |
| Off-ground coco substrate | Very high — light substrate, low water retention capacity | Maximum reactivity but also very rapid salt concentration (EC) risk during heatwaves |
Adapting irrigation: management under heatwave conditions
The basic principle of strawberry irrigation — regular, small frequent volumes rather than large spaced applications — applies even more forcefully during a heatwave. But in extreme conditions, this principle must be recalibrated: frequencies, volumes, timing and water quality all require simultaneous adjustment.
Tensiometric thresholds: lowering trigger values
Under normal conditions, irrigation is triggered at 15 kPa and stopped at 8–10 kPa. During a heatwave, the tension must be kept permanently close to 8–10 kPa — meaning the sensor should never be allowed to climb back towards 15 kPa. The soil must remain permanently well supplied with water to compensate for evapotranspiration that can reach 6 to 8 mm per day at summer peak.
The crop coefficient Kc for strawberry in full summer production sits between 0.85 and 0.9. Applied to a local ETo of 7–8 mm/day in southern growing regions in July, this gives a theoretical demand of 6 to 7 mm/day, nearly double that of a normal spring period. This water balance calculation — ETo × Kc minus effective rainfall — is the basis of rational scheduling, to be complemented by field tensiometer readings.
Timing: irrigate before the heat peak
During a heatwave, irrigation timing matters as much as volume. The objective is to load the plant with water before the heat peak arrives, so it has its water reserves available when transpiration will be at its maximum.
The practical rule: irrigate at night (2am–5am) and early morning (6am–8am). This allows water to infiltrate to the root zone before daytime heat evaporates part of it from the surface. An irrigation triggered at 2pm during a 36°C peak is largely ineffective — a portion evaporates before reaching the roots, and the plant has already been under stress for several hours.
In open field with drip irrigation, cycle management can be handled via a timer or automated control linked to a capacitive sensor. Without a sensor, reading the Watermark tensiometer every morning at 7am remains the most accessible reference method.
Open field with plastic mulch: the reference system, but with constraints
Drip tape under plastic film is the dominant system in France. During a heatwave, moving to daily or twice-daily irrigation is often necessary. Standard T-Tape drip lines (16 mm, emitters at 20–30 cm spacing, flow rate 1 to 2 L/h) are designed for 30 to 60-minute applications — in a heatwave, cycles can be extended or split into two daily passes.
A frequently overlooked issue: water quality deteriorates during heatwaves. Evaporation in pipes exposed to sunlight concentrates dissolved salts. If irrigation water EC exceeds 1.2 mS/cm, risks of root burn and disrupted absorption arise. During high heat, check EC at the drip outlet — and if necessary, dilute or alternate with lower-concentration water.
Furthermore, mulch plays a thermal role in open field: black plastic film under direct sunlight can reach 55–60°C at the surface, penalising shallow roots. In regions with intense summers (Mediterranean basin, Rhône Valley), light-coloured or bi-colour mulch (black underneath, white on top) reduces soil surface temperature by 5 to 15°C and limits evaporation by conduction.
Off-ground substrate systems (coco, peat, perlite): maximum reactivity, heightened EC risk
In off-ground growing, a heatwave requires a radical change in fertirrigation protocol, with two simultaneous and inseparable adjustments: increasing cycle frequency and diluting the nutrient solution.
Frequency and volume: during a heatwave peak (>35°C), switch to 6 to 8 short cycles per day of 5 to 20 minutes each. Daily volume can reach 1 to 3 litres per plant, compared to 0.5 to 1 litre under normal conditions. The objective is to keep the substrate permanently at field capacity, never allowing it to dry out between cycles.
Drainage management: a drainage rate of 20 to 30% must be maintained, or slightly increased. This drainage is not a loss — it is the lever that prevents salt accumulation in the root zone. Suppressing or reducing drainage during a heatwave to "save water" is a mistake that leads to root burn within 24 to 48 hours.
EC monitoring: during a heatwave, plants absorb far more water than mineral elements. If the irrigation formula is not adjusted, salts progressively concentrate in the substrate. Key thresholds to monitor continuously:
- Inlet EC: reduce to dilute the nutrient solution
- Drainage EC: keep below 2.2 mS/cm (absolute alert threshold at 2.5 mS/cm)
- Delta EC (difference between inlet EC and drainage EC): must not exceed 0.2 mS/cm — a larger gap signals active under-irrigation, the plant is transpiring faster than it is receiving water
This delta EC is the earliest indicator of a hydric breakdown in off-ground systems. It allows action before foliar symptoms become visible.
High-pressure misting: active cooling under tunnel
High-pressure misting (50–70 bar) is an increasingly widespread active cooling tool under tunnels, driven by the rising frequency of heatwaves. Its effectiveness rests on a simple physical principle: the latent heat of vaporisation of the droplets cools the ambient air by 3 to 6°C. But its management requires precision, otherwise it creates conditions favourable to Botrytis.
Trigger point: automate activation at T° > 28°C inside the tunnel. Do not wait for 32°C — at that point, pollen sterility is already under way.
Cycle duration and frequency: cycles must be extremely short — a few seconds only, repeated frequently. The objective is for the droplets to evaporate instantly in the air before falling onto foliage or flowers. Wetting of the foliage is precisely what must be avoided: it is the presence of free water on plant organs that triggers fungal spore germination.
Shut-off in late afternoon: stop misting sufficiently early in the afternoon — at least 2 to 3 hours before nightfall — to allow the tunnel and plants to dry completely before night. Botrytis cinerea develops between 15 and 20°C with high humidity: the risk is paradoxically moderate at full heatwave peak (>30°C), but becomes critical at the end of the day when temperatures drop and the air remains saturated. Misting cut off too late turns a thermal protection tool into a vector for grey mould.
For in-depth coverage of strawberry irrigation management outside heatwave conditions, the technical foundations are developed in our article Strawberry irrigation: water requirements and scheduling.
Shading and thermal protection: what professionals actually do
Irrigation alone cannot absorb everything. Beyond a certain level of heat, it is necessary to reduce the plant's thermal exposure directly — on the fruits, on the flowers and on the foliage. Professionals generally combine two or three tools simultaneously depending on their growing system.
Shade nets: rate, installation and limits
Shade nets with 30–50% light reduction are the most versatile tool. Correctly installed, they lower the ambient temperature around the vegetation by 3 to 5°C, which can be enough to drop back below the critical 32°C threshold during moderate peaks.
In tunnels, nets can be installed inside (retractable screens suspended under the plastic cover) or directly on the external structure — a simpler solution but less effective since hot air still circulates under the plastic before being tempered. In open field, hoops with netting can be deployed over rows during peaks (>33°C) — a flexible solution but requiring additional labour.
The line not to cross: a shading rate above 50% significantly reduces photosynthesis, with a direct impact on fruit sugar content (lower Brix), firmness and colour. A strawberry plant under permanent 50% shade produces technically protected but gustatorily inferior fruit. Shading is a temporary peak management measure, not a permanent summer growing practice.
White shading paint on tunnels and glasshouses
White shading paint — a lime or kaolin-based suspension sprayed onto plastic covers — is an economical and reversible thermal solution with growing adoption. Its effect: reducing soil and air temperature under the tunnel by 5 to 15°C by reflecting part of the solar radiation before it enters the structure.
The paint is applied by external spraying onto the plastic covers. It holds for several weeks and can be washed off at the end of the heatwave period. Cost: negligible compared to avoided harvest losses. Its effectiveness is superior to an internally positioned net, as it acts before radiation enters the structure.
Some producers in southern growing regions apply it systematically from mid-June as a preventive measure, then rinse it off in September to regain heat accumulation at the start of autumn.
Tunnel ventilation: the most under-used tool
Managing side vents is the primary cooling lever under tunnel, and often the least well optimised. During a heatwave, full nocturnal ventilation — vents fully open all night — enables effective passive cooling that can lower the internal temperature by 4 to 8°C compared to a closed tunnel. The benefit carries through to mid-morning.
Conversely, closing vents early in the morning (before 9am–10am) retains the night-time cool for a few additional hours before outdoor heat penetrates. This manual or automated vent management is a simple practice but requires rigorous early-morning presence — or investment in thermostat-controlled motorised vents.
For everything related to choosing and configuring growing structures, our article Strawberry tunnels: which structure to choose? covers system comparisons in detail.
Managing the crop calendar: anticipating rather than reacting
Heatwave management is not played out solely in the 48 hours following a weather alert. For a professional grower, the most effective decisions are made upstream — on variety selection, planting windows and harvest organisation.
Everbearing varieties: accepting the summer dip
Everbearing varieties (Charlotte, Mara des Bois, San Andreas, Portola, Albion) produce continuously as long as photoperiod conditions are favourable and temperatures remain moderate. But in the height of summer in warm growing regions, they naturally take a break: flowers abort, runners dominate, and production drops. This is a normal physiological mechanism — not a pathology to correct, but a phase to manage.
In practice, this means avoiding any attempt to force flowering in July–August under heatwave conditions — the plant's resources must be preserved for the September resumption. Irrigation must be maintained at a proper level even during this dip (summer water stress is the enemy of flowering resumption), but nitrogen fertilisation can be lightened to avoid stimulating unnecessary vegetative growth in full heat.
Some producers in Mediterranean growing basins go further: they practise a directed "summer dormancy" — closing tunnels with blackout sheeting in July–August to put plants into semi-forced dormancy, then reopening in September for an autumn production run. This approach requires plants to be in good health at the start of summer.
Non-everbearing varieties and the second cycle: floral induction does not forgive
For non-everbearing varieties in multi-year production, the weeks from late July to early September are critical. This is the floral induction window — the moment when each plant determines the number of flower buds it will form for the following year's harvest.
A plant that has undergone cumulative heat and water stress in July arrives weakened at this window. It induces fewer flowers. It produces less the following spring. And no winter intervention will make up for this deficit.
The technical priority in August is therefore not only to keep plants alive: it is to put them in the best possible vegetative condition for induction — no nutrient deficiency, no residual water stress, no excess nitrogen that would favour vegetative growth at the expense of flower initiation.
Planting windows: avoiding the July heatwave
Planting fresh-rooted runners in summer is a delicate operation even under normal conditions. During a full heatwave, transplant losses can reach 15 to 25% on sensitive varieties such as Gariguette — which is in any case not a variety to manage under summer conditions, as it is structurally suited to early spring production.
The operational rule: avoid planting between 10 July and 5 August in high heatwave risk zones, or shift to cold-stored runners planted later in the season, which tolerate difficult establishment conditions better. Where planting during a heat event cannot be avoided, fine misting over the first few days and temporary shading on young plants are essential to maintain foliar moisture and limit desiccation before rooting.
For a detailed breakdown of planting windows by plant type, our article Fresh runner strawberry: crop management guide covers the specific conduct of the summer establishment period.
Variety sequencing decisions in relation to heat peaks are addressed in our article Staggering strawberry production: varietal calendar.
Harvesting in heatwave conditions: frequency and cold chain
A heatwave does not stop at plant management. It also radically changes harvesting and fruit storage conditions — with direct consequences on the commercial value of the crop.
Daily picking is non-negotiable
Under normal conditions, a picking pass every 2 to 4 days is standard practice depending on the variety. During a heatwave, ripening accelerates dramatically: a fruit at pink stage in the morning can be overripe the following day. Once temperatures consistently exceed 28–30°C, daily picking becomes mandatory — some producers in heated glasshouses pick twice a day at peak.
Harvesting slightly early (fruit just under full colour) can be a salvage strategy: the fruit will continue ripening in cold storage, but will be less fragile in transit than if it had been picked fully ripe at 35°C.
Harvest timing: morning, without exception
Picking in full afternoon heat must be avoided entirely. A fruit picked at 2pm under 36°C arrives in the crate at an internal temperature of 28–30°C — it will take several hours in cold storage to bring it to 4°C, during which gustatory degradation and fungal vulnerability progress.
Harvest in the morning, ideally before 10am, when fruits are still at night-time temperature. This is when firmness is at its maximum, when aromas are best preserved, and when the cold chain can be initiated under the best possible conditions.
Cold chain: zero tolerance during a heatwave
Under normal conditions, strawberries tolerate 2 to 3 hours between picking and cold room. During a heatwave, this window narrows to under one hour. The objective is to reach 2 to 4°C in under 2 hours after picking — which requires having the cold room ready, operational and pre-cooled from early morning, not switched on when the crates arrive.
Forced-air pre-cooling (passing pallets through a cold air tunnel) is the most effective method for large volumes. Without this system, filling crates in thin layers and not stacking more than 3–4 levels high improves cold penetration.
Note: a strawberry picked at 35°C and poorly cooled does not recover its organoleptic qualities in cold storage. Aroma loss and softening are irreversible. The cold chain does not preserve quality — it preserves whatever quality remains at the moment of harvest.
Adaptation comparison by growing system
| Lever | Open field with plastic mulch | Cold tunnel | Off-ground substrate |
|---|---|---|---|
| Irrigation | Daily, extended cycles, trigger at 8–10 kPa | Same + HP misting at T°>28°C | 6–8 short cycles/day, diluted inlet EC, drainage 20–30% |
| Shading | 30% nets on hoops at peaks >33°C | Internal nets or external shading paint | Shading paint on cover or internal nets |
| Ventilation | N/A | Night vents fully open, close at 9–10am | Same + automated temperature sensors |
| Harvest | Daily morning pass | Daily morning pass | Daily morning pass |
| Specific risk | Root burn if bare black film | Amplified greenhouse effect without ventilation | Rapid EC rise if protocol not adjusted |
When the protocol is not enough: the variables that no generic guide can resolve
The rules presented in this article are solid reference points. But in the field, a heatwave rarely strikes on a clean slate — it adds to an already complex situation, and it is the combination of factors that makes each decision unique.
The same 30°C threshold, radically different situations
A 32°C episode during flowering does not have the same impact as an identical episode during full fruit sizing or during the summer vegetative phase. In flowering, pollen sterility is immediate and the loss is direct on the harvest 3–4 weeks ahead. During fruit sizing, it is quality and calibre that collapse. In the summer vegetative phase, a plant in its natural summer dip absorbs the heat better — runner production is already dominant.
Same weather data, different consequences depending on phenological stage — a standard guide cannot make that call without knowing the stage.
Variety makes all the difference
Charlotte and Mara des Bois maintain their cycle better under warm conditions. American day-neutral varieties (San Andreas, Cabrillo, Monterey, Portola) were partly selected for their thermotolerance. In a heat event, these varieties continue producing where a Gariguette has already stopped for several days — and their irrigation thresholds are not calibrated the same way.
Field history recalibrates thresholds
A soil that has been through several consecutive dry summers often has a modified structure — less microbial activity, less organic matter, lower water retention. Soil water tension rises faster after irrigation, and the real trigger threshold must be advanced relative to the standard value. A grower on a new plot and a grower in the fifth year on the same land do not have the same field references.
The Botrytis × heatwave × misting combination
This is one of the most delicate situations. Active Botrytis outbreaks under tunnel at the start of a heatwave episode change the misting decision: running a cooling system too long or too late in the day can aggravate an existing fungal pressure. The trade-off between heat stress and disease risk cannot be resolved by a general rule.
Similarly: should fertirrigation be maintained or reduced if drainage EC climbs to 1.4 mS/cm at peak heat? The answer depends on plant condition, phenological stage, substrate type and inlet water quality. There is no universal answer.
These real-time decisions — on the right information, at the right moment — are precisely what Fraisibot was designed for. Secure your technical decisions under stress conditions: discover all our specialised AI agronomist agents.
Conclusion: heatwave management starts upstream, not in the emergency
Heat stress in strawberry crops is a cumulative phenomenon. Every hour without adapted irrigation, every day without shading, every night at 22°C without ventilation compounds the damage. At 32°C, pollen is already sterile. At 36°C over several days, autumn floral induction is at stake. The useful action window is short.
The tools exist and are well documented — tensiometric scheduling, high-pressure misting, white shading paint, shade nets, adjusted planting windows, early morning harvesting. Their effectiveness depends on coherent combination and activation at the right moment relative to the actual phenological stage of the crop. A blog article on spring frost or a general irrigation guide will never replace a response adapted to your specific situation — your variety, your system, your field history, your 48-hour weather forecast.
Fraisibot, your specialised strawberry AI agronomist, answers your technical questions in real time, at any hour and in any situation. And for climate risk management at the other end of the season, our article Spring frost in strawberry crops: protecting flowering covers the same decision-making stakes in spring.
Access all our specialised agronomist agents: Agronomia — AI agronomic advice 24/7.