During cell expansion, kiwifruit size is built with water. A deficit here is never recovered.
In kiwifruit, size does not come back. During the cell-expansion stage, the gain in fruit size is driven mainly by water, not carbon: cells at that point have an increased capacity for expansion and water uptake, and what does not go in during those weeks will not go in later. The trials that imposed deficits across different stages say it plainly: a deficit here causes severe reductions in fruit volume, weight, yield and water productivity, while the same deficits before or after do not carry that cost (Agricultural Water Management, 2023).
It is awkward news, because it lands in exactly the month when water is most expensive and the temptation to stretch the intervals is strongest. The operational answer is not “irrigate more”. It is to move the saving to where it does no damage.
Why is kiwifruit size a question of water rather than sugars?
Kiwifruit carries large fruit on a very dense canopy with high water use. During cell expansion the fruit behaves, in simple terms, like a reservoir: it grows by filling. Work on Actinidia deliciosa showed that surplus water, CPPU and girdling — alone or combined — increased fruit water content more than fruit dry matter. In that window, the size gain is a water gain.
That has a practical consequence which runs against instinct. A larger fruit produced in this stage is not automatically a better fruit in dry-matter terms — and dry matter is what many payment schemes reward. Size and quality are built in different stages, and they need different irrigation strategies.
Where the saving belongs, stage by stage
The sharpest evidence comes from a rain-shelter drip study that imposed deficits from 15% to 45% across four fruit development stages. In summary:
| Stage | Deficit imposed | Effect |
|---|---|---|
| Stage I and II (early) | up to -25% | Fruit volume and weight increased; no impact on yield, water productivity or chemical quality |
| Stage III (cell expansion) | any | Improves firmness, soluble solids and acidity, but severe reduction in volume, weight, yield and water productivity |
| Stage IV (final) | up to -25% | Improves firmness, soluble solids, acidity and vitamin C without compromising volume, weight and yield |
The recommended strategy that comes out of it is clear: 25% deficit in stages I, II and IV, full irrigation through stage III. That is precisely the opposite of what happens when rationing is decided by the water authority’s schedule rather than by vine physiology.
A second result confirms it from another angle: deficit treatments during the expansion stage impaired yield and water productivity simultaneously. In other words, the water was not even saved efficiently — both sides lost.
Is irrigating to the maximum the safe option?
No, and this is the second counter-intuitive point. In a trial that used sap flow to compare irrigation levels, mean fruit diameter was significantly higher at 68% of crop evapotranspiration than at 100%, and higher than at 57% and 40% as well (Scientia Horticulturae, 2025).
The curve is not monotonic: a mild deficit can beat full irrigation, while moderate-to-severe deficit costs size. Excess water is not neutral either — it feeds vegetative vigour, competition between canopy and fruit, and a microclimate that on a pergola system is already humid, with everything that implies for the main kiwifruit diseases.
Two observations from the same trial are worth carrying into the orchard:
- Fruit refilling happens at night, when competition with leaf transpiration is lower. Which is why a warm, breezy night is not equivalent to a cool one in recovery terms.
- Above 2.0 kPa vapour pressure deficit, stomata close regardless of irrigation level. In those hours, adding water does not buy transpiration: the vine has already decided to protect itself.
How to find the right point without trial and error
The ideal plan — full through stage III, controlled deficit elsewhere — requires knowing two things that growers often lack precisely: what stage the block is actually in today, and how hard the vine is really working.
Phenological stage is estimated through fruit growth checks, and many operations already do that. The second figure is the one that is missing. Soil water content tells you what is in the ground, not what the vine manages to take out of it: with root competition, compaction or high atmospheric demand, a soil that reads “fine” coexists comfortably with a vine in stomatal closure. It is the same misunderstanding that makes reasoning purely in terms of wilting point and available soil water unreliable.
Sap flow measured in the trunk answers the right question: how much water is actually moving through the vine, hour by hour, against how much the atmosphere is asking for. It is the measurement that let the trials above separate 68% from 100% of evapotranspiration, and on farm it is what tells you whether the deficit you planned is the deficit you have. A sap flow sensor measures exactly that, continuously, on individual vines.
The plan for the rest of the season
- Place the block on the phenological scale. While the fruit is in cell expansion, the word “saving” leaves the irrigation vocabulary.
- Move the deficit to the final stage. That is where it pays: firmness, soluble solids, acidity and vitamin C all improve at no cost in weight or yield.
- Apply the pre-harvest deficit early enough. The general rule is several weeks before picking: in trials on a yellow-fleshed cultivar, a three-week suspension during ripening delivered +20% water productivity and up to +2.0 °Brix with no loss of firmness or yield (Plants, 2025).
- Do not chase VPD peaks. In the hours when stomata are shut, extra water does not go in: concentrate application when the vine can use it.
- Verify on the vine, not on the controller. A schedule set in March and never touched describes March’s intentions, not the August you are living through.
Frequently asked questions
When does kiwifruit need water most? During the fruit expansion stage driven by cell enlargement, which is when size is physically built. Trials show that a deficit imposed at this stage severely reduces fruit volume, weight, yield and water productivity, while the same deficits in earlier or later stages do not carry that cost.
Is deficit irrigation always harmful in kiwifruit? No — it depends on the stage. Moderate deficits in the early stages and in the final stage improve firmness, soluble solids, titratable acidity and vitamin C without compromising weight or yield. It is the deficit during full cell expansion that costs size, and that size never comes back.
Does a pre-harvest deficit really improve quality? In trials on a yellow-fleshed cultivar, suspending irrigation entirely for three weeks during ripening raised water productivity by up to 20% and soluble solids by up to 2.0 °Brix without compromising firmness or yield. The practical rule is that a pre-harvest deficit must be applied several weeks ahead of picking.
Does maximum irrigation give the biggest fruit? Not necessarily. In sap-flux work, mean fruit diameter was significantly higher at 68% of crop evapotranspiration than at 100%: the response is not linear. A mild deficit can beat full irrigation, while moderate to severe deficit costs size.
How do you know when kiwifruit vines close their stomata? The signal is sap flow decoupling from atmospheric demand: the day climbs but the vine does not transpire in proportion. The same work observed that above 2.0 kPa vapour pressure deficit, stomatal closure occurs regardless of irrigation treatment, and that fruit refilling starts at night, when competition for water is lower.
Sources
- Deficit drip irrigation improves kiwifruit quality and water productivity under rain-shelter cultivation in the humid area of South China — Agricultural Water Management
- Kiwifruit ecophysiological adaptations under moderate and severe deficit irrigation — Scientia Horticulturae
- Late Water Deficits Improve Intrinsic Water Use Efficiency, Fruit Maturity, and Acceptability in Yellow-Fleshed Kiwifruit cv. Soreli — Plants
- Effects of irrigation and fertilization management on kiwifruit yield, water use efficiency and quality in China: A meta-analysis — Frontiers in Plant Science
