Mid August, and the pergola is sagging. Eight thin bunches last year, forty this year. Ask around and everyone offers the same explanation for why your grapevine has so many grapes: the plant is suffering. Poor soil, no water, no food. It senses the end and throws everything into one last round of offspring.
Good story. It is wrong.
Why does my grapevine have so many grapes?
The cause sits in last year’s weather, not this year’s.
A grapevine lays down this season’s bunches during the summer before. Inside the buds ripening on your vine last year, inflorescence primordia formed, the microscopic precursors of the bunches you are looking at now. How many of them appear depends mostly on how much light and warmth those buds received in that earlier summer. A good summer banks next year’s crop before winter even starts.
In the Netherlands, the summer of 2025 was exactly that. KNMI recorded 743 hours of sunshine against a 641 average, 18.5 degrees against roughly 17.5, and 159 millimetres of rain against 224. The fourth warmest summer since 1901. Textbook conditions for pushing bud fruitfulness up.
Then spring 2026 added a second bonus. With 728 sunshine hours against 567 normal, it ranked among the five warmest springs since 1901, and June averaged 19.1 degrees in De Bilt against 16.2 normal. Warm, bright conditions around flowering help pollination and fruit set. The potential was already there, and the weather let it through.
Does stress really make a vine set more fruit?
As a general rule, no. The research points consistently the other way.
Real drought, nitrogen shortage or a depleted carbohydrate reserve give a vine fewer fruitful buds, poorer flowering, fewer berries and smaller ones. In a two-season study, Guilpart and colleagues found that water and nitrogen status around flowering in year one explained roughly 65 to 70 percent of the yield variation in year two, and it did so through fewer clusters per shoot and fewer berries per bunch. Defoliation trials, which strip a vine of its carbon supply artificially, have produced up to about 50 percent fewer inflorescences the following season.
The underlying idea does exist in biology. Terminal investment says an organism with little future left may commit a larger share of what remains to reproduction. A larger share is not a larger number. Rowan trees declining toward death produced on average 19 percent fewer fruits in their final year than comparable survivors. Koenig and colleagues followed over a thousand Californian oaks for up to 37 years, 70 of which died natural deaths, and found no significant difference in acorn production among the dying trees. What difference there was pointed toward less reproductive investment. Eziz and colleagues, pooling 164 drought studies, found allocation to roots rising while allocation to leaves, stems and reproduction fell, most steeply in woody plants.
For the grapevine specifically, no convincing evidence supports a panic crop. And the myth runs into a plain mechanical problem: this year’s bunches were initiated last year. A vine that goes thirsty in June cannot decide to manufacture twenty extra clusters.
Where the myth comes from
The observation is real. The explanation attached to it is not.
| What you see | What is happening |
|---|---|
| A dry, unfed vine hangs full | Shoot growth collapses faster than the crop that was already initiated. The ratio of fruit to leaf rises, the berry count does not |
| A slightly less vigorous vine outfruits a rampant one | Less vigour opens the canopy, so buds catch more sun. That is balance, not hunger |
| Bunches look denser under drought | Water deficit shrinks the berry before it reduces berry number. Tighter reads as more |
| A neglected vine explodes after a hard prune | The buds were already there. Pruning concentrated stored reserves into fewer shoots |
Too much nitrogen backfires as well. The vine builds leaf, the canopy closes, buds get less light and fruitfulness drops. The response curve is a hill, with an optimum somewhere in the middle and losses on either side of it.
So what does decide the number of bunches?
Five factors, roughly in order of weight.
Last season’s weather. The single largest one. Cornell and the Australian AWRI both tie bud fruitfulness directly to shoot light exposure and temperature in the preceding year. Buttrose demonstrated it under controlled conditions back in 1969.
Weather during flowering. Warmth within a reasonable band helps pollen tube growth. Cold, rain, high humidity and extreme heat all suppress set.
Pruning. Pruning does not create bunches, it selects them. You decide how many already formed buds stay on the vine. Prune hard and you get fewer fruiting shoots and less crop, but stronger shoots with sometimes larger bunches. Prune lightly and you retain more buds, though the vine compensates with poorer budbreak and weaker shoots.
Age and health. A young vine has thin reserves in trunk and roots and carries a heavy crop badly. Once mature, age itself says little. In a Californian Zinfandel comparison, vines aged 40 to 60 actually gave 3.7 kilos and 22.8 clusters more per vine than vines aged 5 to 12. That comparison is not clean, though. The old vines were own-rooted while the young ones were grafted, and they carried roughly three times as many arms and spurs. Age is confounded with training and rootstock there. What it does show is that “old vines crop less” rests on no more evidence than “stressed vines crop more”.
Alternate bearing. In apple, olive, citrus and pistachio, a heavy year genuinely suppresses flower induction for the next one through hormonal signals from the seeds. The grapevine is far less prone to it, because it renews its fruiting shoots every year and you can steer the load by pruning. Skipping a year in a vine usually follows overcropping, disease or leaf loss, so a loss of carbon source rather than a large crop as such.
One thing that does carry over: a light year caused by frost or poor set leaves plenty of reserves and well exposed wood behind, which can produce a big crop the year after. That is carbon accounting, not a death rattle.
A big crop is not a good crop
A garden vine in the Netherlands gets a limited number of sunshine hours to push sugar into its berries. Spread that across forty bunches instead of twenty and everything ripens halfway.
Belgian fruit growing extension puts the practical ceiling for a mature garden vine at roughly 20 to 25 bunches. Above that you get shatter, where berries drop anyway, and poor colouring. In commercial viticulture the same idea is called vine balance, yield set against pruning weight. Too much fruit on too little leaf gives sour berries that colour badly and ripen too late.
In practice, if your vine is overloaded right now:
- Thin down to around 20 to 25 bunches, taking out whatever sits worst in the light.
- Trim the bunches you keep, cutting out the smallest and laggard berries so the rest has room.
- Remove leaves around the bunches, but only as many as needed. Sun on the berry is good, sunburn is not.
- Save hard pruning for winter, between mid November and early January. Avoid the window between mid January and April, when the sap starts moving and the cut bleeds. That is rarely fatal on a healthy mature vine, but it costs sap and vigour, and you can easily avoid it.
The idea that a big harvest signals quality is stubborn in the wine world too. The relationship between vintage, yield and quality is messier than the confident narratives suggest, which you can see in Champagne as clearly as in Limburg. What warming actually does to ripeness, and why that is not simply good news, I set out in Champagne at 14% ABV.
Which animals are grapes dangerous for?
This belongs in any piece about a loaded vine, because the bunches hang at dog height and the fallen berries collect underneath.
Dogs: treat any ingestion as an emergency
Grapes, raisins, currants and sultanas can cause acute kidney injury in dogs, and it can be fatal. There is no known safe dose. Some dogs eat a handful without consequence, others become seriously ill on a small amount, and you cannot tell in advance which dog you have.
Since 2021 there has been a strong suspect for the toxic principle: tartaric acid and its potassium salt, potassium bitartrate. The ASPCA Animal Poison Control Center made the connection after dogs developed the same kidney injury from tamarind and from cream of tartar, with tartaric acid as the common factor. Its content varies widely by grape variety, ripeness and processing, which would explain why one berry is far more dangerous than another. It is not proven. The Merck Veterinary Manual offers a rough risk indication of more than one grape or raisin per 4.5 kilos of body weight, which flags risk rather than defining a safe limit.
The typical course:
- Within 6 to 12 hours: vomiting, sometimes diarrhoea. The Dutch LICG uses a wider window, up to 24 hours.
- After that: lethargy, loss of appetite, abdominal pain, dehydration, excessive thirst.
- Within 24 to 72 hours: kidney damage. Little or no urine production is a bad sign.
A dog still bouncing around after three hours is not in the clear. The damage arrives later.
Call your own vet or the emergency clinic straight away after any ingestion, even with no symptoms. Give the dog’s weight, what and how much, what time, and whether it has vomited. Do not induce vomiting yourself with salt, oil or hydrogen peroxide, since those cause damage of their own. The vet decides on emesis, activated charcoal and intravenous fluids. In the Netherlands they can consult the national poisons information centre, the NVIC. Calling remains useful hours later, because fruit can still be sitting in the stomach. This guide does not diagnose at a distance, so call even if you think it is minor.
Cats and ferrets: likely risk, thin evidence
We know less about cats and ferrets than the warnings imply. The Merck Veterinary Manual records that reports of renal failure in cats and ferrets are anecdotal and that no published case reports exist for those species, though its broader food-hazards chapter does mention renal failure in one cat. The Dutch NVIC states that acute kidney failure is seen mainly in dogs and rarely in cats. The Dutch pet information body LICG puts it at “appear to be susceptible” as well, and adds that why grapes are toxic remains unknown.
That is precautionary advice, not an established syndrome. Not feeding them grapes remains sensible, and a cat or ferret showing symptoms after ingestion belongs at the vet. But anyone claiming grapes are as toxic to cats as to dogs is going further than the sources allow.
Animals we know little about
Restraint fits here. The characteristic syndrome of grape-induced acute kidney injury has been described mainly in dogs. For birds, chickens, horses, rabbits and ruminants it has never been demonstrated, but it has barely been studied either, and those are not the same thing. The Dutch NVIC says in as many words that whether the risk extends to parrots is unknown. Watching blackbirds strip your crop is no cause for alarm. But anyone calling grapes proven safe for those animals is mistaking an empty research shelf for a clean bill of health.
The caveats still apply. Whole berries are a choking hazard for small birds and young animals, so cut or crush them. Grapes and especially raisins are concentrated sugar, which is the real objection for rabbits and for horses with insulin dysregulation or laminitis risk. Rotten and mouldy fruit carries mycotoxins, a risk entirely separate from the grape debate. And what you spray on the vine can be more dangerous than the berry.
Fermentation deserves its own warning. Fallen grapes fermenting on a warm day produce ethanol, and ethanol poisoning in dogs brings sedation, incoordination, hypothermia and low blood sugar. How much alcohol a pile of fermenting garden fruit actually delivers has not been studied, so treat this as reasonable caution rather than a measured risk. Clearing it up takes two minutes.
For leaves, tendrils and prunings there is no comparable case literature. That means “not demonstrated” rather than “safe”, since splinters, obstruction and spray residue remain real. If you have a dog, clear fallen fruit daily. Nothing else is needed.
What to do with a glut
Making wine from garden grapes rarely works in a Dutch summer. Garden varieties like Boskoop Glory and Muscat Bleu were selected for fungal resistance and eating quality rather than sugar, and without heavy chaptalisation you end up with something thinner and sharper than you want to drink. Plan on juice, jam or vinegar rather than a bottle. Unripe grapes are not waste either. Verjus, the juice of unripe grapes, is an excellent kitchen acid and a serious product category in its own right, as I described in Cul Sec 2025. What Dutch producers manage to pull out of these short, cool summers is worth seeing at Domein Holset and Zavel.
Frequently asked questions
Does watering a grapevine less give you more grapes?
No. Water deficit lowers bud fruitfulness for next year and shrinks this year’s berries. What you see is leaf growth collapsing faster than the crop, which skews the ratio and makes the vine look fuller.
Does a bumper year mean a poor one next season?
Not automatically. True alternate bearing is typical of apple, olive and pistachio, much less so of the grapevine, which grows new fruiting shoots every year. What does depress the following crop is loss of leaf area through disease, defoliation or overzealous trimming, because that costs reserves.
How many bunches should I leave on a garden vine?
Belgian fruit growing extension puts it at 20 to 25 for a mature plant. That is a practical rule of thumb rather than a biological constant, since the right number depends on variety, vigour and available leaf area. Go well past it and everything ripens slower and colours worse. Thin in June or July, when the berries are roughly pea sized.
Are a few grapes safe for my dog?
You cannot assume so. No safe dose has been established, and sensitivity varies by dog and by grape. Call your vet after any ingestion, even if the dog seems fine.
Are raisins worse than fresh grapes?
They are certainly more concentrated, and raisins, currants and sultanas generate most of the veterinary poison calls. The Dutch NVIC logged 154 veterinary consultations in 2024 for foods containing raisins or currants against 86 for grapes. Treat both as dangerous.
Sources
Vine physiology
- Guilpart, Metay & Gary (2014), “Grapevine bud fertility and number of berries per bunch are determined by water and nitrogen stress around flowering”, European Journal of Agronomy: doi.org/10.1016/j.eja.2013.11.002
- Carmona et al. (2008), “A molecular genetic perspective of reproductive development in grapevine”, Journal of Experimental Botany: academic.oup.com
- Lebon et al. (2008), “Sugars and flowering in the grapevine”, Journal of Experimental Botany: academic.oup.com
- Bennett, Jarvis, Creasy & Trought (2005), “Influence of defoliation on overwintering carbohydrate reserves, return bloom, and yield of mature Chardonnay grapevines”, AJEV 56(4):386-393: ajevonline.org
- Buttrose (1969), light and temperature effects on bud fruitfulness: journals.uchicago.edu
- Cornell CALS, “Grapes 101: Bud Fruitfulness and Yield”: cals.cornell.edu
- UC Davis, “Flowering and Fruiting of Grapevine”: wineserver.ucdavis.edu
- AWRI, “Understanding Grapevine Yield”: awri.com.au
- Oregon State Extension, “Understanding Vine Balance”: extension.oregonstate.edu
- Riffle, Palmer, Casassa & Dodson Peterson (2022), “Vine Age Affects Vine Performance, Grape and Wine Chemical and Sensory Composition of cv. Zinfandel from California”, AJEV 73(4):277: ajevonline.org
Stress response and alternate bearing
- Takeno (2016), “Stress-induced flowering: the third category of flowering response”: pubmed.ncbi.nlm.nih.gov
- Eziz, Yan, Tian, Han, Tang & Fang (2017), “Drought effect on plant biomass allocation: a meta-analysis”, Ecology and Evolution 7(24):11002-11010: pubmed.ncbi.nlm.nih.gov
- Declining fruit production before death in rowan, Annals of Forest Science (2018): annforsci.biomedcentral.com
- Koenig, Knops, Carmen & Pesendorfer (2017), “Testing the Terminal Investment Hypothesis in California Oaks”, The American Naturalist 189(5):564-569: pubmed.ncbi.nlm.nih.gov
- Martínez-Lüscher & Kurtural (2021), source-sink carry-over: pmc.ncbi.nlm.nih.gov
Dutch weather data
- KNMI, “Zomer 2025”: knmi.nl
- KNMI, “Lente 2026”: knmi.nl
- KNMI, “Mei 2026”: knmi.nl
- KNMI, June 2026 monthly report: knmi.nl
Grapes and animals
- ASPCA Animal Poison Control, “Toxic component of grapes and raisins identified”: aspcapro.org
- MSD/Merck Veterinary Manual, “Grape, Raisin, and Tamarind Toxicosis in Dogs”: merckvetmanual.com
- Cornell Riney Canine Health Center, “Grape and Raisin Toxicity”: vet.cornell.edu
- Wegenast et al. (2022), tartaric acid as suspected toxic principle, JAVMA: pubmed.ncbi.nlm.nih.gov
- NVIC (UMC Utrecht), “Vergiftigingen door voedsel”, 2024 figures: nvic.umcutrecht.nl
- LICG, “Praktisch: vergiftiging bij huisdieren”: licg.nl
Growing
- fruitABC, grape growing guide, on bunch numbers per mature vine: fruitabc.be
Last verified 21 August 2026. This guide gives general information and does not replace veterinary advice. If you suspect poisoning, call your own vet or an emergency clinic.