Low fruitfulness and poor fruit set...why choose just one?
As the tallies are coming in this harvest, one thing across the board seems to be clear: there just isn’t very much fruit out there. Our own management company, which operates predominantly in West Sonoma, is clocking in around 50% normal yield. While that number is a little extreme compared to other areas, low yield seems to be a common refrain up and down the state.
How do we get to final yield?
In order to get to the bottom of this, I want to break up yield formation into three chapters: fruitfulness, fruit set, and sizing. Let’s work backwards in time:
Sizing: Beginning with berry sizing, sometimes you get through fruit set only to have berries put on minimal weight. Excessive water stress, heat events or periods of high evaporative demand (VPD, vapor pressure deficit) can reduce vascular water potential needed for berry growth and/or increase berry water loss from sizing fruit during the period from fruit set to lag phase. This is the most sensitive period for berry growth and hence for poor fruit sizing when conditions aren’t favorable. After veraison, these conditions can shut down or reduce sugar loading and lead to smaller berries, though the effect on final berry size is less pronounced than during initial berry growth.
Fruit set: Backing up a developmental step, prior to berry growth, poor fruit set or shatter is of course a regular occurrence these days, especially with budbreak occurring earlier and earlier in recent years. This year, shatter was prevalent in California due to a remarkably early budbreak and a chilly April and May.
The problem with 2026 wasn’t only poor fruit set, but rather poor fruitfulness to a larger degree.
Fruitfulness: Taking another step back in time, fruitfulness is basically what you start with. At flowering the year prior to any given season, future buds are already differentiating into cluster primordia, although the extent of differentiation is still debated. If you are concerned about fruitfulness for the coming year, as soon as canes are lignified you can send your buds in to be dissected and quantified for number of clusters and general size. Excessive shading or stress during bloom in year one can limit your fruitfulness in year two. I believe that the low yield we saw in 2022 was due to drought conditions in the Spring of 2021, although once everything fried on the vines that September, no one cared to quantify the total loss resulting from low fruitfulness.
The number of flowers on each cluster, however, is determined during the current season’s spring during budbreak and period of rapid shoot growth. This is where we first started to notice that something was skimpy about 2026. Bloom was popping, but with very few flowers per cluster.
Weirdly warm weather
Backing up a little, I’ve been following temperature data closely this year mostly to try to make sense of why budbreak was two-plus weeks earlier than usual this year. Typical explanations seem to fail me. 2024 had very warm soil temperatures and a fairly early budbreak. 2026 was still earlier. I’d always heard that warm soil temperatures kicked off an early budbreak but across the board, I’m not seeing anything distinctive about this year’s soil temps. 2020 and 2021 also had warm soil temperatures, but these were also drought years evidenced by their volatility (water conducts heat better than air). I would think that you need both warmth and heat, however nothing about this data seems to be a slam dunk.
Average air temperature also didn’t seem to bring about anything noticeable about 2026, as evidenced by this mess of a graph below. Although this big spike here post-budbreak caught my attention. And here’s where I think the low fruitfulness came from.
Some Background
The reason being that high heat during budbreak and the period of active shoot growth prior to bloom has been shown to cause reductions in yield. Research out of WSU in 2010 subjected breaking buds to cool, ambient (control), warm, and hot temperatures over the course of three years. For the hot treatment, buds were heated with a continuous-flow of warm air, topping out at 40°C (104°F). The warm treatment was kept halfway between the hot treatment and ambient temperature. Flowers per cluster on the basal cluster were reduced by 28% in the hot treatment and 20% in the warm treatment[i]. The apical cluster was found to be largely the same across treatments, so this behavior seems to affect primarily basal cluster position. This may explain some of the wonky uniformity that characterized 2026.
The warm and hot treatment also caused shoots to grow more rapidly, which is said to limit fruitfulness due to an increased competition for assimilates early in the growing season.[ii] During this period, shoot tips are a much stronger sink than developing inflorescences, so if warm temperatures speed up shoot growth and hence the need for nutrients and carbohydrates at the shoot tip, you may then see fewer flowers at bloom.
In fact, Gibberellic acid, which stimulates shoot elongation and is upregulated during these warmer periods of rapid growth, is applied as an early spray in some table grape varieties in order to thin flower numbers and allow for sparse clusters with heavier berries to form.[iii]
Occam’s razor
And this all tracks with what we saw early this year.
Phenology is all about accumulation. The Winkler growing degree days (GDD) starts at April 1st by definition. This year given that we were already well into bloom by April 1st, I ran the accumulation from January 1st to track how heat units accumulated during the winter. The results are staggering.
Since I don’t have great data about exact timing of budbreak across all the sites I work with, I broke this accumulation out by months. Almost all sites showed 50% to 100% more heat accumulation in February leading to an early budbreak. Then March saw 300% more heat accumulation than any other year since 2020. This second warm spell is what caused rapid shoot growth and limitations to flower development from the start. Here are some sites in the North Coast:
And here are some sites from Napa:
And Paso Robles:
Oddly enough, places very close to the ocean saw markedly higher heat accumulation this winter, which is odd as coastal regions are usually more buffered than inland areas. Edna Valley for instance shows the starkest increase of all the locations I polled for this post. Morro Bay, although not a wine region per se, saw double the amount of heat accumulation in each month from January to July. Temperatures at Point Lobos (likewise not a wine region but very coastal) also showed very high heat accumulation during the winter months. I’d be curious to see how this affected Avocado and strawberry growers, which are cultivated along these marine corridors.
Shattering the bed
After a warm February and very warm March, we got a very cold April, with average temperatures barely climbing above 50°F in some locations. You can read more on that here if you’d like. Long story short, if you happened to go into bloom with full inflorescences, you probably didn’t go into May with much of anything. All this makes for small potatoes in 2026.
A warning shot?
This coming winter we are supposed to be experiencing a super El Niño with warming ocean temperatures. Given that we just saw warm coastal temperatures and what they potentially did to this year’s yield, I’m inclined to recommend more generous pruning. Leave more buds to counteract the reduced fruitfulness and poor uniformity. If we’re going to get another hot and vigorous spring, we may see 2026 all over again in 2027.
[i] KELLER, M., TARARA, J.M. and MILLS, L.J. (2010), Spring temperatures alter reproductive development in grapevines. Australian Journal of Grape and Wine Research, 16: 445-454.
[ii] Bowen, P. A., & Kliewer, W. M. (1990). Relationships Between the Yield and Vegetative Characteristics of Individual Shoots ofCabernet Sauvignon’Grapevines. Journal of the American Society for Horticultural Science, 115(4), 534-539.
[iii] Dokoozlian, N. K., & Peacock, W. L. (2001). Gibberellic acid applied at bloom reduces fruit set and improves size of’Crimson Seedless’ table grapes. HortScience, 36(4), 706-709.
Data from Western Weather Group and Davis Instruments Weatherlink
