September 2026 was drier and thirstier than September 2025 across the Central Valley. On the blocks we read in both years, almond trees finished the month carrying less stress than they did a year ago.
We read 294 California blocks through September, 8,311 block-days across almonds, wine grapes, prunes and a few other crops. Sacramento Valley almonds spent the month coming down from August. The regional average moved from −6.1 Δbar in the last week of August to −2.1 at the end of September, and the share of blocks in high stress or beyond fell from a third to under a fifth.
North Coast vineyards stayed tighter all month and reacted hard to the one real heat wave, in the week of September 7. Almonds took the same jump in demand and kept relaxing.
How to read the numbers
Stem water potential is the tension a tree’s water column is under at midday, measured in bars. A tree with all the water it can use reads close to its fully watered baseline, which is what a well-watered tree of that crop would read under the same day’s heat, sun and humidity.
Every number below except apple and citrus is a Δbar, the distance between what a tree measured and that fully watered baseline. Zero means the tree kept up with the day. −8 means it finished eight bars short of a fully watered tree. Because the day’s weather is already inside the baseline, a block in June can be compared with the same block in September. We walk through this in more detail in How to Read a Stem Water Potential Number.
The bands are our published scale. No stress above −2, minimal to mild from −2 to −6, moderate from −6 to −8, and high stress or beyond at −8 and drier.
The almond network
Every figure aggregates block averages. A block is the unit a valve actually controls.
| Region | Blocks | September avg | Median | Driest tenth | Wettest tenth | No stress | High stress or beyond |
|---|---|---|---|---|---|---|---|
| Sacramento Valley | 133 | −3.9 | −3.0 | −11.5 | +1.5 | 42% | 22% |
| Northern San Joaquin | 67 | −3.7 | −2.5 | −9.2 | +1.3 | 46% | 12% |
Both regions averaged under four bars below a fully watered tree for the month. In August the Sacramento Valley averaged −7.7, so September roughly halved it. The spread stayed wide. The wettest tenth of Sacramento Valley blocks read at or slightly above the baseline, and the driest tenth averaged −11.5 Δbar. About a fifth of the region’s blocks (22%) still spent September in high stress or beyond.
Our Southern San Joaquin almond coverage is too thin to publish as a regional reading. Those blocks read close to baseline for the month.
Week by week in the Sacramento Valley
| Week of | Almond (Δbar) | High stress or beyond | Reference ET (mm/day) | Average high |
|---|---|---|---|---|
| August 24 | −6.1 | 33% | 7.71 | 93°F |
| August 31 | −4.6 | 26% | 4.66 | 78°F |
| September 7 | −4.0 | 26% | 7.05 | 92°F |
| September 14 | −3.4 | 16% | 5.47 | 85°F |
| September 21 | −2.5 | 12% | 4.94 | 85°F |
| September 28–30 | −2.1 | 18% | 6.87 | 91°F |
The trees came down in one direction for four straight weeks while the weather swung back and forth over them. The week of September 7 brought the month’s heat. Reference ET jumped from 4.66 to 7.05 mm/day and afternoon highs went from 78°F to 92°F. Almond blocks kept easing right through it, from −4.6 to −4.0 Δbar, and the high-stress share held at 26%.
The heat returned for the last three days of the month. This time the high-stress share ticked up, from 12% to 18%, while the regional average kept improving. Some blocks moved back into high stress when the demand came back.
We can’t yet say from our own records how much of the recovery is post-harvest irrigation. Stage tags are grower-maintained and they lag. 119 of the 133 Sacramento Valley blocks still carry the Hull Split tag set in mid-August. Pump sensors show that about 60% of Sacramento Valley almond blocks with a pump sensor ran water in each of the middle weeks of September, close to last September’s share, with a median run of 20 to 24 hours per block that ran.
The vineyards felt the heat week
Our 56 North Coast wine grape blocks averaged −5.0 Δbar for September, median −4.7, with 25% of blocks in high stress or beyond and 20% at no stress. They ran tighter than the almonds all month.
| Week of | North Coast wine grape (Δbar) | High stress or beyond | Reference ET (mm/day) |
|---|---|---|---|
| August 24 | −5.5 | 24% | 4.50 |
| August 31 | −4.8 | 13% | 3.14 |
| September 7 | −5.7 | 32% | 5.13 |
| September 14 | −4.8 | 27% | 3.42 |
| September 21 | −4.3 | 21% | 3.46 |
| September 28–30 | −4.9 | 29% | 4.15 |
The same heat wave reached the North Coast. Reference ET there rose from 3.14 to 5.13 mm/day, and the share of vineyard blocks in high stress or beyond went from 13% to 32% in a single week, the highest of the month. Growers answered it. Two-thirds of North Coast vineyard blocks with a pump sensor ran water that week (67%, up from 51% the week before), and the stress still climbed. It took two cooler weeks to work back down to 21%.
By the last week of September only a quarter of those blocks ran water (25%), as harvest moved through the region, and the late heat pushed the high-stress share back up to 29%. Fewer blocks reported in those last three days, so read that final row as a direction.
The same blocks, one year apart
Year-over-year numbers mislead when the network grows, so this table holds only blocks we read in both Septembers.
| Crop and region | Blocks | Sept 2025 | Sept 2026 | High stress or beyond, 2025 → 2026 |
|---|---|---|---|---|
| Almond, Sacramento Valley | 63 | −4.3 | −2.9 | 29% → 13% |
| Almond, Northern San Joaquin | 28 | −5.2 | −2.2 | 21% → 7% |
| Wine grape, North Coast | 23 | −6.4 | −4.5 | 30% → 17% |
| Prune, Sacramento Valley | 17 | −2.4 | −2.3 | 0% → 0% |
The weather pointed the other way. The same weather stations, compared month to month across the two years, recorded a September with more evaporative demand and almost no rain.
| Same stations, September | Reference ET 2025 → 2026 (mm/day) | Average high 2025 → 2026 | Rain 2025 → 2026 |
|---|---|---|---|
| Sacramento Valley (78 stations) | 5.16 → 5.68 | 90.4°F → 85.6°F | 26.4 mm → 0.4 mm |
| Northern San Joaquin (33 stations) | 4.97 → 5.27 | 89.4°F → 85.8°F | 3.8 mm → 0.2 mm |
| North Coast (19 stations) | 3.72 → 3.90 | 84.2°F → 82.0°F | 10.9 mm → 4.9 mm |
In the Sacramento Valley that is 10% more evaporative demand than last September, and close to no rain against about an inch last year, most of it on September 9 and 10. Afternoons ran about five degrees cooler, the same cooler and thirstier pattern we measured in August. By the weather alone, September 2026 should have been the harder month on the trees.
On the blocks we read in both years it was the easier one. The share of Sacramento Valley almond blocks in high stress or beyond fell from 29% to 13%, the Northern San Joaquin from 21% to 7%, and North Coast vineyards from 30% to 17%. Prunes held level.
Whatever made the difference happened inside the orchards. Harvest timing and the water that went back on afterward are the likely places to look, and our stage tags are too far behind to separate the two. The pump sensors give one hint. In the weeks of September 13 and 20, Northern San Joaquin almond blocks that ran water ran it for a median of 24 and 20 hours, against 13 and 10 hours in the same weeks last year. That is pump run time across the network and says nothing directly about how much water reached the root zone, but it moves in the same direction as the trees.
A schedule built on reference ET alone would have planned for a harder September than last year on these blocks.
Prune
Eighteen Sacramento Valley prune blocks averaged −2.1 Δbar for September, median −1.4. Over half (56%) spent the month at no stress, and none averaged high stress for the month. In August this fleet was split down the middle, with a third of blocks at no stress and 39% in high stress or beyond. September closed that gap as the dry blocks came back toward baseline.
Apple, olive, citrus
Our California coverage of these crops is too thin this month to publish a network reading. Our apple network is concentrated in New York, and this report covers California.
What we are watching in October
October opens warm. The forecast through October 11 has highs in the low 90s across the Central Valley and no rain. Reference ET runs near 4.9 mm/day in the Sacramento Valley, 5.3 in the Northern San Joaquin, 6.1 in the Southern San Joaquin and 4.1 on the North Coast.
For almond and prune blocks, that is late-season demand on trees that still carry their leaves and are setting up next year’s buds. We went through the published research on that window in our post on post-harvest irrigation. The blocks to watch are the ones that slipped back into high stress in the warm last days of September, 18% of Sacramento Valley almond blocks in that final week.
On the North Coast, harvest is moving through, and the early October warmth lands on the blocks still hanging fruit. Those were the blocks pushing the high-stress share back up as September closed.
FloraPulse measures stem water potential continuously from inside the trunk, block by block. See what California orchards and vineyards are reading this week.
About the data: Data from the FloraPulse sensor network, 294 California blocks and 8,311 block-days, September 1–30 2026, from continuous microtensiometer measurements inside the trunk. Block averages only, anonymized and aggregated by region. North Coast covers Napa and Sonoma and is reported separately from the Central Valley. Weekly rows run Monday to Sunday. Year-over-year comparisons use only blocks, and only weather stations, reporting in both Septembers. Pump figures are pump run time and do not measure water applied. Learn more at florapulse.com.