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Midsummer rain is not an autumn verdict

7 minJames

Revised 11 augusti 2026

A wet Midsummer is easy to remember. The table is moved under a roof, the birch leaves drip, and a few months later the forest is generous enough that the rain acquires an explanation. The dry Midsummer that preceded a thin autumn is remembered too. A rule has formed: wet Midsummer, good autumn.

It is not a foolish rule. It has named two things that plainly belong in the same story: water in summer and fruit bodies in autumn. But it compresses a landscape, a season and several kinds of fungus into one remembered day. The question is not whether rain is relevant. It is what must happen between a shower around the solstice and an autumn fruiting for the saying to carry more than a good memory.

Midsummer is a date with weather on both sides

Midsummer is not a stable weather event. Since 1953, midsommarafton has fallen on a Friday between 19 and 25 June. SMHI describes skurar as a classic feature of the holiday, but also records sharp contrasts inside the same weekend and across the country. On midsommarafton 2023, for example, heavy rain fell in Norrbotten while conditions were more stable in the south. That is not a technical objection. It is the centre of the problem. A saying about Sweden has to survive rain falling in one län and not another.

A rain gauge records precipitation at its station. SMHI makes hourly, daily and monthly precipitation observations available, and its gridded daily precipitation record extends from 1961. Those are strong materials for asking what fell at a given place and time. They do not turn a Midsummer shower into a measurement of the water available to a fungal mycelium later in the season. Between gauge and forest floor lie canopy, soil, slope, drainage, roots and the weather of July and August.

Nor is a wet day necessarily a wet June. A month can end near its normal total after one violent åskväder and many dry days; another can be steadily wet without providing a memorable afternoon. Sweden’s precipitation is especially variable in summer, when short convective showers can put much water on a small area. The folk rule chooses the memorable part of a month. The mechanism, if there is one, has to work with accumulated water and its persistence.

Rain has to become stored water

The necessary mechanism is ordinary hydrology. Rain has to reach the ground, enter a soil volume that can retain it, and remain available despite runoff, plant use and evaporation. SMHI writes the water balance as precipitation, runoff, evapotranspiration and change in storage. Rain that runs down a track or is held on needles and evaporates has still made Midsummer wet; it has not necessarily altered the later water balance of the soil.

Barrskog makes the distinction particularly plain. Some rain is intercepted by crowns and returns directly to the air. SMHI notes that in dense conifer forest roughly a third, and in some cases as much as half, may be caught by trees and evaporate. The arrangement of rain matters too: many brief showers create more interception loss than the same quantity falling in fewer long spells. Meanwhile evaporation is generally greatest in summer, when energy from the sun is available and vegetation is active. A warm, windy fortnight after the holiday can spend a good deal of what the rain supplied.

So the useful variable is not rain alone but water balance over time. SMHI’s drought material makes the same point in a different language: soil moisture is governed by precipitation, evaporation and the soil’s capacity to hold water. A precipitation index can describe whether rain was unusually abundant, yet cannot on its own account for evaporation. That is why a national millimetre rule would be false economy. The same amount has a different meaning on a thin, freely drained ridge above Skäralid than in a sheltered hollow with deeper, finer soil.

The Swedish evidence for this variation is unusually good at the terrain scale, though it does not report a sensor reading from every spot. SLU’s Markfuktighetskarta models long-run wetness from terrain, climate and soil information, trained on Riksskogstaxeringen field data. It distinguishes places expected to stay wet for much of the year from ground whose wetness changes more readily with weather. That is habitat context, not a report that a particular patch is moist today. It nevertheless explains why the same Midsummer rain can be retained in one piece of ground and forgotten in another.

Fruiting has a calendar and a crop

“Good autumn” conceals two different observations. One is phenology: when fruit bodies appear. The other is productivity: how many species or fruit bodies are recorded, or how much biomass is produced. They need not move together. An autumn can begin late and still be substantial; it can begin early and be brief.

The long Swiss forest-plot study by Straatsma, Ayer and Egli is the source most often invoked for the weather part of this argument, and it deserves its full shape. Across 21 years of repeated observation, productivity was correlated with precipitation accumulated from June through October, while the timing of appearance was associated with summer temperatures. That result gives Midsummer a place, but a modest one: June belongs to a multi-month water window. It does not single out the solstice, and it is from a Swiss plot rather than Swedish barrskog.

Büntgen, Kauserud and colleagues likewise connect precipitation amounts and temperature means to fungal activity, productivity and timing. Their point is not that one replaces the other. Moisture and temperature act together, and their relative importance changes with place, season and the fungi being counted. In a boreal mixed forest in eastern Canada, Pinna and colleagues made weekly surveys precisely because fructification phenology cannot be reduced to a calendar date. Krebs and colleagues found evidence that above-ground mushroom production in the southwestern Yukon responds to rainfall. These are useful northern comparisons, but Yukon and eastern Canada are not Uppland or Skåne. Forest composition, soils and the annual course of heat and water are part of the result.

The rule is strongest when spoken this way: early-summer rain can be one contribution to the conditions from which autumn fruiting later emerges. It becomes weak when “can be” turns into a calendar guarantee, or when fruiting time is confused with the size of the crop.

The Nordic phenology record does not isolate the holiday

The best-known Nordic phenology study, Kauserud et al. (2008), used roughly 34,500 dated herbarium records to examine autumnal fruiting in Norway from 1940 to 2006. It found substantial change in the timing of fruiting and related the work to monthly temperature and precipitation anomalies across Norway’s main regions. The scale matters. It is good evidence that climate and season have moved the fungal calendar. It is not a test of rain during a single Swedish holiday against the abundance of a later autumn.

Andrew et al. (2018) took a still broader European view, relating fruiting phenology to geographic and climatic variation. Koskinen et al. (2023), working at a Norwegian research station, examined how fruiting fungal communities respond to climatic factors and forest-habitat characteristics, including whether related groups respond similarly. Both approaches point away from one universal weather rule and toward responses structured by species, habitat and climate. They are more faithful to the ground than a single national saying, even if they are less satisfying around a coffee table.

Swedish precipitation records strengthen the objection to the calendar, not because they show rain irrelevant, but because they show why a day is a poor summary. SMHI’s national climate indicator is built from more than 1,200 time series and reports seasonal and annual precipitation for Sweden as a whole. Its own method notes the work needed to handle station changes, gaps and local differences. That is appropriate for describing a national climate signal. A fruiting test needs something else: local rain and temperature series paired with repeated autumn observations from fixed forest plots, with habitat and survey effort recorded.

The national record also shows that Sweden is not becoming a single, simply wetter mushroom country. Compared with 1961–1990, summer precipitation has generally increased in the 1991–2020 normal period, while autumn precipitation changes are small and go in both directions depending on region. A wet Midsummer and a wet autumn are therefore not interchangeable categories. Nor does a wetter seasonal mean tell us how water was distributed among downpours, dry spells and warm winds.

The rule can be tested without being turned into a forecast

A fair test would begin by refusing the vague word “good.” It would specify a response: perhaps weekly fruit-body counts in permanent plots, separated by ecological group, from late summer through autumn. It would pair those records with nearby precipitation and temperature observations, and where possible with a soil-moisture measure or a carefully described soil-water model. It would then ask whether rain during the Midsummer window retains any association with the autumn response after July and August weather, site wetness, forest type and observer effort are included.

The test has a clear failure condition. If the apparent Midsummer effect disappears once later summer water balance and habitat are accounted for, the saying is a compressed description of a longer season, not an independent trigger. If it remains across repeated years and comparable Swedish forest plots, it has earned a narrower claim. Even then it would describe a condition that has sometimes mattered in those plots, not a promise about what will be found in any autumn.

Settled: water availability and temperature influence fungal fruiting; precipitation must be understood alongside evaporation and stored soil water; and Sweden’s Midsummer weather is highly variable in time and space. The phenology literature supports all three. Not settled: a published Swedish test showing that rain specifically around Midsummer, rather than the rest of summer’s water balance, independently explains an autumn crop. I could find no such test. Until it exists, the old rule is worth keeping as a question asked of the whole summer, not as an answer delivered in June.

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