Skip to content

A blåbär year is decided before the berries turn blue

7 minJames

Revised 11 August 2026

Four in ten open-pollinated blåbär flowers set fruit in one recent study of Swedish hemi-boreal forest. That is not a poor count made on an unlucky patch. It is a reminder of what the blue carpet in July conceals: a berry is the last surviving part of a much longer sequence.

The plant is Vaccinium myrtillus, the low, angular-stemmed blåbär ris that can make a whole forest floor look continuous and still behave unevenly from one patch to the next. A year with much fruit is not one event, and not simply the consequence of a rainy June or a warm May. The crop has passed through at least four different gates: the formation of shoots and flowers before winter, flowering after winter, pollen transfer while flowers are open, and the development of the fruit that follows.

Each gate is exposed to different weather, at a different date, and sometimes in a different year. That is why an account of a “blåbär year” based on the weather during picking season is usually backwards. By the time the berries colour, much of the decision has already been made.

The crop begins in the previous year’s shoot

A blåbär flower does not appear from nowhere in spring. Vaccinium myrtillus flowers on the previous year’s shoots. This small piece of plant architecture puts a winter between part of the crop’s preparation and the crop that is later counted. The number and condition of those shoots set a ceiling on what spring can make possible; spring cannot turn a stem without flower-bearing capacity into a heavy crop.

Selås, Sønsteby, Heide and Opstad (2015) studied annual growth rhythm and flower formation in V. myrtillus directly, rather than treating the berry count as an immediate response to the weather of the same summer. That is the correct scale of the question. A visible berry crop combines an old structural decision with a new reproductive one.

This also separates two things that are often run together. The extent of blåbär ris is a property of the forest floor built over years. The number of berries on that ris is an annual outcome. Riksskogstaxeringen reported that the national cover of blåbär ris had been broadly unchanged after a decline to 2010, while its annual programme separately counts blåbär and lingon. Stable cover is not stable fruiting. A patch can retain its familiar green floor and carry few berries in a particular year; it can also carry a striking crop without having expanded by a centimetre.

Canopy and stand history belong to this slower clock. Work summarized by SLU finds that blåbär ris in dense, spruce-dominated stands has less above-ground biomass than ris in pine stands. That does not make pine a guarantee and spruce a verdict. It means that the plant entering the next flowering cycle is not independent of the light environment and the history of the stand.

The useful distinction is between capacity and crop. Capacity accumulates in shoots, rhizomes, light and years. Crop is what remains after spring and early summer have had their say.

Anthesis is a short exposed interval

The second clock runs much faster. Once the pale flowers open, the crop meets a brief, exposed interval of anthesis. An open flower can be damaged, fail to receive adequate pollen, or later abort its developing fruit. A large count of flowers is therefore an inventory of potential, not of berries.

Frost is the bluntest example. SMHI calls the isolated frosts that can occur well into early summer järnnätter, while also noting that the old calendar dates attached to them are folklore rather than statistically privileged days. The important fact for blåbär is not the saint’s day or the parish saying. It is whether frost coincides with flowers that are open and vulnerable.

SLU’s old national bluebär observations show the shape of this plainly. In 2016, the researchers judged that ris in southern Sweden had flowered through an early warm period and largely avoided night-frost damage, while later flowering in colder and more elevated ground farther north met less favourable conditions. In 2011, SLU linked frost nights during flowering, and drought during kartbildning, with a low national crop. In 2017, it again described frost during the important flowering period as having killed many flowers in parts of Norrland.

These are observations of particular seasons, not a frost rule that can be carried unchanged into every barrskog. They do establish the mechanism: frost and bloom have to overlap. They also establish the inconvenient local part. A regional weather summary does not tell us the temperature of every flower under every canopy, beside every stone, or across each small change of slope. The 2016 account itself noted large variation within broad regions.

That variation matters because flowering is a moving target. A warm spell can advance it; a cold period can interrupt its progress; plants in the same län need not be at the same stage. Calling a May frost “bad for blåbär” is broadly sensible. Treating it as a complete explanation of a later crop is not.

Pollination is a separate gate

A flower that survives frost has not yet made a berry. Pollen has to arrive in a form and quantity that can do the reproductive work. The forest floor is not silent at this point. Blåbär flowers are visited by bees and hoverflies, and the visitors differ in their abundance and effectiveness.

Bartholomée, Björnberg, Smith and Kendall (2024) tested this in Swedish hemi-boreal forests rather than assuming that all insect visits are equivalent. Their flower-visitor community included several bee and hoverfly taxa. Bumblebees were the most important pollinators in that study because they combined high abundance with high effectiveness; Andrena bees were also effective, while hoverflies contributed to a lesser extent. Most important, the researchers found strong pollen limitation: only 40 percent of their open-pollinated flowers set fruit.

The number is worth keeping in its frame. It is a result from that study, not a national conversion rate from flowers to berries. Its force lies elsewhere. It shows that open flowers can remain unfruitful even in a forest where pollinators are present. A flower count alone is therefore an incomplete measure of the coming crop.

There is a further complication in the word “pollinator activity.” It is tempting to reduce it to a pleasant afternoon. But activity comes from an assemblage, not a single switch. Naturvårdsverket stresses that pollinator groups differ in the times and weather conditions in which they are active, and that their differences can complement one another. In spring, a bumblebee colony is also at a particular stage: the overwintered queen begins the colony before workers later appear. The overlap between bluebär bloom and this changing insect community is biological timing, not merely air temperature.

Jacquemart’s three-year study from the Belgian Ardennes found that supplemental hand-pollination increased both fruit and seed set in V. myrtillus, significantly so in one year, and identified unfavourable weather including frost as a possible stronger cause of low fruit and seed set in the two species considered. It is valuable evidence for the separation of pollen and weather effects. It is not Swedish barrskog. The forest, latitude and weather regime are wrong for turning its result into a number for Småland or Västerbotten.

Fruit development carries spring forward

After successful pollen transfer, the plant still has to carry kart to a ripe berry. This is the part of the story that makes a good flowering display an unreliable early verdict. SLU’s forest-phenology dataset follows flowering, kartbildning and berry ripening in bluebär and lingon, alongside temperature sums derived from climate stations at each observation site. The design itself acknowledges the sequence: the relevant observations are not collapsed into one midsummer count.

Drought can matter here without having been the reason for a rich or poor flowering. In its 2023 national observations, Finland’s Naturresursinstitut described both weakened pollination after cold early summer and frost nights, and a separate effect of prolonged drought: berries became smaller and some were aborted. The institute also found unusually large local and regional variation in pollination outcome. Finland is not Sweden, but it is a nearby boreal case that illustrates why the word “weather” is too coarse. Cold nights, insect flight conditions and water during fruit development are not interchangeable explanations.

The lag is therefore long in two senses. There is the carry-over from the preceding season’s shoots to spring flowers. Then there is the passage from flower to kart to ripe fruit. A report made during bloom can say something about that stage of the chain; it cannot honestly settle every later stage. A count of ripe berries settles the final outcome for the measured place and year, but not which single link was decisive.

This is also why annual totals should be read cautiously. Riksskogstaxeringen’s field work is national and systematic, but it is a sample. Its annual design includes roughly 12,000 field plots and the flora and fauna inventory counts blåbär and lingon. That makes it valuable for describing broad patterns and their uncertainty. It does not make a national average a description of one familiar slope.

A blåbär year is a chain rather than a verdict

Settled: the crop is not set at ripening. Vaccinium myrtillus carries the previous season into the next through its shoots; open flowers face frost risk; the Swedish field experiment shows that fruit set is strongly constrained by pollen transfer; and developing fruit can still be lost or reduced after flowering. The best evidence treats these as distinct stages rather than asking which one weather variable “makes” a bluebär year.

Also settled: large-scale records and local ground answer different questions. SLU’s phenology observations, the flower–kart–berry measurements used in its former berry forecasts, and the permanent work of Riksskogstaxeringen are serious records. They show real variation and can reveal broad associations. None replaces repeated observation on a particular patch.

Not settled: the relative weight of the gates in every Swedish forest type and every year. The strongest direct pollination experiment is Swedish hemi-boreal forest, but it does not provide a national fruit-set constant. Jacquemart’s pollen study is from Belgium. Finland’s drought-and-frost account is close in climate but still a different monitoring system and country. The flower-formation study is directly on the species, but no one study reduces the previous summer, winter snow, spring frost, insects and summer water to one portable rule.

A bluebär year is thus not a signal that arrives in July. It is a record, finally visible in July, of conditions that began before the previous summer had properly ended. That is a better explanation for the old experience of a familiar patch: the ris can look much the same from one year to the next, while the crop does not.

Sources