Alder (Alnus) is the first tree to release pollen into the air in Slovenia — often in the middle of winter, while snow still covers the ground and every other tree is bare. For sensitive people this means the pollen season actually starts months earlier than most expect: the first symptoms can appear as early as January, when no one is thinking about pollen at all.
What alder is and where it grows
The most common species here is black alder (Alnus glutinosa), which grows near water — along streams, rivers, and on damp soil — and the somewhat rarer gray alder (Alnus incana), more common in mountain and foothill areas. Alder belongs to the birch family (Betulaceae), together with birch, hazel, and hornbeam, with which it shares related allergenic proteins.
Alder is wind-pollinated: it releases pollen from catkins that form in late autumn and open on the first mild days, before the tree grows leaves. Because there are hardly any other pollen sources at this time of year, wind carries alder pollen over long distances, and concentrations can be very high locally, right along the watercourses where alder likes to grow.
Season and distribution in Slovenia
Alder in Slovenia flowers from January to March, and in mild winters flowering can start as early as late December. Because the start of flowering is closely tied to weather, it can shift by a few weeks from year to year.
Since alder thrives best near water, pollen concentrations are highest in the Ljubljana basin and along major rivers such as the Ljubljanica, Sava, and Drava, as well as in the pre-Alpine foothills in the north of the country, where the soil stays damp all winter. Alder often flowers at the same time as hazel or just before it, while birch follows later, typically from April onward.
Modeled data for Slovenian cities
For Slovenian cities, pelud.net obtains alder pollen data from the European CAMS model (via Open-Meteo), not from measurements at a local monitoring station as is done in Croatia. The level shown is therefore a modeled estimate, calculated from weather and atmospheric data, rather than a direct count of pollen grains in the air. The estimate is a useful and generally reliable guide for planning your day, but it is not equivalent to a measured value. Official pollen monitoring in Slovenia is carried out by NLZOH. For more on how we calculate the risk estimate, see the article How We Calculate the Risk.
Symptoms
- watery discharge and a blocked nose
- sneezing, often in bouts
- itchy, watery eyes
- itching of the palate and throat
- worsening asthma in sensitive individuals
Because alder flowers in the middle of winter, many people mistake the first symptoms for a cold. The key difference is that allergy symptoms don't clear up after a few days — they persist for as long as alder is flowering.
Cross-reactivity with birch and food
Alder's major allergen (Aln g 1) is closely related to birch's major allergen (Bet v 1) — both belong to the PR-10 protein group. Cross-sensitivity between alder and birch is therefore almost the rule: people allergic to birch pollen frequently react to alder as well, and vice versa. This relatedness also extends to other trees in the order Fagales, mainly hazel and hornbeam.
Because of this relatedness, some sensitive individuals also develop oral allergy syndrome — itching or mild swelling of the mouth after eating apple, hazelnut, or stone fruit such as peach, cherry, or plum. Reactions are usually mild, and cooking or baking generally destroys the proteins responsible, which is why cooked or baked fruit is often better tolerated than fresh. In especially sensitive people reactions can be more pronounced, so if cross-reactivity is suspected, testing by an allergist is worthwhile.
Treatment
Because alder flowers so early, many people overlook it and start treatment too late.
- Second-generation antihistamines help with sneezing, itching, and discharge; they work best when started before the season peaks.
- Nasal corticosteroid sprays work best when started as early as December, ahead of the expected start of flowering.
- Eye drops help with allergic conjunctivitis.
- For asthma, bronchodilators and inhaled corticosteroids are important, used as directed by a doctor.
Specific immunotherapy (SCIT or SLIT) is, because of the close relatedness to birch, often targeted at the birch allergen as the leading representative of the Fagales family, which indirectly covers alder as well. An allergist decides on suitability and choice of treatment based on testing. Treatment lasts several years and typically starts outside the season.
Practical tips for sensitive people
- Follow the daily pollen estimate on pelud.net — data for Slovenian cities is updated several times a day.
- Because the season starts in winter, don't wait for the first warm days to begin preventive measures (sprays, antihistamines).
- Ventilate during hours with lower concentrations, and avoid spending long periods right next to watercourses during the flowering period.
- After spending extended time outdoors, wash your face and hair and change clothes to reduce how much pollen you bring indoors.
- For more general advice on reducing exposure, see the article How to Protect Yourself from Pollen.
Note
This is not medical advice. To assess your own sensitivity to alder, birch, or related allergens, and to choose the right treatment, consult your doctor or an allergist.