The ‘Second’ Allergy Season: Why Autumn Brings Weed Pollen & Spores

Most hay fever advice focuses on the grass pollen peak between May and July — and for good reason, since grass pollen is the most common trigger of seasonal allergic rhinitis in the UK. But for a significant number of people, symptoms return (or never fully resolve) from late August into October and beyond. If you experience hay fever in September, sneezing through the early autumn, or worsening nasal and eye symptoms as the weather turns damp and cool, you may be reacting to a different set of triggers altogether: weed pollens, mould spores, and a rising tide of indoor allergens. This guide explains what drives the UK's ‘second’ allergy season, how to distinguish allergic symptoms from an autumn cold, what a targeted testing strategy looks like, and how to reduce your exposure at home during the months most allergy resources forget about.

Why Autumn Can Feel Like a Second Allergy Season in the UK

The UK pollen calendar does not end in July. While grass pollen counts drop sharply after mid-summer, three categories of aeroallergen rise or persist into autumn — creating what many patients and clinicians informally call a second allergy season:

• Weed pollens. Nettle (Urtica), dock (Rumex), and mugwort (Artemisia vulgaris) are common UK weeds whose pollen seasons extend from June into September. Mugwort, in particular, peaks in late July through September and is a well-recognised cause of late-summer rhinitis and cross-reactive food symptoms (Allergy UK, 2025).
• Mould spores. Outdoor mould counts — especially Alternaria alternata and Cladosporium herbarum — rise during the warm, damp conditions of late summer and autumn. The mould spore season in the UK typically runs from July through to October, with a peak in September and early October, particularly after periods of rain followed by warmth. Fallen leaves, compost heaps, and agricultural stubble all contribute to spore release (NHS, 2025).
• Indoor allergens. As temperatures drop, people close windows, turn on central heating, and spend more time indoors. This increases exposure to house dust mite allergen (which accumulates during the warm, humid summer months), pet dander, and indoor mould — particularly in poorly ventilated bathrooms, kitchens, and bedrooms. The result can be a seamless transition from outdoor pollen symptoms to indoor perennial symptoms, making it feel as though hay fever never ended (Allergy UK, 2025).

For patients with perennial rhinitis — year-round nasal symptoms driven by non-seasonal triggers — autumn often marks the point where symptoms shift or intensify, rather than a distinct new ‘season.’ Understanding which triggers are responsible requires careful attention to timing and, in many cases, specific IgE testing to identify the relevant sensitisations.

Key Triggers: Weeds, Mould, and Changing Indoor Exposure

Weed Pollens: Nettle, Mugwort, and Ragweed

In the UK, the most significant late-season weed pollens are nettle and mugwort. Both produce lightweight pollen that can travel considerable distances in the air. Mugwort is of particular clinical interest because of its association with cross-reactive food allergies — a phenomenon sometimes referred to as the mugwort–celery–spice syndrome, where sensitisation to mugwort pollen proteins is associated with oral symptoms when eating celery, carrots, coriander, cumin, and certain other foods (Allergy UK, 2025).

Ragweed allergy UK: ragweed (Ambrosia artemisiifolia) is currently uncommon as a wild plant in the UK, but isolated populations exist — particularly in southern England — and climate projections suggest its range is likely to expand over the coming decades. Ragweed pollen is one of the most potent aeroallergens globally, with a season running from August to October. UK patients who spend time in continental Europe (where ragweed is much more prevalent, especially in Hungary, France, and northern Italy) or North America may become sensitised. If you experience autumn allergies that seem disproportionate to the UK weed pollen count, ragweed sensitisation is worth considering — particularly if you have a travel history to affected regions (NHS, 2025).

Mould Spores: The Invisible Autumn Trigger

Moulds reproduce by releasing microscopic spores into the air. In the UK, outdoor mould spore counts peak in late summer and autumn — driven by damp conditions, decaying vegetation, and agricultural activity. The two outdoor genera most commonly associated with respiratory allergy are:

• Alternaria alternata — considered the most clinically significant outdoor mould allergen in the UK. Sensitisation to Alternaria is associated with both allergic rhinitis and asthma, and has been linked to severe asthma exacerbations in sensitised individuals (BSACI, 2024).
• Cladosporium herbarum — one of the most abundant airborne moulds worldwide. It is found on decaying plant material, soil, and textiles. Cladosporium counts can be extremely high during warm, damp days in late summer and early autumn.

Indoors, Aspergillus and Penicillium species are the primary mould allergens. These tend to be present year-round but become more problematic in autumn and winter when homes are sealed against the cold, humidity rises from drying clothes, cooking, and showering, and ventilation decreases. Visible black mould in bathrooms and around window frames is a common sign of elevated indoor mould exposure.

Back-to-School Indoor Triggers

September marks a significant change in indoor exposure patterns for families. Children return to classrooms where dust mite allergen, pet dander (carried on clothing from homes with animals), and mould can be present at higher levels than in a well-managed home environment. The so-called ‘September asthma epidemic’ — a well-documented spike in asthma-related hospital admissions in the UK during the second and third weeks of September — is thought to result from the convergence of increased allergen exposure, circulating respiratory viruses, and reduced adherence to preventer medications during the summer holidays (NHS, 2025; BSACI, 2024).

Symptoms and Overlap With Colds

One of the most common challenges in autumn is distinguishing allergic rhinitis from a viral upper respiratory tract infection — the common cold. Both can cause sneezing, nasal congestion, a runny nose, and fatigue. However, several features can help separate the two:

If your autumn symptoms include prominent itching, last longer than two weeks, recur at the same time each year, or worsen in specific environments (outdoors on damp days, or indoors in dusty or poorly ventilated rooms), an allergic component is worth investigating. Your GP or pharmacist can advise on initial management, and a specific IgE blood test can help identify or exclude sensitisation to the relevant aeroallergens (NHS, 2025).

Testing Strategy: What to Include in an Autumn Allergen Profile

If you suspect that your autumn symptoms are allergic in origin, a targeted specific IgE blood test can provide objective laboratory data on your sensitisation profile. A well-chosen autumn panel might include:

• Weed pollens: mugwort (Artemisia vulgaris), nettle (Urtica dioica), plantain (Plantago lanceolata), and — where clinically relevant — ragweed (Ambrosia artemisiifolia)
• Mould species: Alternaria alternata, Cladosporium herbarum, Aspergillus fumigatus, and Penicillium chrysogenum (covering both outdoor and indoor moulds)
• House dust mite: Dermatophagoides pteronyssinus and Dermatophagoides farinae — the two most common dust mite species in UK homes
• Pet dander: cat (Fel d 1) and dog (Can f 1) epithelial allergens — relevant if you share living space with pets or visit homes with animals
• Grass pollen: even if you think your main issue is autumn triggers, including timothy grass (Phleum pratense) can help confirm or exclude grass pollen as a contributor — particularly if your symptoms begin in late summer and blur into autumn

Some patients may also benefit from a total IgE measurement, which gives an overall picture of IgE production. While a high total IgE can be associated with atopic conditions, it is not diagnostic on its own — its main value is providing context alongside specific IgE results (BSACI, 2024).

If you are considering a broad allergy blood test panel, look for one that includes both seasonal and perennial aeroallergens. This helps distinguish between true seasonal hay fever (grass and weed pollens), mould-driven symptoms, and perennial rhinitis caused by year-round indoor triggers — a distinction that significantly affects management.

Home Strategies for Autumn Allergens

Once you know — or strongly suspect — which allergens are driving your symptoms, you can focus avoidance strategies where they are most likely to help. The following measures are supported by UK allergy guidance and are practical for most households (Allergy UK, 2025; NHS, 2025):

Reducing Mould Exposure

• Keep indoor humidity below 50–60% — use a hygrometer to monitor levels. A dehumidifier may be helpful in particularly damp rooms.
• Ventilate kitchens and bathrooms by opening windows or using extractor fans during and after cooking, showering, and drying clothes.
• Avoid drying clothes on indoor radiators — this releases significant moisture into the air. Use a tumble dryer with external venting, or dry clothes outdoors when conditions allow.
• Remove visible mould with a damp cloth and appropriate mould cleaner. Address the underlying damp cause (condensation, leaks, poor insulation) where possible.
• Clear fallen leaves from around your home promptly — decomposing leaves are a significant source of mould spores.

Managing Dust Mite Exposure

• Wash bedding at 60°C or above weekly — this kills dust mites and removes accumulated allergen. Lower temperatures clean the bedding but do not kill the mites.
• Consider allergen-proof (encasing) covers for pillows, duvets, and mattresses. These create a barrier between you and the reservoir of dust mite allergen in bedding.
• Vacuum regularly using a vacuum cleaner with a HEPA filter — this reduces airborne allergen during and after vacuuming compared to standard filters.
• Reduce soft furnishings in bedrooms where practical: hard flooring produces less dust mite habitat than carpet, and minimising cushions and soft toys reduces allergen reservoirs.

Reducing Weed Pollen Exposure

• Check the pollen forecast — the Met Office provides daily pollen counts that include weed pollen data from late summer onwards.
• Shower and change clothes after spending time outdoors on high-count days — pollen can cling to hair, skin, and fabric.
• Keep windows closed during peak pollen hours (typically morning and early evening) and when mowing or disturbing vegetation.
• Wear wraparound sunglasses outdoors to reduce pollen contact with the eyes.
• If you are sensitised to dust mites as well as pollen, combining outdoor and indoor avoidance strategies is particularly important in autumn, when both triggers may be active simultaneously.

Where Diagnostic Testing Helps

A specific IgE blood test does not diagnose allergy on its own — but it provides objective laboratory data that, when interpreted alongside your clinical history by a qualified clinician, can significantly improve the focus and effectiveness of your management plan.

What Testing Can Show

• Sensitisation to specific allergens. A positive specific IgE result (reported in kU/L) indicates that your immune system has produced IgE antibodies against that particular allergen. This is evidence of sensitisation — a necessary condition for IgE-mediated allergy, though sensitisation alone does not confirm clinical allergy.
• Pattern identification. Testing across a panel of aeroallergens can reveal whether your sensitisation profile fits a seasonal pattern (e.g., weed pollen only), a perennial pattern (e.g., dust mite and pet dander), or a mixed profile — which is common in patients with autumn symptoms.
• Ruling out specific triggers. A negative result makes IgE-mediated allergy to that allergen less likely. This can be just as valuable as a positive result — helping you and your clinician narrow the list of triggers and avoid unnecessary avoidance measures.

What Testing Cannot Show

• It cannot predict the severity of a future reaction. IgE levels correlate with the probability of clinical allergy, but not reliably with how severe a reaction might be.
• It cannot detect non-IgE-mediated mechanisms, irritant responses, or conditions such as vasomotor rhinitis where no allergen-specific immune response is involved.
• It cannot replace a clinical consultation. The test report provides the laboratory data; your GP or allergy specialist provides the clinical interpretation, diagnosis, and management plan.

Questions to Ask After You Receive Your Report

When you take your results to your GP or allergy specialist, consider asking:

• “Do my positive results match the timing and pattern of my symptoms?”
• “Are any of these sensitisations likely to be cross-reactive rather than primary allergens?”
• “Based on these results and my history, would you recommend any changes to my current treatment?”
• “Should I consider a referral for specialist allergy assessment or immunotherapy?”
• “Are there specific avoidance measures I should prioritise based on which allergens showed the highest levels?”

Glossary

Specific IgE

Immunoglobulin E antibodies directed against a specific allergen. Measured in a blood sample and reported in kU/L. Indicates sensitisation to that allergen.

kU/L (kilo units per litre)

The standard unit used to report specific IgE levels. Higher values generally indicate greater sensitisation, but clinical significance depends on symptom history and clinical context.

Sensitisation

The presence of IgE antibodies against an allergen in the blood. Indicates the immune system has responded to that allergen, but does not automatically confirm clinical allergy.

Cross-reactivity

When IgE antibodies produced against one allergen also recognise a structurally similar protein from a different source — for example, mugwort pollen and celery.

Perennial rhinitis

Year-round nasal inflammation caused by non-seasonal triggers such as dust mite, pet dander, or indoor mould — in contrast to seasonal rhinitis driven by pollen.

Aeroallergen

An airborne substance capable of triggering an allergic response — including pollens, mould spores, dust mite particles, and animal dander.

Frequently Asked Questions

Summary

The UK allergy season does not end in July. Weed pollens — including mugwort, nettle, and in some cases ragweed — continue into September. Mould spore counts peak in the warm, damp conditions of late summer and early autumn. And as people move indoors and close windows, exposure to house dust mite, pet dander, and indoor mould increases. For many patients, this combination creates a ‘second’ allergy season that is just as troublesome as the grass-pollen peak — but far less widely discussed.

If your symptoms persist or return in autumn, a targeted specific IgE blood test can provide objective data on which allergens you are sensitised to. This information — interpreted by your GP or allergy specialist alongside your clinical history — can help focus your avoidance strategies, guide treatment discussions, and clarify whether your symptoms are seasonal, perennial, or a mix of both.

Sources

• NHS — Hay fever overview, allergic rhinitis, and mould allergy patient information (2025): nhs.uk/conditions/hay-fever
• NHS — Anaphylaxis overview and when to call 999 (2025): nhs.uk/conditions/anaphylaxis
• Allergy UK — Pollen calendar, mould allergy factsheet, dust mite guidance, and mugwort cross-reactivity information (2025): allergyuk.org
• BSACI — Guidelines on the management of allergic rhinitis and mould sensitisation in asthma (2024): bsaci.org
• NICE — Allergic rhinitis management guidance and asthma quality standards (2024): nice.org.uk
• Met Office — UK pollen forecast methodology and weed pollen monitoring (2025): metoffice.gov.uk