The short answer
June and September. June is when grass pollen peaks and overlaps with lingering tree pollen. September is ragweed season, and Ontario gets some of the highest ragweed concentrations in Canada.
The full calendar
Ontario’s allergy season for dogs runs from April through October, with three distinct waves:
April to May is tree pollen. Birch, poplar, alder, ash, and maple all release pollen in this window. Dogs with tree pollen sensitivities will start itching as soon as temperatures stay above 10 to 15 degrees consistently. Symptoms tend to be moderate compared to what comes next.
June to July is grass pollen. This is when many dogs hit their worst stretch. Grass pollen counts peak in June, and if your dog is also sensitive to trees, there’s a nasty overlap in early June where both are still circulating. Dogs who are allergic to grass get a double hit from both airborne pollen and direct skin contact.
August to October is ragweed. Southern Ontario and Quebec get hammered by ragweed, which starts in late July and peaks in September. Ragweed pollen can travel hundreds of kilometres on the wind, so even urban areas like Oakville with less ragweed growing locally still get high counts. Mould spores also rise in this period, adding another trigger.
The in-between months
November through March gives most allergic dogs a break. Freezing temperatures kill off pollen sources, and snow covers mould. But some dogs react to indoor allergens like dust mites, which are a year-round problem. If your dog itches just as badly in January as in June, the issue might be food-related instead.
Why this matters for treatment timing
If you know your dog’s allergy pattern, you can start treatment before symptoms hit. A dog that always flares up in June should start Apoquel or get a Cytopoint injection in late May. Waiting until they’re already miserable means playing catch-up.
Your vet can help you map out a seasonal plan based on your dog’s specific triggers. Allergy testing can identify exactly what your dog reacts to if you want to get precise about it.
And since Ontario’s pollen season overlaps almost perfectly with tick season, spring is a good time to make sure both allergy and parasite prevention are sorted.
Key takeaways
- June (grass pollen peak) and September (ragweed peak) are the worst months for allergic dogs in Ontario.
- The full allergy season runs April through October, with three overlapping waves.
- Ragweed in southern Ontario is some of the worst in Canada.
- Indoor allergens like dust mites can cause year-round symptoms that mimic seasonal patterns.
- Starting treatment before the bad months hit is more effective than reacting once symptoms are in full swing.
References
- Kleenex Canada. “Pollen Calendar: Types of Pollen by Month.” kleenex.com
- The Weather Network. “Pollen Report: Toronto.” theweathernetwork.com
- CBC News. “The Four Seasons of Hay Fever.” cbc.ca
- CP24. “Ontario Lab Predicts ‘Intense’ Allergy Season for Toronto.” cp24.com
- PetMD. “Dog Seasonal Allergies: Causes, Symptoms, and Treatment.” petmd.com