Hay fever is caused when the body creates allergic antibodies to certain allergens like tree pollen, grass pollen and weed pollen. They all produce microscopic particles of pollen that's easily carried around by the wind. That ease is why pollen ends up being all around us in the summer months. Flowers are also an allergen, though their heavy, sticky pollen is a lot more difficult to move around, making them a much rarer allergen.
Paying attention to the time or frequency of your symptoms is a good way of figuring out what causes your hay fever. The most common allergen is grass pollen and those affected notice symptoms between May and July. Tree pollen season is between February and June, while weed pollen causes symptoms between June and September. Those that notice hay fever symptoms all year-round are usually affected by indoor allergens, such as house dust mites, pets or mould. Pets can very easily bring all kinds of pollen inside, so that might be worth bearing in mind when noting down the timing of your symptoms.