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Hayfever is largely seasonal, based on the allergen that triggers it, but there will usually be pollen in the atmosphere before and after this period. Generally this means late Spring through to late Summer or early Autumn. The "hayfever season" varies depending upon which country you are in. In Britain (the Northern Hemisphere), the main hayfever season runs from the about the beginning of June until mid-July, or even early August. In New Zealand (the Southern Hemisphere and about halfway around the world from Britain), the seasons are sometimes not as distinct and do not feature the great variations in temperature that you get in some other countries, making it harder to define a "hayfever season". Typically this includes Spring and Summer for New Zealand and runs from September to February. Hayfever sufferers gradually learn what constitutes their personal "hayfever season" and are able to make appropriate preparations for it with antihistamines. The incidence of hayfever generally subsides with the onset of colder weather. In the Northern Hemisphere, pollen is usually released by trees first, then grasses, and then weeds, with mould being most prevalent during the Winter months. Summer is the only season that typically has triggers for all four of these major allergen groups. Perennial allergic rhinitis sufferers, on the other hand, tend to have their condition triggered by indoor allergens so they may suffer throughout the year. The four major outdoor allergen groups are:

• Trees - tend to flower in Spring time with the increase in temperature so spring is the season when people in the US experience hayfever triggered by the pollen of trees and shrubs. There is some overlap into Summer because of tree species like the pine. While pine trees may produce a prolific amount of pollen, the proteins involved are much less allergenic than, for example, ragwood, which produces comparatively less pollen. Pine tree pollen is also heavier, meaning that it tends to be distributed straight down rather than scattering widely. In the Eastern and Midwestern United States, people with allergies to tree pollen suffer most during early Spring when birch, elm, maple, and poplar produce pollen. Other trees that provoke allergies are the hardwood deciduous trees like the oak, ash, hazel, elm, pecan, hickory, box alder and the mountain cedar that produce allergenic pollen. In the United Kingdom, it is estimated that between 15% and 25% of people with hayfever are allergic to birch pollen.
• Grass - about half of all hayfever sufferers have a sensitivity to grass pollen, which is probably not surprising when you consider that this is where hayfever originally got its name. The white tip at the end of green grass is the flower that produces the pollen. The longer the grass, the more pollen is produced so frequently cutting your lawns can prevent the flower heads from forming and pollinating. During July and August of the Northern Hemisphere Summer, grasses are a major irritant. Even at the beginning of September, some types of grass can still cause problems, where the duration of their period of pollination is dependent on the weather. Each type of grass has its own flowering period and these can often overlap. Of the more than 1200 species of grass that grow in North America, the most allergenic pollen comes from species such as the Kentucky bluegrass, Johnson grass, Timothy grass, Bermuda grass, Redtop grass, Sweet Vernal grass and Orchard grass. In the United Kingdom, there are many species of grass that can cause trigger the allergies of hayfever sufferers, including Timothy, Cocksfoot, Fescue, Meadow, and Rye grass.
• Weeds - usually flourish in the US from mid-Summer to Autumn (Fall), although this duration depends on the weather to some extent. Plantain is often the first weed to appear, with ragweed following in the first or second week of August. Ragweed is a hairy, coarse-looking plant with unattractive flowers that derives its name from the ragged appearance of its leaves. A native of North America, it also grows on every other continent except Antarctica. The ragweed is a common cause of hayfever in the US and has a long season that includes parts of Autumn, during which it produces its potent aero-allergens. Once the temperature at night drops and more wind and rain is generated, ragweed pollen begins to diminish. This is significant because it has been estimated that as many as 75% of hayfever sufferers are sensitive to ragweed pollen. In North America, other weeds like the Sagebrush, Redroot Pigweed, Russian Thistle (tumbleweed), Lamb's Quarter and English Plantain produce allergenic pollen causing problems for hayfever sufferers. In the United Kingdom, other weeds such as nettles, dock, and Mugwort can also trigger hayfever.
• Mould - the "mould season" in the US can last from Spring to late Autumn, peaking from July to late August as more mould and fungus spores become airborne. In Autumn, the increased moisture from rain and the drop in temperature increases the growth of mould and fungi. Even though the Winter snow will lower the outdoor mould count, some moulds thrive on the vegetation that has died. As the leaves on the ground rot, more mould spores are released. Mould spores can also be located around hay, compost heaps and straw, even in subfreezing temperatures. In the South and West Coast of the US, for example, some outdoor moulds can be found all year round. Mould consists of many cells that grow into branching thread-like hyphae that form the mycelium of a fungus. Mould releases microscopic fungal spores that can sometimes pass through the protective mechanisms of the nose and the upper respiratory tract to enter the lungs. They thrive in humid weather and places like damp basements or bathrooms with poor air circulation. During the winter months, as the internal heating is turned on, it is mould in the moist areas inside the house that can create perennial problems. The moulds most commonly found indoors and outdoors in the US are Alternaria and Cladosorium (Hormodendrum), but other common moulds include Ascospores, Aspergillus, Penicillium, Helminthosporium, Aureobasidium (Pullularia), Epicoccum, Fusarium, Mucor, and Rhizopus. Sometimes certain foods can have detrimental effect on mould allergies. The obvious foods are mushrooms and cheeses but dried fruit and foods containing yeast, soy sauce, or vinegar can also produce allergic symptoms. If you have an allergy to the drug penicillin, however, it does not mean that you will necessarily have a respiratory allergy to the mould Penicillium that penicillin is derived from.