Pollen Counts

For Northern California pollen counts, please visit the  AAAAI.org national pollen page.

Northern California Stations of interest are:

Below are images of pollen-producing plants that are often implicated in pollen-related allergic respiratory disease in Northern California. Some are felt by allergists to be of major, and others, of minor importance. At the bottom of the page have been added images of stinging insects with a brief comment about them. These images are presented for informational purposes to help you identify some sources of your allergic disease or asthma.


Juniper/Cedar/Cypress are all related and are major causes of allergic disease in Northern California. (Pictured above: a large Juniper bush) These trees and bushes pollinate from January to May, and even can pollinate into June! These pollens can be moderately allergenic. They are very important sources of pollen in the City of San Francisco.


Acacia is a heavy pollen that pollinates in February and into March. Its bright yellow blossoms are highly visible in the spring and are often thought by patients to be the source of their allergies, but it is only mildly allergenic and often gives either slight or no reaction on skin testing. It is a heavy pollen, only important near the tree.


Birch is very easy to identify with its white bark (see small picture at left above). It is not native to California but has been planted extensively for decorative purposes. Birch pollen is highly allergenic and plentiful during its pollen season, usually March through mid-April. If you have one nearby, it may one of the causes of your springtime allergies.


Pines (above: Monterey pine) produce a large pollen grain that has a waxy coat on it. The yellow pollen that you see on your car from February to May of each year is probably pine. Although highly visible, the waxy coating makes the pollen much less allergenic than many others (3% positive skin tests among allergic subjects in the Bay Area (RMH and DFG, Ann Allergy 1985; 55:678-9) although it may cause respiratory symptoms by an irritant, non-allergic effect.


Mulberry trees are very fast-growing. Their leaves are used to feed silkworms in China. Mulberry pollinates only for just over a month from March to April of each year. The pollen is small and very plentiful when present. It is of relatively low allergenicity but sometimes causes many respiratory complaints, including asthma.


Oak is an indigenous and very ubiquitous tree. The insert at left is of a large oak tree at Stanford Medical center. The oak pollen season is quite long, from March to mid-April; some species even pollinate into May here in California. Its pollen is considered moderately allergenic, but because of the high concentration of pollen in the spring, it is a very important allergen. It can cause significant clinical hay fever and asthma in Northern California.


Pictured: annual bluegrass, Poa annua. Grasses are the most important group of plants causing pollinosis in this area. Grass pollen grains are large and highly allergenic, and in some areas, grass pollen counts can get very high. Skin tests reactions to grass are often quite strong in allergic patients confirming that it is an important allergen. Pictured above is grass that wasn’t mowed. Wild and unmown grass produces more pollen than a well-mowed lawn, but the pollen flies everywhere, and your symptoms may be from grass pollen that has traveled a fair distance.


Walnut is another highly allergenic pollen that starts to pollinate at about the same time as grass but, as for most trees, has a shorter, more intense pollinating season. The picture above shows a pollen-producing brown “tassel” (catkin) and walnuts beginning to grow. Walnut is highly allergenic but is a heavy pollen that doesn’t fly too far. Some patients complain of problems even out of season, and it is possible that particles from twigs and leaves can also cause allergies.


Olive is another highly allergenic pollen. Like walnut, it is heavy and is only important if it is close by, although some patients have trouble just driving by large stands of olive pollen on expressways and roads! Olives start pollinating later in the spring than many other trees and grasses and cause a lot of allergy symptoms, including severe asthma.


Privet is related to olive, is quite allergenic, and pollinates as the olive pollen season is ending. The plant can be a tree, bush, or hedge (privet comes from the word “private,” suggesting its use in hedges). It has a sweet smell and has become more important in San Jose in recent years, probably because the trees have been planted more frequently as houses are built closer together, and privacy has become a concern. Privet planted 10-20 years ago have probably also become larger and put out more pollen in season. It can be found growing wild near creeks.


Chinese Elm, Ulmus parvifolia, blooms in the fall and is a moderately allergenic tree. It is an important tree if it is near your workplace or home. There is also a spring-blooming elm which can cause symptoms in February and into March in Northern California, but it is not as prevalent as the fall-blooming elm.


Pictured above: honeybee (Apis Mellifera), yellowjacket (Vespula species), Paper wasp (Polistes fuscatus), and yellow hornet (Dolichovespula arenaria). Of the thousands of insects in No. California, only a few are capable of inflicting venomous stings. Honeybees, yellow jackets, wasps, and hornets are of major clinical importance in insect sting allergy.

Honeybees live in large hives, often man-made wooden boxes painted white. These hives are commercially managed for honey production and pollination. Only the female stings. Bees have squat, hairy bodies with bright yellow or black stripes and are found around flowers or other blooming plants, particularly clover. Her stings can be identified because she is the only stinging insect that has a barb on her stinger, which is left in the skin when she pulls away or is brushed off.

Yellowjackets nest in paper cells found in walls, logs, compost heaps, or underground. They are slightly smaller than honeybees and have yellow stripes on their abdomen. Yellowjackets exhibit aggressive behavior towards food, particularly meat, rotten fruit, and sweet drinks, and have a characteristic, rapid, side-to-side flight pattern before landing.

Wasps build their nests under eaves and rafters. The nest is a horizontal comb of paper cells. Wasps have a characteristic narrow, pinched waist which separates the upper chest area from a cigar-shaped abdomen. Most wasps are nearly hairless and are black, red, or brown.

Hornets: In the U.S., there are no true native hornets. Dolichovespula Arenaria and Dolichovespula maculans two large yellowjackets are called yellow hornets and white-faced hornets. Nests are football-shaped and are found in trees or shrubbery in wooded or open areas. Hornets are usually larger than most other stinging insects of the same family and have black bodies with yellow or white markings.

Avoidance of stinging insects starts with awareness of where their hives or nests might be and avoiding them. Wear light-colored clothing (insects are attracted to dark clothing), and wear shoes when walking on grass. Avoid wearing wool, leather, or suede – insects can smell these odors. Don’t use perfumes and hair sprays. Avoid picnicking outside, and do not carry fruit, meat, or sweet drinks, which will attract these insects. When they are nearby, avoid sudden, violent motions which will alarm them.

These pictures are for informational purposes only. Insect sting allergy is a complex problem, and this website cannot cover the subject adequately. If you have had an allergic reaction to an insect sting, check with your local primary care physician or allergist for more information; visit the websites www.aaaai.org or www.acaai.org.Sources: first three images (honeybee, yellowjacket, and paper wasp) are for free use within the limitations outlined in the links and the fourth (yellow hornet) required a release which was given.