What kinds of pollen cause hay fever?

Hay fever is a malady we generally don't associate with winter.  Even in Northern California, the rainy season damps it down for a while (just in time to catch respiratory viruses instead).  Still, depending on what you're allergic to, hay fever can strike almost year-round in warmer climes.  If the humidity is down and the temperature is mild, get ready to buy allergy medication.

Assorted pollen grains (public domain, from
Dartmouth Electron Microscope Facility)

Allergic responses in the airways are caused by all sorts of microscopic particles: dust, mold spores, animal dander, and pollen are major players.  When it comes to pollen, you often hear people blaming various plants -- goldenrod, daisies, sunflowers, wisteria, and even roses have been named by sufferers.  They don't usually include the most likely culprits, such as oaks, evergreens, and grasses.

The major deciding factor in whether a plant triggers hay fever is how it's pollinated.  Plants tailor their pollen, like their flowers, to their pollinator of choice.  For a large class of plants, the main pollinator is simply the wind; they're called anemophiles (anemo means "wind", phile means "loving").  The rest rely on animals of some kind, self-pollinate, or clone themselves some other way.  Insect-pollinated plants are referred to as entomophiles (you might recognize entomo from "entomology", the study of insects).

In order for pollen to cause respiratory allergies, it has to be airborne.  For anemophiles, that's the whole idea: make light, powdery, often small pollen that is picked up and carried easily by the wind.  Some conifers go so far as to put "wings" on their pollen grains.  Flowers are small and quite simple, as their task is largely to get as much exposure to air currents as possible.  Male flowers throw lots of pollen out in the hope that a grain or two will fall on a receptive female flower, somewhere.  (Fun fact: airborne pollen is quite effective at encouraging cloud formation.)¹

Entomophiles, on the other hand, have insects to handle distribution for them.  They make less pollen, and it's more nutritious (to encourage foragers); that added protein and fat content makes the grains heavier.  They're covered with a sticky substance called pollenkitt², which is barely present on pollen made to be windborne.³ The size of the grains doesn't seem to have much to do with the type of pollinator⁴ (in fact the smallest pollen commonly listed is from forget-me-not, an insect-pollinated plant)⁵, but the heavy, sticky nature of insect-borne pollen makes it much less likely to be wafted up into the air.  These plants also make showier flowers, with bigger structures to serve as billboards for their pollinators.  Those structures are what we typically think of as flowers, with color, petals in various arrangements, and sometimes insect-luring scents and nectar.  Sweet-smelling flowers are insect- or bird-pollinated, so unless you stick your nose deep in and take a big whiff, the pollen is unlikely to bother you.

That isn't to say animal-pollinated flowers can't cause hay fever at all... when there's enough of a particular entomophile around, airborne pollen can build up to the point that it can sensitize some people.⁶ But it's unusual.

So what should you be leveling your glare at between tissues?  Look for the plants which don't sport showy flowers.  Conifers, like pine, juniper, and spruce.  Olives, oaks, sycamores, mulberries, ginkgoes, all those trees where you never see a "real" flower on them.  Grasses, with their spiky panicles that rise way above all the leaf blades to get a good shot at the wind (that includes food plants like wheat, rice, and corn).  The rattier weeds, like pigweed, ragweed, Russian thistle, and many other roadside nuisances.  Some culprits are widely planted landscape plants, such as xylosma or London plane tree.  Take care with the plants you choose for your own landscape, and try to lean toward those with conspicuous flowers, or strictly female plants.

As a side note, many people believe that buying local honey can help with seasonal allergies -- but if bees don't visit the plants that put out the worst pollen, how could it?  Not all the pollen that bees collect is from the plants they visit; some windblown pollen settles on them and their preferred plants, and gets picked up by the static charge on their body hairs.⁷ So honey that is largely from clover or orange blossoms will still have small amounts of pine, ragweed, or other anemophilous pollen.  As to whether it helps, there's only a little research out there on honey and hay fever, using rather small patient groups, and the results seem to be mixed.^8,9The bottom line: if allergies bother you, and you don't have a problem with honey, it's worth a try.  Worst case, you get a sweet little silver lining to your seasonal sniffles.

¹ Pollen grains are efficient cloud condensation nuclei
² Pollenkitt - its composition, forms and functions
³ The fine structure of the exine in relation to the stickiness of angiosperm pollen
⁴ Pollen-size comparisons among animal-pollinated angiosperms with different pollination characteristics
⁵ Wodehouse R P, 1935 Pollen Grains: Their Structure, Identification and Significance in Science and Medicine (New York: McGraw-Hill)
⁶ Dolichandrone platycalyx: New entomophilous pollen--A report on pollen sensitization in allergic individuals.
⁷ Collecting of Pinus (Pinaceae) Pollen by Osmia Bees (Hymenoptera: Megachilidae) ⁸ Effect of ingestion of honey on symptoms of rhinoconjunctivitis
⁹ Birch Pollen Honey for Birch Pollen Allergy - A Randomized Controlled Pilot Study