Pollen Alert and Hay Fever

If bermudagrass flowers like this one are left to release pollen in the air, they will cause "hay fever". That's why common bermudagrass is not permitted for planting in Clark County, Nevada.

Pollen Alert

That’s what we see on our advisories in the early spring regarding pollen from mulberry, ash and junipers. The pollen alert continues through the “pollen season” as we go from mulberry to pine to olive. Pollen season might last until May. Right now we’re in “mulberry season”. Some pollen like mulberry is light enough and can be pushed by the wind and carried by pollinators like honeybees. This type of pollen causes “hay fever” while others are considered “sticky” or “heavy”, too large to travel long distances in the air and not considered allergenic.

Male flowers from the 'Bonita' ash tree. Because this ash tree is male, it produces only male flowers. Great for producing no seed but not if you dont want pollen.

Hay Fever

            “Hay fever” was a misnomer from the start. It was an old historic association with the cutting of hay in the spring and not paired with flowering of plants that cause the release of pollen. Back then pollination by plants was not studied much. Tree and shrub pollen was not considered responsible for “hayfever”. Most allergenic pollen comes from uncontrolled, wild grasses growing where rain was available. These allergies were caused by pollen floating in the air but the idea of “pollen fever” never caught on. Until recently people with severe symptoms were told to move to the desert Southwest where “pollen fever” was never considered much of a problem. Maybe that was the case back then, but they are wrong now!

Common bermudagrass flowering and it will seed next spreading pollen and seed everywhere. When bermudagrass escapes mowing, it creates pollen, allergies and seed.

Desert and Hay Fever

            As people moved to the desert Southwest and started planting more and more “desert trees” like Acacia, Mesquite and Palo Verde, “pollen fever” (aka, hayfever) developed into more of a problem. Typically, trees and shrubs with large showy flowers like oleander do not contribute much to the “hayfever problem”. This pollen is heavy or sticky and did not travel in the air far from the flower and the plants released pollen too large to cause “allergy problems”. Most of the “problem pollen” comes from “non-showy” flowers commonly found on olive, mulberry, pines, ash trees, mesquite and the like.

Oleander flowers are quite "showy". Showy flowers are not typically allergenic because the pollen is large and/or sticky.

Planting Restrictions

            Now we have planting restrictions in population dense Clark County that prohibit the planting of male mulberry trees or olive trees that produce lots of fruit. So, is it “illegal” to plant mulberry trees in Clark County? Yes and no. It is “illegal” to plant male mulberry trees but not the female trees.

Mulberry flowers are called "catkins". Flowers in mulberry are like ash trees; they are either male or female depending on the sex of the tree.

How about olive trees? Yes and no, but for a different reason. Olive trees always have both male and female parts in the same flower so we focus on the so-called “fruitless mulberries” and hope that these trees reduce “hayfever” in large communities. Mulberries, similar to ash trees, are bought as either male trees or female trees. As I have always said, plant sex is much more interesting than animal sex because of its diversity.

Olive trees are both male and female so their flowers, unlike mulberry and ash, contain both male and female parts.