Where is all that yellow pollen coming from?

Anyone living in Phoenix will have noticed a coating of  bright yellow on neighborhood streets, parked cars, sidewalks, and yards this spring. It’s everywhere.   So what is it?  And perhaps more important, “Does it cause allergies?”  In April through May the answer is usually the Palo Verde tree.  Palo Verde (which is Spanish for green pole, named for it’s green trunk and branches) is the state tree of Arizona and produces a conspicuous bright yellow flower in the spring.  As to causing allergies, the Palo Verde tree is one exception to the rule.   Pollen in the air causes allergy symptoms: runny nose, itchy, watery eyes, stuffy nose, congestion, etc.   As a rule, plants that go to the trouble to produce beautiful, fragrant, conspicuous flowers do so to attract  insects or birds which they rely on to distribute there pollen to distant plants.    These flowers contain a heavy, sticky pollen designed to stay put in the flower until carried away by bees and the like, and so very little is in the air to cause allergy symptoms.    On the other hand, plants that rely on the wind to carry their pollen around produce small, inconspicuous flowers, but lots of light, dry pollen that floats easily in the breeze and to your nose and eyes.   Palo Verde is an insect pollinated tree and therefore should not be a major allergen.  However, as can be witnessed on the sidewalks and streets of  Ahwatukee, Chandler, and Tempe this spring, there are so many trees producing  flowers that the sheer volume causes pollen to find it way into the air, particularly when the wind blows.