Cities could reduce their pollen count by planting street trees that produce very little of it, writes Thomas Leo Ogran, author of the book Allergy-Free Gardening. American cities weren't always hotbeds of springtime sneezing. Rather, he writes, they became much more allergenic in the 60s and 70s when Dutch elm disease killed many of the low-pollen American elms that lined the streets in cities across the country. Their replacements were highly allergenic, and are responsible for the wheezing, drowsiness and watery eyes that we now associate with this time of year.