Why goldenrods don’t make you sneeze: the biology of pollination

Image: Not being allergic to Solidago juncea. © Stephen Heard

Note: This is a science outreach piece belonging to a series I wrote for the newsletter of the Fredericton Botanic Garden. I’d be happy to see it modified for use elsewhere and so am posting the text here under a CC BY-SA 4.0 license . If you use it, though, I’d appreciate hearing where and how.

One of my favourite autumn sights is a field of goldenrods, blazing yellow and alive with insects flitting and buzzing from bloom to bloom. I’ve learned, though, that not everyone agrees with me – especially sufferers of seasonal allergies, who tend to recoil from goldenrods rather than rejoice in them. But if you’re sneezing, goldenrods aren’t the culprit – and there’s some interesting biology behind understanding why.

Plants have an interesting problem: mating without moving. Imagine yourself as an apple tree, say, rooted to the ground, and yet unable to reproduce unless you can exchange pollen with another apple tree many metres or even kilometres away. It’s a lonely thought!

Some plants overcome their immobility simply by mating with themselves (self-pollination). Most, though, have co-opted partners to take care of moving their pollen. For some plants, pollen is carried by insects; for others, by birds or bats; and for still others, by wind. Most of the biology of flowers can be understood with the realization that they have evolved to be good at getting their partners to transfer pollen. To start with, insect-, bird-, or bat-pollinated flowers need to signal their availability to pollinators – which explains their colourful displays and beautiful fragrances. (It also explains the quite revolting fragrances of a few flowers that have evolved pollination via carrion-feeding flies!) Most such flowers also reward pollinators for responding to their signals, which explains the production of sweet nectar (although a few disreputable orchids offer no reward, instead mimicking female wasps to draw visits from naive and perhaps desperate males). Plants using wind to carry pollen are quite different: they don’t need to signal the wind, so their flowers are often tiny and inconspicuous. But they do need to release pollen so that it can be carried aloft and, with time and a lot of luck, find its way to a receptive flower on another plant of the same species. This is, for instance, why corn tassels (male flowers) are at the top of the stalk.

So what does all this have to do with allergies? Pollen, too, has evolved to enhance its own transport. In insect-, bird- and bat-pollinated plants, pollen grains are large and sticky so they adhere to the pollinator’s body long enough to be carried from flower to flower. In wind-pollinated plants, in contrast, pollen grains are small and light so they can be carried long distances in even a gentle breeze. So which kind of pollen might find its way up your nose? Exactly! When you sneeze, it’s wind-borne pollen you’re reacting to: birch in spring, for instance, or grass in summer. (And yes, both have flowers.) What about autumn? Those beautiful goldenrods are insect-pollinated, and their pollen is heavy, sticky, and won’t find its way to your nose unless you unwisely inhale a bee. But lurking unnoticed beside the goldenrods is ragweed, an annual whose tiny greenish flowers release copious amounts of wind-borne pollen. Goldenrods and their flamboyant floral displays end up taking the blame for allergies that are really due to its understated cousin.

The good news? Feel free to enjoy goldenrods in nature, and feel free to add them to your own garden. There are native species through most of North America, around 200 all told. We have about 20 native goldenrods in New Brunswick, and some are really gorgeous: the lush rough-stemmed goldenrod (Solidago rugosa), the delicate downy goldenrod (S. puberula), or my favourite, the elegant and (surprisingly) white-flowered silverrod (S. bicolor). No sneezing required!

© Stephen Heard (sheard@unb.ca) Sept 3 2015 but licensed CC BY-SA 4.0 . CC BY-SA 88x31

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