14 August 2009

Fruit of the Gods? I Don't Think So!

I often hear people, when talking about a non-native plant species, say, "I hate that plant." To this, my response is always, "Don't hate the plant, hate the people who brought it here." Plants obviously don't have the ability to know that they are invading an area to which they are not native, so why hold them responsible? Even today, when invasive species are well known and discouraged in landscaping and erosion control projects, some people still intentionally introduce them. Is it the plant's fault? I don't think so.

This being said, there is one genus that I truly hate. Well, hate is a strong word, and I do still have a degree of respect for this genus, so let's just say that I dislike this genus. The genus to which I am referring is Ambrosia; the Latin name refers to the fruit of the gods (why, I have no idea). In this part of the country, there are two common species of ragweed: A. artemisiifolia v. elatior and A. trifida. Here, I will discuss the latter.

"It isn't attractive," you say, "but Scott, why do you dislike it so?" Here's why... Ambrosia trifida is known more commonly as Giant Ragweed (or Great Ragweed). I learned at a young age, when I thought that ragweed was the yellow-plumed plant covering old-fields and blooming in August and September, that I was severely allergic to the pollen of ragweed. I later learned that this yellow-flowered plant that I saw flowering when hay-fever was setting in, is actually goldenrod (Solidago spp.), which has heavy pollen that is spread by insects rather than by the wind, as the pollen of ragweed is spread. There were many late afternoons in my youth that I was laid out on the couch with a wet washcloth on my eyes to reduce the swelling, struggling to suck in every precious breath. Even as recently as 10 years ago, before I was introduced to the miraculous nasal spray Flonase, I would go to bed on a humid August night wondering if I would be able to breath until I woke up in the morning. Yeah, ragweed and I don't get along.

All of this said, it is still quite an impressive plant. Giant Ragweed, a member of the Sunflower Family (Asteraceae), can reach heights of nearly 20 feet tall, dying back annually and doing it all again by starting as a new seedling the next spring. The rough-textured, deeply 3- to 5-lobed opposite leaves can also produce a short-lived rash if you walk through enough of them, which I certainly have. The flower structure is also unique in Giant Ragweed. Most of the flowers are staminate, meaning that they are only male flowers. The pistillate, or female, flowers, are few and at the base of long spikes of staminate flowers. The flowers lack petals, causing them to be unattractive to insects as pollinators. That's alright... the wind has no problem dispersing the tiny, lightweight pollen grains. Giant Ragweed often forms large near-monoculture colonies in disturbed and alluvial soils, and grows in 47 of the continental United States (how did Nevada get so lucky?). It is said to be native throughout much of its range.

Above, you can see the individual staminate flower heads, consisting of green phyllaries and yellow pollen-covered clusters of stamens, representing disk flowers. Below is a typical leaf of Giant Ragweed.

After walking through a field of Giant Ragweed, my clothes are often yellow as a result of being covered in pollen. I also often see puffs of yellow raining down from the plants when I bump into them. The result can be seen below, as pollen grains cover anything with which they come into contact.

More impressively, below is a photograph of a pollen grain of Giant Ragweed under a scanning electron microscope. This photo is from http://pro.corbis.com/Enlargement/Enlargement.aspx?id=IH056326&ext=1, © Lester V. Bergman/CORBIS. You can see the mace-like appearance of the individual pollen grain. The spikes allow the grain to cling onto anything it touches, and also severely irritate the inside of my nose. I'm sneezing just thinking about it!

Even though Giant Ragweed causes me and countless others so much discomfort, hopefully you can see why it is still difficult for me to truly hate this species.


Janet Creamer said...

Cool post. I have never seen a pollen grain from ragweed.

Thanks for the comment on the wasp post. I love cicada killers and recently found one's burrow. I am going to try to get some pics of it this week.

Scott said...

Thanks Janet. Good luck with the Cicado Killer photos.