Portraits of Wildflowers

Perspectives on Nature Photography

Giant ragweed

with 14 comments

Giant ragweed; click to enlarge.

People with respiratory allergies have at least one reason to dread the coming of autumn, for that’s when ragweed chooses to flower. Its flowers are small and inconspicuous, but these plants don’t rely on insects to spread their pollen. Instead, they release their pollen into the air in the expectation that it will be blown to other receptive plants of the same species. Because that’s literally a hit-and-miss proposition—with much more miss than hit—ragweed releases large amounts of pollen, so that even if the hits amount to only a small percent, a small percent of a large number is still a success. Unfortunately a portion of all the misses ends up in our noses and causes us plenty of suffering. (I say us because I’m susceptible.)

While other species of ragweed stay relatively close to the ground, in central Texas we have one, Ambrosia trifida, that justifies its common name of giant ragweed. This plant grows erect and can reach heights of 10 ft (3 m). Not only that, but it can form large colonies that line roadsides and fill fields. I found a lot of it (in both senses of lot) beginning to flower on September 9 on the same property where I photographed some peppervine flowers and buds earlier in the summer. This is the lot on the east side of US 183, a block south of Braker Lane, adjacent to Costco and Wendy’s, that’s about to be redeveloped, so I went back to see what else I could photograph before it’s too late.

This giant ragweed plant’s central stalk is vertical, but it has many branches that give the overall effect of a candelabra. You’re seeing only the upper portion of the plant here; note the angle at which I had to look up to include the top of the plant, an indication that it was a few feet taller than I am.

For more information about Ambrosia trifida, including a clickable map showing the great many places in North America where the plant grows, you can visit the USDA website.

© 2011 Steven Schwartzman

Written by Steve Schwartzman

September 15, 2011 at 5:55 AM

14 Responses

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  1. I recently identified ragweed for the first time, but I’m puzzled: up here in Vermont, I’ve got pictures of what looks like ragweed flowering back in July, and from what I can tell of it now, it’s gone to seed. That seems quite a bit early, no? http://musingsfromdave.blogspot.com/2011/09/use-these-in-sentence-ragweed-question.html Am I way off base here in thinking that the July stuff, was flowering? What do y’all think?


    September 15, 2011 at 6:51 AM

    • You raise a good question, Sarah. What I’ve learned about native plants I’ve learned in my (warm) little corner of America, central Texas, so I confess that I know almost nothing about what goes on elsewhere, especially in a part of the country with as different a climate as the one you have in Vermont. I just looked at the website of the Lady Bird Johnson Wildflower Center and found that there are at least seven species (and several varieties) of Ambrosia listed for the United States. It’s conceivable that one or more of those flower earlier than the fall.

      And I’ve also observed that when the conditions are right, some plants have no qualms about flowering outside their “normal” bloom period. (I’ve grown fond of repeating a line that’s apparently well known among botanists: Plants don’t read field guides.) For example, this past year I saw some goldenrod (which as usual is just beginning to flower now) flowering away in my neighborhood in the cold of January, way past its “normal” time.

      What I’d suggest is getting in touch with a botanist at a local college to see if you can find out more about the species in your blog. If you find an answer to your question, please let us know.

      Steve Schwartzman

      September 15, 2011 at 7:39 AM

    • Sarah, our giant ragweed here in N. Illinois has long since flowered and caused headaches as well, most in mid August, so your intuition is probably right!

      Chris Zeiner

      September 15, 2011 at 9:05 AM

  2. I too am allergic to ‘a trifida.’ Interestingly, Nevada seems to be the only ragweed free state on the mainland. How does that happen? 😀 ~ Lynda


    September 15, 2011 at 7:45 AM

    • I guess we could call it the luck of the draft, i.e. breeze, since ragweed is wind-pollinated. You raise another good question; I’ve sometimes wondered why a species is found in location X but not Y even if Y isn’t all that far away and has similar conditions. But things do change, and I’ve become aware from looking at old books that some species are now more widespread in the US than they were a hundred years ago. So maybe Ambrosia trifida will eventually end up in Nevada. Another possibility is that it’s already there but hasn’t been noticed. For example, there have been times when I’ve looked at the USDA map for Texas for a certain species and have seen a blank for a county where I’ve observed that species growing.

      Steve Schwartzman

      September 15, 2011 at 8:34 AM

  3. We had our first hard frost here in central Minnesota last night, so my local ragweed should be on its way out. Woo Hoo! August and September are pretty miserable for me.

    I find it amusing that so may people blame goldenrod for their misery. Goldenrod has big, heavy, yellow pollen and blooms at the same time as ragweed, but it is only insect pollinated – the only way to get goldenrod pollen in your nose is to shove it in! Yet allergy medicine ads often feature goldenrods.


    September 15, 2011 at 9:17 AM

    • Hard to think of frost when yesterday’s high here was over 100°. And our ragweeds are just beginning to crank out pollen. I’m glad you can breathe better.

      You’re right about the mistaken blame attached to goldenod, which is also just getting started here. In fact I saw my first flowering plants of the season yesterday.

      Steve Schwartzman

      September 15, 2011 at 9:40 AM

  4. Until your ragweed posts this past week, I had no idea what ragweed looked like. And your post made me more conscious of the fact that I probably should have been more curious about the plant that causes me misery for a couple months of the year (along with the rest of the allergy club). I happened to be taking a quick walk at the park yesterday and suddenly realized that the field (big, big field) of beautiful, dense yellow blooms that I stopped to admire is RAGWEED (as I started sneezing). It made me look at them in a whole new light, a somewhat less appreciative light. But also empowering – when my allergies are really bad, I can avoid that trail. Thanks for highlighting ragweed in your blog.


    September 18, 2011 at 10:30 AM

    • Sorry for your suffering. At least giant ragweed is large enough that you can see it at a distance and try to avoid it (though you may not be able to stop breathing the air that carries its pollen). The other ragweeds are smaller and less conspicuous and therefore harder to avoid. I sometimes get zonked by all of them when I go out photographing in the fall, so I’ll commiserate with you.

      Steve Schwartzman

      September 18, 2011 at 11:10 AM

  5. […] contrast to yesterday’s picture, giant ragweed can sometimes look less linear and more scraggly, especially at the top, as seen […]

  6. […] Pond in north Austin on July 30th, I came across some paper wasps at their nest on a dried-out giant ragweed plant, Ambrosia trifida. I took a couple of dozen pictures, and for the last group of them I zoomed in as […]

  7. Sounds like this is the spam bot of the plant world. Or did spammers study the pollination habits of plants to get their ideas for spamming?


    February 7, 2014 at 5:30 PM

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