Sunday, March 10, 2013

Ragweed or Cosmos?

A row of ragweed sprouts next to one Cosmos seedling.

Cosmos seedling has a few lateral rootlets off the primary root.

Ragweed has long shallow root with rhizomes.

Ragweed on the left, Cosmos on the right.  Don't confuse the two!

Last summer I let a young plant grow in my flowerbed.   It looked to me like a Cosmos (C. sulphureus) seedling.   When I noticed it again, it was not a beautiful, glowing Cosmos, but a mature ragweed.  I pulled it up, of course, but too late.  This February, I had hundreds of little ragweed babies popping up in that flowerbed.  Arrgh!

To be sure I would never make the same mistake, I took these photos of a cosmos seeding next to the ragweed sprouts.  I might still confuse the foliage, but there is NO WAY that I will mistake what happens under the soil with the roots!  The Cosmos seedling has only a small set of lateral rootlets of the primary root.   The ragweed has a clever, extensive underground system of threadlike roots and rhizomes that supports multiple plants.   To get it under control, I dig up each bit of ragweed and collect as much of the root system as possible.  I declare that this flowerbed will eventually be ragweed free.


Cristy said...

Thanks for posting this. This is my first year to grow cosmos, so I was wondering how to tell it from weeds. Now I know.

Sally said...

Hello Cristy! I am glad to hear that this was useful. I figured I could not be the only one who had trouble telling these guys apart. We love cosmos - so many butterflies and bees use it for nectar. You'll be able to save seeds for next year, too.

Sharon K. Yntema said...

I'm writing 3 years later, but I'm still having trouble distinguishing them without uprooting them. I think the leaves are much denser on the ragweed, but it is still strange to me how similar the foliage is. Also how different this giant cosmos looks compared to "regular" cosmos plants, which are so feathery.