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Saturday - December 11, 2010

From: Austin, TX
Region: Southwest
Topic: Invasive Plants, Wildflowers
Title: Need to Control Giant Ragweed in Wildflower Field in Austin, Texas
Answered by: Marilyn Kircus

QUESTION:

I have an acre pond around my business park planted with several different kinds of wildflowers. I let all the vegetation grow until the first frost, because I have wildflowers that grow throughout the summer. Is there some way I can control stocky weeds like ragweed - some are an inch around and 8 feet tall - and other large weeds from taking over the area?

ANSWER:

The ragweed you are talking about is Ambrosia trifida var. texana, Texas Giant Ragweed. I know your pain. I once spent an entire hot, sweaty morning at Brodie Wild using my big loppers to cut down a 20 X 30 foot area that had these guys growing about 18 inches apart and 10 feet tall and maybe 2 inches around.

This plant is an annual and starts growing really early. In fact farmers have actually accidently selected for a variety that will sprout earlier than normal and is resistant to post-emergent herbicides.

I’m not sure what wildflowers you are growing and when they can be expected to sprout. But your first line of defense is to pull out the seedlings. That just takes your two fingers, or, if you are smart, you can throw a party and give everyone a picture of the felons and let them go find them. Maybe a prize for the guy with the most bodies. And don’t forget the pizza. Here is a link to an article with pictures of the seedlings. Note the first pair of leaves are spoon shaped.

http://www.utextension.utk.edu/publications/wfiles/W119.pdf

They will sprout over several weeks so keep looking. If you are really lucky, you can mow them down before anything else comes up. But I expect you’ll have other plants sprouting at the same time.

The next plan is to go pull them out or cut them down in the early summer, well before they produce that terrible pollen or set seeds. You may have seeds sprouting for years, so you’ll have to be vigilant.

A third defense is to mulch your area. But if many of your wildflowers are annuals, you will also inhibit them from growing because the mulch prevents the seeds from reaching the dirt and getting enough moisture to sprout. You can get free chipped wood mulch from many counties. Just be sure to let it compost for several months before you spread it or the composting organisms will steal nitrogen from the soil at your plant’s expense.

There are some chemicals recommended for giant ragweed, but even Monsanto suggests that you will need multiple treatments of different herbicides. Other articles talked about how it has developed glyphosate resistance in some areas. So chemicals may not do much good and could harm the other plants.

I don't know what your other weeds might be but agressive pulling will take care of them also. Just be sure you know what your seedling or emerging wildflowers look like so you don't accidently pull them.

Ambrosia trifida var. texana