GARDENING

White top is a weed of concern for Florida


Ragweed parthenium — also called carrot weed and white top — is listed as one of the most invasive weeds in the world. (Courtesy of R. Stamps/UF)

Published: Saturday, June 8, 2013 at 6:01 a.m.
Last Modified: Thursday, June 6, 2013 at 10:07 a.m.

Simply said, a weed is anything growing where it is not wanted. That means that if a volunteer tomato plant germinated in a flower bed and it is not wanted there, it is a weed.

Therefore, a plant's designation as a weed may vary depending on location in the landscape and the gardener.

A few years ago, I visited a garden in Ocala and saw a beautiful specimen of a plant known to us in Antigua and Barbuda as whitehead broom. I asked the gardener about the plant, and she informed me it was purchased at a native plant sale. I recommended she keep a close eye on it as it is very prolific and very soon there will be lots of seedlings popping up in the garden. Several weeks later, she informed me that she removed the plant because numerous seedlings were taking over the garden.

Interestingly enough, I attended a talk and there was whitehead broom, Ragweed parthenium — also called carrot weed and white top — on the list as one of the most invasive weeds in the world.

White top is a member of the Asteraceae family and is native to the Caribbean and Central and South America. It was first identified in Florida in the 1980s, and is now present in 23 counties in Florida. This herbaceous, annual weed has deeply lobed leaves, grooved stems and small white flowers.

White top is an abundant seed producer with up to 30,000 seeds per plant. Dispersal is by seeds transported mostly by animals, equipment, moving water and, possibly, wind. White top is a very competitive, drought-tolerant weed, and can thrive in a wide range of soil types.

According to University of Florida professor and Extension cut foliage specialist Robert H. Stamps, white top is a serious threat to native flora and fauna in natural areas.

It is poisonous to mammals, and health problems such as asthma, bronchitis and eye irritation have been reported. Considerable economic losses are possible due to reduced crop yields.

White top can be controlled in the home garden by hand pulling, mowing plants before they flower and go to seed, and avoiding transportation of seeds.

A number of herbicides, including glyphosate, will provide effective control.

For more information on Ragweed parthenium, visit http://edis.ifas.ufl.edu/pdffiles/EP/EP44800.pdf.

My advice to gardeners is to be sure to research the characteristics of each new plant you intend to put in your garden. You can unintentionally introduce a problematic weed that is difficult to control and can spread easily; thereby creating negative environmental and economic impact.

Norma Samuel is the Urban Horticulture Agent for the UF/IFAS Marion County Extension Service. Contact her at norma.samuel@marioncountyfl.org.

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