Mexican Sunflower-(Tithonia): To Plant or Not To Plant

(Picture of Tithonia by Jeff Schalau)

Sharing is fun, especially when it comes from the garden. Of course I enjoy sharing produce with others, but my real love is in sharing flowers. It is an activity I can do with my wife that brings joy to our family, friends, and neighbors. I plant and raise the flowers, and my wife arranges them in mason jars to give away to whoever needs them. Maybe they just had a baby or just lost a loved one. Maybe they are sick or just had surgery. Or maybe they just need a pick-me-up or something to smile about. My wife and I were blessed to share our flower arrangements with 47 people last year.

I am always looking for new flowers to share. One flower I started using two years ago is Tithonia Rotundifolia of the Asteraceae family, commonly known as Mexican Sunflower. It has much the same open growth habit as of regular sunflowers. It grows over 6 feet tall and has vibrant orange/red, 3-to-4-inch flowers with a yellow center. They are beautiful in arrangements.

This year I hoped to still use Tithonia, but first I wanted to check out the allelopathic detriment Tithonia may have on other plants around it. The 2024 CCMG manual glossary defines allelopathy as: “The excretion by some plants’ leaves and roots of compounds that inhibit the growth of other plants.” 

Jeff Schalau, an agent of agriculture for the University of Arizona, reported, “In my garden this year, the Mexican Sunflower (Tithonia rotundifolia) looks to have had serious allelopathic effects on my adjacent lettuce bed. The lettuce plants within four feet of the Tithonia are noticeably stunted.” 

(Picture of the affected lettuce)

(Plot growing the same lettuce seed but isolated from Tithonia)

Good thing I don’t grow lettuce. Although Tithonia can have negative effects on other plants (Schalau also mentions legumes and cereals), it appears that it is only within close proximity. In my own garden, beans and tomatoes were planted about 10 feet away from the Tithonia but produced great, as did chard, peas, and peppers.

So of what benefit is Tithonia? As the North Carolina state extension Gardener Toolbox discusses, one of its ecological positives is its role as a pollinator. It is commonly used in gardens to attract butterflies, hummingbirds, songbirds, and other pollinators. I remember in my garden last year I had to pick them early each day before the plants were covered with bees. 

Now that I know Tithonia better as a great pollinator and understand the precautions needed to decrease allelopathic effects, I will invite this beautiful flower back into my yard. Who knows? Maybe my wife and I will share 50 arrangements this year.