A sunflower at the LandHealth Native Plant Nursery in 2020 (Image by Steve Jones)
Sunflowers are known to be tough and sturdy, and they come in all shapes, colors, sizes, and uses! Some branching and single-stem sunflower species work well as cut flowers, others grow massive in height to provide privacy and shade, and then there are some that also produce forage like seeds for wildlife. But did you know that these bright and beautiful sunflowers, a symbolism of strong loyalty, have a potentially harmful toxin to harm its neighbors?
Sunflowers are “allelopathic”. Allelopathy is the study of the chemical interactions of plants, and plants that are allelopathic contain toxins that may stunt the growth of nearby plants (note: “may stunt”, not “will stunt”). These toxins are found in all parts of the plant, including their stems, seeds (hulls), leaves, flowers, etc., so the sunflowers’ seedlings are not affected and lighten the competition for the next generation! Just as sunflowers come in different forms, their levels of toxin vary as well.
“But sunflowers can’t possibly be a villain of our gardens!” You might think, and you are right. Sometimes, there is good in bad, and allelopathic plants can be both beneficial and harmful depending on the situation. But prepare to add two mouthfuls of new vocabulary in that mind of yours! Hyperaccumulator and phytoremediation.
A sunflower field in the distance of power plants. According to some scientists, they are capable of cleaning up radiation. (Image from: http://environmentaffairs.blogspot.com/2018/06/sunflowers-cleaners-of-environment.html)
Hyper-accumulators are plants that have the ability to absorb high concentrations of particular toxic materials, metals or metalloids such as lead, arsenic, zinc, copper, etc., in their tissues, and that accumulation can rack up to thousands of times greater than what is the usual normal level for most plants. Phytoremediation (“phyto-” is Greek for plant, and “-remediation” is remedy) is a “...process that employs various types of plants to remove, transfer, stabilize, and/or destroy contaminants in our soil, water and air. Compared to other cleanup methods, such as soil excavation or pumping polluted groundwater, phytoremediation has become a clean, cost-effective and environmentally-friendly way to reclaim and reuse land that has been tainted by poisonous chemicals and heavy metals.” (Blumenthal, 2021) Sunflowers can actually be a handy addition to cleaning your garden soil-- it’s like a hidden secret power!
There are different types of phytoremediation! (Image by: Anamika Kushwaha)
As you garden, checking your soil for pollutants is important as if you do find high levels of dangerous toxic materials, then human and animal health can be at risk. And if you are still worried about the toxins found in sunflowers and some plants, there are species that can, however, grow and resist allelopathic flowers and trees. Some examples are the Black-Eyed Susans (Rudbeckia spp.), Tickseeds (Coreopsis spp.), Coneflowers (Echinacea spp.), Coral Bells (Heuchera spp.), and of course, other Sunflowers (Helianthus spp.). Even if you have allelopathic-resistant plants, it’s always recommended to cut the sunflowers (all of it, including the roots!) in the fall, and can be used as compost for next year. Happy gardening!
Black-Eyed Susan at the LandHealth Native Plant Nursery (Image by: Brenda Vong)
Gardener, L., 2021. When Sunflowers Kill!. [online] Laidback Gardener. Available at: <https://laidbackgardener.blog/2017/05/08/when-sunflowers-kill/>.
Willis, R.J.. (2007). The History of Allelopathy. The History of Allelopathy. 1-316. 10.1007/978-1-4020-4093-1. https://www.researchgate.net/publication/281739374_The_History_of_Allelopathy
Reeves, R.D., Baker, A.J.M., Jaffré, T., Erskine, P.D., Echevarria, G. and van der Ent, A. (2018), A global database for plants that hyperaccumulate metal and metalloid trace elements. New Phytol, 218: 407-411. https://doi.org/10.1111/nph.14907
B. Stone, A., 2019. Phytoremediation Plants Used to Clean Contaminated Soil - Countryside. [online] Countryside. Available at: <https://www.iamcountryside.com/growing/phytoremediation-plants-clean-contaminated-soil/>.
Rascio, Nicoletta and Navari-Izzo, Flavia, 2011. Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?. Plant Science, Volume 180, Issue 2, Pages 169-181, ISSN 0168-9452. https://doi.org/10.1016/j.plantsci.2010.08.016.
Blumenthal, S., 2021. Sunflowers To The Rescue!. [online] Farmers' Almanac. Available at: <https://www.farmersalmanac.com/sunflowers-to-the-rescue-15614>.
Question of the Day. 2021. Sunflowers and Nuclear Meltdowns. [online] Available at: <https://www.askqotd.com/sunflowers-and-nuclear-meltdowns/>.