Hi, Greg! Thank you for your question to AskExtension/Ask a Master Gardener. It is an excellent question; most people don’t realize that sunflowers can have an herbicidal impact on some other plants. Still … sunflowers are a magnificent plant. I adore them!

A little background: as Master Gardener Volunteer from Hamilton County Ohio, I volunteer my time to research and answer questions for fellow Ohio gardeners. To help ensure accuracy, I consulted several research-based sites to verify the following response. You will find links to the best of those sites at the end of this response should you be interested in reading more.

You are correct that sunflowers emit chemicals that can inhibit the growth of some, but not all, other plants. The name of this group of naturally occurring chemicals is sesquiterpene lactones (SL) and is a plant defense mechanism referred to as allelopathy. All parts of the sunflower plant including its roots, stems, leaves, stems and seeds contain this chemical.

You could certainly plant sunflowers on the edge of your garden; they are marvelous for attracting pollinators and for keeping some insects at bay. I would suggest that you plant a “barrier” plant or plants next to the sunflowers that are resistant to sesquiterpene lactones. (How wide, you may ask? That would depend on the variety of sunflower you choose. Taller varieties will, of course, have wider root structures, and your aim will be to insulate SL sensitive plants from your sunflower roots and seed-drop.) A few ideas for SL resistant plants are: thyme, rosemary, basil, carrots (and other root crops), corn, legumes, dianthus, vinca, lemon balm, lantana, coral bells, dead nettle (prettier than the name suggests!), daylily and dahlia. Plants that do not do well as neighbors of sunflowers include tomato, lettuce, kale, broccoli, cabbage (and other members of the Brassicaceae family) and some grasses.

Just FYI, the majority of plants that produce sesquiterpene lactones are in the Asteraceae family, including echinacea (coneflower), rudbeckia (Black-eyed Susan), daisy and aster. These plants are good companion plants for sunflowers but may also add SL to soil.

On a different note, I stumbled upon an interesting article in my research that doesn’t relate to answering your question, but had good information about succession planting for sunflowers and varieties of sunflowers, so I thought I would toss it in. You can find it here or through the link below.

I hope I have answered your question. If you need clarification, or if my answer raised additional questions, I would welcome you to write back. If you respond to this email, your response will come directly to my inbox.

I hope you are enjoying garden planning during these cold bleak winter days. I know I am! Spring will be here before we know it! Wishing you great garden success!

----------------------------------------------------------------------

Articles for your consideration:

Allelopathy in the Home Garden – Penn State Extension (Description of allelopathy) https://extension.psu.edu/allelopathy-in-the-home-garden

Allelopathy of sunflower plant – Toronto Ask a Master Gardener https://www.torontomastergardeners.ca/askagardener/allelopathy-of-sunflower-plant-itself/

How to Grow Sunflowers at Home – University of Illinois Extension (Has info about varieties and succession planting) https://extension.illinois.edu/blogs/good-growing/2022-03-18-how-grow-sunflowers-home-great-addition-any-garden

https://extension.oregonstate.edu/news/sunflowers-burst-scene-new-personalities