Allelopathy of sunflower plant itself

(Question)

I have been trying to do some research about the allelopathy of sunflowers. I read a helpful response from one of the master gardeners who was replying to a person that had a birdfeeder and sunflower hulls would drop beneath it, preventing many plants from growing in that location because of the allelopathy of the hulls.
However, I have also heard that the sunflower plant itself (while growing) can be allelopathic. Is this true? Or, is it only the remains of the sunflowers (once the sunflowers have stopped growing and/or died) that prevent other plants from growing? I don’t believe my climate zone would really affect the answer to this question so I won’t worry about including it.
To give a bit more backstory – I work for a non-profit organization that starts and grows school gardens in our community, teaching kids and families how to grow and prepare their own food as well as providing fresh food to the greater community. Sunflowers are a favorite of many children that spend time in these gardens, but our staff has disagreed time and again about how many sunflowers to let reseed in various garden areas. We have cut down on the number of sunflowers we grow in the past few years because of this issue, but I’m curious to know more information.
Thank you for your help!

(Answer)

Thank you for your interesting question. The link for the article you speak about is https://www.torontomastergardeners.ca/askagardener/plants-that-will-survive-sunflower-seed-hulls-toxicity/

Sunflowers (Helianthus spp.) produce natural plant chemicals (sesquiterpene lactones) that are fairly effective at helping this species fend off competitors trying to weasel into its growing territory. This natural plant-to-plant defence (called “allelopathy”) has the potential to create friendly-eco products for weed management for field crops.

Sunflowers are not toxic for humans or animals.

I quote from Sesquiterpenoids Lactones: Benefits to Plants and People

Sesquiterpenoids, and specifically sesquiterpene lactones from Asteraceae, may play a highly significant role in human health, both as part of a balanced diet and as pharmaceutical agents.

Sesquiterpene lactones and other secondary metabolites are not produced by plants for the benefit of humans, but rather for their function in the plant. They are mostly found in leaves and flowering heads of plants.

Sesquiterpene lactones are functional compounds and are therefore liable to change in concentration during plant development according to the plant’s needs. For example, when a plant undergoes floral transition it is likely to produce more defensive compounds to protect its investment in reproductive structures.  In terms of human use this has its implications for the field holding capacity of a crop; that is its ability to stay in an edible state for a long time prior to harvest.

Many sesquiterpene lactones from sunflowers reduce the germination rates of plants from other families to reduce competition in the locale:

  • Tomato, pepper & potato family -Solanaceae, tested on tomato (Solanum lycopersicum).
  • Grass family – Poaceae, tested on barley (Hordeum vulgare) and wheat (Triticum aestivum)
  • Mustard family – Brassicaceae, tested on watercress (Lepidium sativum)

Evidence suggests that plants in monocultures (crops) are likely to produce more volatile alleochemicals, and that plants show greater response to them.