Do Sunflowers Use Chemical Warfare?

Today's post was inspired by a question I received from Debra, whose husband told her that sunflower plants ruin garden soil. She wanted to know if this was true.

Sunflowers are highly competitive and they're willing to use chemical warfare to get ahead of the plants they're competing with. 

In nature this is called #allelopathy, and many common plants, like corn, wheat, sweet potatoes, soybeans, etc., use it. 

If you're in central Texas, you may have noticed that nothing grows beneath our native "cedar" (juniper) tree. Same concept. 
You'll notice the same phenomenon under some oaks, apples, walnuts, lantanas and many others.

Sunflowers use natural "allelochemicals" to inhibit the growth of plants around them. Reducing competition ensures that they get more of the resources they need (physical space, water, nutrients, sunlight).

This doesn't mean sunflowers are bad to grow in a garden, or that they're harmful to your soil, but it's something to be aware of since some plants are more susceptible to allelochemicals than others.  

Seeds and seedlings will be most effected, but I have also seen it mentioned that beans and potatoes are particularly sensitive to sunflower allelochemicals.

As gardeners we sometimes use allelopathy to our advantage, like when we apply corn gluten meal as a weed preventer on our lawns.

I have sunflowers towering over my zucchini and tomatoes, and they seem to be uneffected, but they were also planted before the sunflowers.

If you're concerned about the effects sunflowers may have on your other garden plants, I recommend putting the dead sunflower plants in the compost pile (or the trash) at the end of the season versus letting them stay in the garden where they might effect your next crop. Another precaution is to get your veggie crops growing before introducing sunflowers into your garden, or plant them somewhere other than your vegetable garden.

All of this is a bigger concern if you're planting acres of sunflowers on land that you plan to use for acres of wheat, rye, etc. the following season.

Knowing this, I will still make a place for sunflowers in my garden, and I recommend you do, too.

For more Sunflower facts -- like why they always face the sun -- check out Bud, Bloom, Seeds -- The Evolution of a Sunflower

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