Monday, September 23, 2013

Are sunflowers allelopathic?


With self sown sunflowers spring up all over the garden I was reminded to tell you about my sunflower experiment that I conducted earlier this year. I have found sunflowers to be great companions to the vegetables - offering valuable shade over the hot summer. They seem to love the heat and even their large leaves seem unaffected by the harsh rays. Their height means they give plenty of room for other plants to grow beneath, even climb. You can read more about their use in our garden here.

Contrary to my experiences I have found a number of references claiming sunflowers to be allelopathic. Meaning they produce chemicals the inhibit the growth of other plants around them. As this has not been my experience I thought I would conduct a little experiment.


I took some fresh sunflowers and shredded them up in blender, making a highly sunflower concentrated potting mix. The thought being that if sunflower was indeed allelopathic its effect would be strongest and most noticeable on the germination and development of seedlings. Not knowing which part of the sunflower plant is apparently allelopathic I included flower, seed, leaf, stem and root in the mix. I sowed from seed a set of 4 plants - a flowering (broccoli), a root (beetroot), and a couple of leafy (lettuce and spinach). Along with a set of control plants grown in a various soil mixes.


What was amazing is that not only did the plants germinate, they did amazing well - equal to that of the control. Which I found quite surprising. Even though I have not noticed any negative effects from growing vegetables alongside sunflowers I was expecting there to be a bit of truth to the allelopathic claim. With the mix containing such a high concentration of sunflower - one third -  I thought even a minor effect would have been noticed, but there was none. Concluding sunflowers as non-allelopathic.

As with all experiments I have made some assumptions: 1. The effect would be strongest on germination and seedlings; 2. It would affect one of the chosen plants; 3. The effect would still be present after the sunflower had been killed and shredded. Regardless my findings do confirm my observations and personal experiences, and as a result I will definitely continue to grow them as a shade providing and beautiful companion.

8 comments:

  1. This is a great idea! I have some self sown sunflowers growing over my basil at the moment. I let them grow because they're so pretty but I'm happy to hear that they won't harm the basil and may actually help it grow better.

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    1. Don't you just love the surprise of self sown plants. Of course as long as they don't over do it and become weeds. I hope the sunflower and basil become good friends, let us know it they do.

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  2. I grow sunflowers each year in my veggie patch. I love them and love the shade they provide my veggies. I had never heard that they might inhibit the growth of other plants and I must say that like you I had only experienced them as helpful garden flowers. Thanks for your experiment. I have a large number of seedlings in and cant wait till they tower and shade their veggie friends.

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    1. Even though I am fairly confident in my experiment, it is afterall just an experiment. So it is wonderful to hear of other people having similar observations and experiences.

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  3. Good to hear the positive results. I'm hoping to have a bumper yield of sunflowers and have limited space and garden beds so this is great news.

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    1. That's what I like most about sunflowers, they look big, but take up very little ground space. All the best with the new companions and do let us know how they go.

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  4. We grow sunflowers in the community garden where I have my allotment to attract the good bugs and encourage bees. Apart from that of course they look so pretty. Veggies grow quite happily around the bed of sunflowers.

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  5. I love your experiment! I too have never heart of them affecting other plants, but I'm glad of your findings :) I've planted a whole packet of them across my main bed this year.

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