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I read that sunflowers can have up to 3m deep dense roots.

Is it true that sunflowers improve soil structure, by penetrating its roots through the ground?

  • ALL plants improve soil structure, some just do it better than others. If you take soil without any plants, and leave it there for a decade or two it will become almost like sand. – black thumb Sep 10 '18 at 5:50
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No, not really. If that were true, then any plant would improve soil structure, because they all have roots that penetrate the ground, to a greater or lesser extent. In many respects, long term, permanent plants like trees contribute to soil structure because of the mycorrhizal system that builds up around and between them, and because the soil is not regularly disturbed by major digging, but annuals such as sunflowers,no. Sunflowers also are a greedy crop - they use a lot of nutrients and may leave the soil depleted, and that ongoing depletion whilst they are present may deprive other plants grown nearby.

Generally, what improves soil is the addition of organic materials; as for the structure, this is maintained by using a no dig method, and just applying organic material on top, without disturbing the soil itself. Even with a no dig method though, it is necessary at times to break the soil structure, perhaps in order to plant, or remove weeds, to remove roots of crops after harvest in order to replant, or to break up heavily compacted soil.

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  • There are different varieties of sunflowers, but in my experience, they pull out easily with fairly small root balls - 6-9in across perhaps, rather than 3m. If you want to improve the soil with annuals, grow legumes (beans, clover, etc) – winwaed Sep 7 '18 at 14:06
  • Thanks on answering! Your claims somewhat disagree with NSA findings on sunflower roots: sunflowernsa.com/magazine/articles/default.aspx?ArticleID=3055 What is you comment on that NSA's article? – Aleksandar M Sep 7 '18 at 23:14
  • Its aimed at sunflower crop growers, and many plants will put down tap roots and do the same job, so there's nothing particularly special about them for home gardens, its only that that article is focussed on sunflowers, not generalised planting. Potatoes are said to break up heavy soil too, for instance, so that article does not materially change my answer as given. – Bamboo Sep 8 '18 at 8:19
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I haven't heard that sunflowers specifically improve soil structure before, but it's certainly possible. It's especially that a specific sunflower variety improves soil structure, although finding the details of that variety may require a bit of research.

I have heard that sunflowers will remove heavy metals from soil and it's easy to find articles such as this one that mention research into that area.

A much less detailed article about rotating sunflowers and sugarcane in Australian discusses the potential benefits for that specific application.

Cover crops are a long respected strategy for improving a variety of attributes of soil. From the perspective of gardeners, soil "structure" is one specific attribute. It seems likely that sunflowers may be an attractive way for farms to improve their soil since sunflowers grow well with minimal water and have a commercial value. For a specific gardener, it's worth considering the specific goals you have, your priorities, and what is the best way to achieve them. If your primary goal is to remediate heavy metals then sunflowers may indeed be a good choice. If the primary goals are to fix nitrogen and improve soil structure, then a legume or other green cover crop may be better than sunflowers. If you want a cover crop that's going to be low-water use, have pretty flowers with edible seeds, and may have some soil benefits then Sunflowers seem like a decent choice.

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