Garden Logic – understanding correlation and causation in our gardens and landscapes

This home landscape is managed using science-based methods; the only routine additions are water and arborist chip mulches.

Upon reading this post’s title, you may be inclined to stop right there. (That’s why I have an eye-catching photo to lure you in.) While logic may seem irrelevant to your enjoyment of gardening, I can guarantee that reading this blog post will challenge many seemingly logical assumptions you’ve heard or read about. Recognizing unsubstantiated assumptions and avoiding their pitfalls means you can make wise choices about how you care for your gardens and landscapes.

You can find this and thousands of other silly correlations at

A few definitions are needed before we get started:

Correlation refers to variables whose changes mirror one another. For instance, the addition of nitrogen fertilizer to container plants is correlated to plant growth: as nitrogen levels increase so does plant growth. You can also have inverse correlation, where the variables move in opposite directions. An example is water availability in soil and planting density: the more plants you have in a specified area, the less water is in the soil.

Plant growth is correlated with increased nitrogen and other nutrients (from Xu et al. 2020)

Causation takes correlation one step further: it establishes that one of those variables is causing the change in the other. Using the same examples, we know through published evidence that the increase in nitrogen is causing the increase in plant growth, and the increase in planting density is causing the decrease in soil water because of competing roots. These relationships are obvious to us, but what’s important is that these causative effects have been established through scientific experiments.

Inverse relationship between planting density and soil water content (from Shao et al. 2018)

Sometimes scientific evidence doesn’t exist to demonstrate causation. That may be because it’s impractical or impossible to run an experiment that tests for a causative effect, or it may be because the experiments just haven’t been conducted yet. The latter is the unfortunate reality for those of us interested in managing gardens and landscapes: there is no major funding agency that supports field research for us. There is research being done, but it’s on a small scale with a shoestring budget…so the body of literature develops very slowly. In such situations, we must rely on established applied plant physiology and soil science to ask whether a suggested correlation might be elevated to causation.

Something caused these arborvitae to fail…but what? Research is slow to catch up to our observations of landscape failures.

Which brings me to my current source of online irritation: the constant blaming of tree failure on mulch volcanoes. Yes, tree failure is definitely correlated with mulch volcanoes – because lots and lots of newly planted trees fail. But is the mulch to blame? No one seems to care much that there is NO published work to show that mounds of appropriate mulch materials will somehow kill otherwise healthy trees. Instead, observers jump to the conclusion that thick layers of wood chip mulch kill trees. They are elevating correlation to causation in the absence of either experimental research OR known plant physiology. In fact, there is published research to show that thick layers of arborist wood chip mulch enhance tree establishment and survival. And there are many poor planting practices that increase the likelihood of tree failure. But it’s easiest to blame the wood chip mulch, though it’s merely masking a multitude of planting sins.

Not interested in mulch volcanoes? Well, there are lots of other examples of garden and landscape management practices or phenomena that fall into the logical fallacy camp. I’ve linked to appropriate references, when available, that go into more detail:

All of these products, practices or phenomena are correlated with some anecdotal observation (increased yield, healthier soil, plant failure, etc.) that elevates them to causative relationships. But no science.

I’d encourage you to think objectively about your closely held beliefs about your gardens or landscapes. Are you sure that what you’re doing is actually beneficial? How do you know there’s a cause-and-effect relationship? I’m not going to talk you out of your cherished beliefs – but if you are a science-based gardener, you might talk yourself out of them instead.

Willow screams in pain
What is its source of anguish?
More research needed!

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and a Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

11 thoughts on “Garden Logic – understanding correlation and causation in our gardens and landscapes”

  1. This is one of the most difficult things to teach people. This failure is seen everywhere, but is understandable when you recognize that 50% of American’s have below average intelligence…

  2. As a nursery worker and teacher I always get questions about Companion Planting – my basic answer is there are other factors more important to vegetable garden design (crop rotation, access to water, sun/shade, trellising) that need priority. I was therefore happy to come across Jessica Walliser’s book “Plant Partners” after reading her book on attracting beneficial bugs. I haven’t finished it yet but it DOES have 10 pages of articles cited, don’t know about peer review status, but still. Let me know what you think.

    1. Hi Kathie –
      I haven’t read the book so I can’t address the validity of the citations. Popular books, unverified websites, and magazines are questionable sources for science. What I would expect to see for a science-based book is citations for recent journal articles that are relevant to home gardens and landscapes, and related publications by recognized experts in applied plant sciences. I can tell you that there aren’t many reliable sources out there, as I am also working on a peer-reviewed Extension manual reviewing companion plants. Most of the articles are related to growing monocultural crops and these aren’t relevant to homes and gardens.

  3. I think it is important to determine if native plant enthusiasts ARE in fact claiming native plants are “superior”; especially when universities are massively funded by corporate entities, and as you point out yourself there is so little actual research to use for home owners. What do you think we mean by that, and are we actually saying that? No. Most of us are pushing for more native plants because for the most part we are living in places pretty much devoid of them. All plants have homes, and functions in their ecology (unless manipulated by humans in ways that prevent that), and if you compare one plant with another, “superior” then becomes a human subjective statement.

    When the current paradigm of experimental design pretty much excludes looking at how whole systems work, and field observations are dismissed out of hand as anecdotal, we are missing all those circumstances where animals and plants that do not accept laboratory (or field experiments) well are left out of the equation. We also (in North America) have a great deal of difficulty getting long term experiments done because most work is funded and done in 2-3 years by the span of a particular graduate student’s tenure as a student (as the person to run the experiment). So how do we study a birch tree (300 year life span) and really know much about it? This mindset means many ecological relationships are either not studied, or dismissed as correlation or anecdotal.

    Most of us are beginning to recognize the many examples we do have (established by science) of co-evolved relationships and the number of animals that need very specific plants to survive. I do it because I want to help prevent Anthropocene extinction.

    It is not a matter of “superiority” it is a matter of preventing extinction in the one place a person has any control any more…their garden (unless governed by a HOA, lol). I think how the question and argument is made is important. You have taught me how important the structure of a habitat can be, and I have passed that on. For those of us humans interested in “doing no harm” ecologically, and improving habitat quality, choosing local native plants is not a “superior” but a “safe” choice, but in a changing climate, all bets are off. I want to make clear I am not a native plant purist by any sense of the word. But I certainly promote, and celebrate observing plants for more than their beauty or “keep up with the Joneses” value. Particularly since the science is telling us that most invasive species come from our ornamental garden efforts. People who want to keep things simple and not carefully research every plant for its potential to cause harm become purist native plant gardeners…there are also the snobs who want to make the “superiority” argument, but those are competitive gardeners. Definitely more science is needed…and more publicly funded science is essential to free scientists to get back to basic research, which is deep inquiry without a specific economic goal in mind.

    1. I’m not going to address the attacks made on university research and funding, which really have nothing to do with the issue. There are lots of system-level publications looking at landscape biodiversity and the presence of native and nonnative woody plants that I reviewed in 2015 and which linked in this post ( I refer readers to this and other posts in the blog that can be found by using the search bar on the left hand menu and search for “native plants.”

        1. The point is you were using straw man arguments; I am not funded by “big” anything. My work is done on shoestring budgets, generally on the donations I receive for my outreach programs. You weren’t arguing about the merits of the research that exists. The literature I reviewed IS ecological literature. It is a systems approach to landscacpe biodiversity. I don’t see how I’ve dismissed ecology when basically the entire review was looking at landscapes and gardens as ecological systems.

  4. Ha! I did need that “eye-catching photo” to read the article. Then the article itself is about assumptions and bias! Goodness that is funny. I wanted to share that you were right. [Of course!]

    This is my first comment on the blog, because the articles are so thought provoking for me. Thank you for your work.

  5. Thank you! I’ve read thru the last several years posts and have come away with a better understanding of what’s happening in my garden. I’m a plant nerd and collector and have tried several of the home made potions, added amendments to planting holes, used 10-10-10 fertilizer, planted trees too deeply, unknowingly planted invasives, etc. your science based and common sense articles really opened my eyes. Only wish had found your blog earlier. I started gardening for pretty flowers years ago only recently into growing vegetables. There is so much bad information to weed thru! Looking forward to reading your books and I’m getting a soil test next week 🙂 gardening in central Florida

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