Not All Gardening Advice Is Correct. Anecdote Is Not Evidence. Correlation Is Not Causation. But We Often Treat It That Way. Just Because I, Or Anyone Else, Says Something Is True Doesn’t Make It True. Sadly Many Gardeners Wrongly Believe All They Read.

Early blight Of Tomatoes Doesn't Exist In the UK
Early blight Of Tomatoes Doesn’t Exist In the UK. And Correlation Is Not Causation

An excellent example would be all the UK gardening blog posts where people talk about early blight on potatoes or tomatoes. They know it’s true because they’re read about it. The only thing is …. it’s not true, it’s a gardening myth. We don’t get early blight in the UK. It’s a problem that Americans get that doesn’t exist in the UK.  It’s the same with tomato pests. They are often plagued with tomato cutworm. We do get a few types of cutworms here but in all the years I grew tomatoes commercially I never once saw cutworm damage. 

So when people start writing about how tomatoes in the UK are suffering from early blight, ignore them. They are wrong. We don’t get it here. The information about early blight is relevant to America not here. 

Another example would be the argument about traditional digging and No-Dig and which is best. Personally I prefer No Dig and can give my reasons. They include the fact that it improves soil structure, yields and quality improve and that it is lower cost and easier on the back. Those are verifiable reasons based on scientific and verifiable evidence.  But it doesn’t prove it is best.  

Why, because if we say “best” we have to define what we mean by best. It might well be the traditional digging is best in some circumstances. For example it’s “best” if you want to use digging as a form of exercise. It will burn off plenty of calories. It’s “best” if you want to remove stones from the soil. No Dig doesn’t reveal the stones that are below the soil so isn’t best in that case.  

However, if I want the best method to sequester carbon in my garden, science indicates that No Dig is best. 

It’s not just about what someone says. Sometimes it’s drawing the wrong conclusions from poor evidence. Correlation is not necessarily causation. 

Correlation Isn’t Causation


The silliest example of this illustrates it nicely. When the wind blows hard windmills always rotate fastest. Therefore we could argue, because they are mechanical devices and called mills, the wind is caused by windmills rotating. But it wouldn’t be true. 

Another example would be that because my granny smoked until she was 93 it was the smoking that kept her alive. Therefore we could argue that we should all smoke if we want to live longer. 

Of course these things are obvious .. says he with a smile! It’s like the evidence about gardening and obesity. During lockdown many more people took up gardening and many people put on weight. Therefore gardening makes you put on weight. (Of course it might be that the increased number of gardeners and increased obesity isn’t connected). 

The thing is there is a lot of bad advice out there. Some of it is spouted by TV gardeners who may have a wonderful TV garden, but actually have no experienced except reading the script that is given to them. If the scriptwriter gets it wrong so does the TV gardener. 

Their garden is of course a theatrical stage and far too often maintained by a team of gardeners rather than the presenter.   

Years ago TV gardeners were all knowledgeable gardeners. Most had served a gardening apprenticeship eg Alan Titchmarsh. Another example is Geoff Hamilton who worked on various nurseries, took a National Diploma in Horticulture at Writtle College and later started a nursery and garden centre before fronting BBC Gardeners’ World. Both were experts. Sadly not all TV gardeners are so knowledgable. 

So my advice is to research what you hear. Ask yourself if it makes sense. Read books by knowledgeable authors that have a background in gardening and horticulture. And don’t just think that, because they have written a gardening book or ten, they know what they are doing. Some use ghost writers to write the books in their name. I ghost wrote one myself some years ago. But I can’t tell you whose name is on the cover! 

Sadly it’s not easy to sort the good advice from the bad and there is a lot of bad advice, especially  online. I sometimes read information and have to check it out by researching academic research or referring to people I trust.  I’m lucky in that I know a few well known gardeners and, if in doubt, can ask them. 

Part of the reason I started this gardening website was to make it easier for people to find reliable gardening information. With my decades of owning a nursery, and years working in agricultural and horticultural colleges, I the advantage of experience. But even then I still have to double check some information before publishing it on my website! 

Enjoy your gardening. 

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