Garden Variety: Do your homework before heeding gardening advice

Gardening is full of anecdotal reports, folklore, old wives’ tales and urban legends. Even experts sometimes have trouble discerning between good recommendations and faulty ones when information originates from other supposed experts.

A recent publication about planting trees in square holes is a good reminder of how unreliable information can sound better than it is and how quickly misinformation spreads. Gardeners seeking the best methods for plant care should evaluate recommendations by looking for the science that backs them.

The faulty recommendations for tree planting were originally published less than a month ago in a British newspaper. The author claims that planting a tree in a square hole rather than a round one eliminates the possibility of a tree’s roots wrapping around in a circle within the hole, which is a known problem. The story was quickly rehashed by other major media outlets and is still making its way around social media.

Tree roots wrapping around in circles within a planting hole is referred to as the “container effect.” Anyone who has pulled a rootbound plant from a pot has witnessed this type of root growth. When a root hits the hard edge of a container, it cannot pass through, so it must turn sideways and will end up following the round shape of the side of the container. The same thing happens when the soil at the edge of a planting hole is impenetrable.

It doesn’t matter to a tree whether the hole it’s planted in is square or round. Instead, roots respond to hard surfaces they come into contact with and alter their growth patterns accordingly. Roots growing in a square hole with hard edges will still wrap around the inside of the hole.

The best way to prevent the container effect is to refill a planting hole with the soil that was in it instead of adding compost or otherwise changing the soil. Multiple studies support this, and evidence is easily found in peer-reviewed scientific journals dating back to the early 2000s.

The square hole recommendation lacks peer-reviewed scientific research, which would likely easily prove it wrong.

Gardeners trying to decide between good recommendations and anecdotal advice should always look for the science supporting the theory. Search online or visit the library, and consider peer-reviewed scientific journals and science-based books as the best references.

Journal articles should reference the studies that support or disprove the theories being discussed. Studies should be based on the scientific method — observation, hypothesis, experimentation and analysis.

With research on plants, there are a few additional things to keep in mind. First, look at the age of the studies that are available. Modern technology has disproven many earlier theories about plant growth. Second, look to see if the study has been replicated in different soil types and climates to eliminate other causal factors or influences. Third, consider the intent of the researcher or publisher. If they are trying to sell you something, there is greater potential for bias than if the work is published by an individual or institution interested in research and education.

The good news about the square-hole planting recommendation is that the method is unlikely to hurt any trees or cost gardeners any money if utilized. You can even skip buying a new shovel, unless you really want one.

— Jennifer Smith works in regulatory horticulture and has worked as a horticulturist for various government entities. She has experience in landscape design and maintenance and as an educator.

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