Since 1789, the constitution has been amended 27 times and I am here to say what I think should be the 28th amendment: no. more. amendments. … to planting holes! Okay so here me out because you may not have heard about this but it’s a hot button issue for me. The concept of “amending planting holes” is referring to when people add soft soil like compost or other organic matter to holes in their garden before putting in the tree or plant. It is a common misconception that this is a good practice and will solve all your soil problems. Gardeners need to stop amending planting holes and though this really may not need to be added to our country’s constitution, maybe we could petition to include it in the pledge of allegiance? Or the National Anthem? We can circle back around to that later…
All kidding aside, if you’ve ever planted something yourself, you know it’s not as easy as “dig a hole, plant a tree.” If you want your plant to flourish and your garden to thrive, there is a lot of prep work involved including choosing the perfect location, deciding on the right plant, and even making sure you’ve dug the right sized hole. What comes next is what to fill your hole with. That’s the part that people tend to diverge in method.
In college I remember reading this book about soil from Carl Whitcomb, an expert in plant root systems with a PhD from Iowa State University and inventor of RootMaker products. At the time, what I was reading seemed like a pretty boring subject. Little did I know, Dr. Whitcomb’s findings would become very important to me. His research showed that adding organic material to the holes of new plantings is not very helpful to the plant. Individual plants do best in native soil – no amendments!
Dr. Whitcomb hypothesized two ideas as to why compost could be harming your planting hole. For one, the combination of native soil and composted soil can be confusing for the roots of the plants. The different sizes of soil mixed together suck moisture from the root ball, causing its growth to stagnate. Another reason compost doesn’t help the planting hole is because the new plant doesn’t have any motivation to venture out from the rich, amended soil it was stuck in. It is gaining all its nutrients from the enhanced soil so its roots don’t want to grow and settle into the surrounding natural soil. Why would they?
Luckily, Dr. Whitcomb is not alone in his findings about amending holes. According to Linda Chalker-Scott, Ph.D. Extension Horticulturist and Associate Professor at Washington State University, “No scientific studies to date show any measurable benefit of soil amendment except in containerized plant production. Plants grown in native soil consistently showed better root establishment and more vigorous growth.” In her article, she gets into the nitty gritty of plant physiology and soil water relations but her findings are essentially the same as Dr. Whitcomb’s. Dr. Chalker-Scott writes, “Roots react much in the same way as they do in containers: they circle the edge of the interface and grow back into that more hospitable environment of the planting hole.”
So you might be wondering, if the scientific and horticulturist community agree that amending planting holes with organic material is nothing but a myth, why do we see this still happening everywhere? The answer is, as it unfortunately tends to be, money. According to Dr. Chalker-Scott, “This outdated practice is still required in the specifications of architects, landscapers, and other groups associated with landscape installation. It is still recommended by garden centers and gardening articles. And there is a multi-million dollar soil amendment industry that has little interest in debunking this myth.” She calls to action, as do I, green industry professionals everywhere to spread the word and take a stand against non-sustainable practices like these.
The good news is, foreign organic matter is not completely useless in your garden. It has its place! Dr. Whitcomb recommends using mulches such as pine straw and nuggets on the top of the soil, once the hole has been filled with native soil and the plant is in the ground. This type of organic material will break down over time and add nutrients to the soil from the top down, which actually mimics natural processes.
Also, compost is very useful in beds for large numbers of plantings. The native-only-soil approach applies to single holes dug for single plants whereas tilling compost with native soil can give your large plant beds lots of opportunity for healthy root growth.
So have I won you over to my cause? No more amendments… to planting holes! Don’t do it. I promise, you will thank me later.