Tree descriptions refer to two classes of trees: some with shallow roots and others with deep taproots. Those with taproots have most of their roots deeper in the ground, where they get most of their water and nutrients. These deep-rooted trees are claimed to be better for gardens because digging in the garden will not damage their root system, and there is less competition with other landscape plants.
Some trees, like oaks, do start out life with a large taproot. If you look at some year-old seedlings you can clearly see the carrot-like root. Non-taproot seedlings have a fibrous root system similar to perennials. In the first few years of life, trees with taproots do form a strong, deep root but as the tree ages it also forms a traditional fibrous root system. Over time the fibrous system becomes the main root system of the tree and the taproot starts to shrink and may even be lost completely in old age.
All trees have a shallow root system. Roots need to find water, air and nutrients, which are most abundant in the upper few inches of soil, and this is where tree roots grow best. Some trees have a more open root system and when a gardener digs in the soil the root system is not as obvious. Others, like junipers, make a very dense root system that can be difficult to plant into.
Assume that all trees have shallow roots and each time you dig near a tree you will damage some of these roots. Luckily, trees can easily grow new roots, provided that not too many are damaged.
Since tree roots are shallow, never add more than 2 inches of soil over their root system at any one time. Building deep flower beds on tree roots will harm them and can kill the tree.
Written by Robert Pavlis, author of three books, , Garden Myths , Soil Science for Gardeners and Building Natural Ponds.
This article is Myth #76 from Garden Myths: Book 1, available in the UK online from Amazon here: https://www.amazon.co.uk/Garden-Myths-Mr-Robert-Pavlis/dp/1542465222/ref=sr_1_1?dchild=1&keywords=garden+myths&qid=1605707945&sr=8-1
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