Literally, the word ‘tap’ describes both essential functions, that this first root to grow from a seed is designed to do - go deeply into the soil (mechanically ‘tap’ its way down into the soil) and then secondly be able to provide a consistent supply of water (become its ‘water tap’) so essential for a plant’s early establishment and long-term health. However, once the taproot in a nursery is cut or forced into a pot, it will not regrow to reach the depths of its genetic potential.
The carrot for example, so familiar to most of our salad plates, is planted as a seed to enable (in good soil) its taproot to go 10ft or more in the first few months before it begins to focus on the leaves. The part of the root we see and eat is the upper storage structure - which has few lateral roots along its sides as it is the taproot (and mycorrhiza when present) which provides all that the plant needs to grow healthy leaves to create the carbs stored in the carrot we so enjoy eating. Incidentally, the leaves also make good eating!
A tree’s taproot comes in 3 forms when grown from seed – up to 6ft in depth - up to 10 to 15ft and then those that are known to go down as far as they can (400ft is the deepest known). Because the emerging seedling prioritizes this primary root, if we want to be able to grow trees in city landscapes that will have the means to live long lives, be stable and be able to go through long periods of drought without people having to water them, then we growers of trees must prioritize the skill and practise of enabling the seed to fulfill the taproot’s potential.
The problem is that these days we sow seeds in containers for later transplant, and this physically prevents this wonder of a root from exploring down into the depth of soil and subsoil.
The physical stability of a street tree or a tree near our home is compromised so that we can transplant it to its final home. This rightly becomes a safety factor in cities where the winds speed up over open ground. A sight we see so often in the media when there has been a strong windstorm is large uprooted trees but with a shallow and very small root mass. When a tree has a deeply grown anchoring tap root they do not fall over - they bend as they have evolved over the millennia to do.