Taproots are the umbilical cords of the tree world.
They serve as a lifeline from day one as a tree seed sprouts, growing straight down or shifting course slightly as minor obstacles are encountered. As the root tip encounters an inhospitable environment that it can’t avoid (too wet, too dry, too high pH, etc.), it’s growth stops. Root buds sprout and develop higher up, many near the soil surface. These new root branches, horizontally growing and radiating outwards from the trunk, become the framework for the permanent root architecture that will influence the trees survival until it’s old age.
Now imagine the same seed sprouting in a hard plastic pot. The taproot emerges, goes straight down, until…it runs into the bottom of the pot, makes a right turn, hits the side of the pot, makes another turn…you get the idea. The taproot is never terminated. The horizontal side roots that emerge face the same fate, and the end result is a rootbound mess. Plant that same tree directly in the ground, and the imprint from the pot will be there for the rest of its short, unhappy life.
Eventually, the poor structure of the pot-imprinted root system means the tree will be knocked over in the wind, or the circling roots manage to girdle the main trunk before that happens. Back when I was a tree consultant, it seemed to me like it took about 10 years before the mangled root system, usually combined with too deep of planting, did the tree in. Just enough time to invest all the time and water to get the tree established, but not long enough to start receiving most of the benefits of having an established tree.
Now imagine another scenario. A tree seed sprouts in a nice, loose, rich garden bed. The taproot goes down, and down, and down. It never encounters a region of the soil that stops its growth, so it never stops growing. The buds that lead to horizontal roots at the soil surface are thus never stimulated to sprout. At the end of it’s first year of growth, it is just one long root, like a carrot. When that tree gets dug up, this single long root shank breaks or is later cut. Next year, root branches are stimulated to grow at last year’s cut—much lower than the surface. One long carrot-root with a cluster of roots below is the final result. When this tree gets replanted again, the planter follows standard planting practice, with the cluster of roots near the soil surface. This leaves the upper part of the root shank sitting up above the surface exposed to the elements. Not ideal.
The solution as a tree grower is to mimic the natural process outlined at the top of this essay. Put the seed in a location where it will eventually encounter the stress it needs to stop growing and allow horizontal roots to sprout higher up, with some near the soil surface.
It turns out that exposure to air is enough to terminate the growth of the taproot. There are fancy plastic pots that have been engineered to allow “air pruning”, rather than circling of the roots. They are fascinating from a design perspective, with each pot company claiming advantages over the other. But they are also over-engineered and unnecessary, when you realize all that is you really need for air pruning is some ¼” hardware cloth at the bottom of a raised bed.
So that’s what we grow seedling trees in. A raised bed with ¼” hardware cloth as a bottom. The result is a communal bed full of seedling trees with a root architecture designed to last.