Plant In Square Holes
Makes sense once they describe the problems.
"Traditionally, trees were planted in round holes, perhaps because their trunks are round, as is the spread of their canopies. It was just one of those seemingly obvious, unquestioned assumptions. But here’s what happens to the tree’s roots when you plant them in a round hole, especially one filled with lots of rich compost and fertilizer, as the old guide books suggest. The little sapling will rapidly start growing new roots that will spread out into the rich, fluffy growing media, giving you excellent early success. However, once they hit the comparatively poorer and compacted soil at the perimeter of the hole, the roots will react by snaking along the edge of the hole’s edge in search of more ideal growing conditions.
Eventually, this spiralling action around the limits of the hole will create a circular root system, with the plants essentially acting much as they do when grown in a container. Once the roots mature they will thicken and harden into a tight ring, creating an underground girdle that will choke the plant, eventually resulting in the severe stunting and even death of your treasured tree.
The very simple and counter intuitive act of digging a square planting hole will dramatically reduce the chances of this happening. This is because systematic planting trials have shown that roots are not that good at growing round corners. When they hit the tight, 90-degree angle of your square hole, instead of sneaking around to create a spiral, they flare out of the planting hole to colonise the native soil."
I know from looking at the root structure of my planters at the end of the season, that yes plants will continue to grow roots until they fill up the container. Perhaps I'll try planting my vegetables this year in more loosely dug holes and instead of filling them back in with a handful of compost, just use the dirt instead.