Wow.... didn't know my comment would cause such an argument.
I didn't say that deep horizontal roots are not a good thing. Indeed, deep roots are crucial for moisture deprivation stress and relieving high top soil temperatures, as well as comfortably anchoring the plant. I was simply replying to the fact that when you feed the plants with liquid fertilizers or by cultivating in amendments into the topsoil, its the upper lateral roots that draw in the most nutrients, as deep as 2 ft down, while the lower vertical roots and tap roots draw in a lot more water in ratio to the nutrients they are pulling up. In conditions where I have been unable to dig deep holes, I have dug wider instead. Like when solid granite deadpan was less than 2" below soil level. Its an effective alternative.
I think you and Fdd have different soil compositions right? One sandy loam and the other red clay? Not sure what Fdd has for local there, but it looks nice and rich and well ammended.
I dig my holes as deep as I can, and ammend all the way down. The roots will grow wherever there is water, air, and nutrition.
In general I have found, the larger the hole, the larger the roots/root system, the larger the stalks and amount of foliage, and the better the results. So just a very large whole in general is good. I haven't been growing for 20 yrs, but I have been around growing, and plants, and gardening my entire life, and have always been taught that the deeper you Can go, why not? As well as, the wider you Can go, why not?
When I have put the plants in the ground, I usually don't find the end of the root systems, they just keep going, quite a bit farther than the holes I dug, but the holes I dug have the densest root masses. For example, the raised beds for my larger plants are 3x3x2, I dug down into the level soil about 18-20" avg and about 4 1/2 ft by 4 1/2 ft wide, the top of the raised bed sits 18 inches above the ground, so my hole including the raised bed is about 3' deep... there is pics of a hole I dug in my outdoor thread. I hit a vein of granite that was made up of a good amount of larger pieces of granite with cracks and fissures between them, I decided to stop there even though it was only 15-17" deep, because getting the granite out would have really sucked balls, and the cracks and fissures provided good drainage. I expect the roots would go down into the cracks there following the water, but also out to the sides, because water would run off to the sides over the granite as well, and there is plenty of amended soil to inhabit. I widened the hole and amended laterally as an alternative,
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I put my sensi star in that bed, and now she is trying to flower.... so Im not sure how much her roots will grow out, as Im not sure if she will reveg, I hope she does though.