Q: My neighbor grows her tomatoes in pots about 1.5' deep. I always heard that the height of the plant above ground should be equal to the height below ground - the roots.
Yet she gets better vegetables than I do and I
plant in the soil. Her plants grow over 3' tall but obviously the roots can't.
Please advise. (Now that I think about it, I can't imagine our
tall trees having roots going that deep - maybe she is right)
If you are growing in native soil, it may not be friable enough or rich enough , even with amendments.
Your neighbor is probably
using a good potting soil, which is better for tomato plants than hard soil.
As for depth, roots do not grow as deep as a plant is tall. As you've
observed, this would mean that tree roots would be dozens of feet down, and I assure you they are not.
Instead, roots grow outward, and can be several
feet deep in some cases, but they're rarely down more than 5' around these parts.