Nonstructural carbohydrates (NSC) are one of those quirks of nature that perplex gardeners when they come across these knobby underground things. They are a part of the plant but not a root per se even if they are underground. Scientist don’t agree on why NSC exist but one thing that is known is they don’t provide structural stability for the plant (hence nonstructural) and carbohydrates and nitrogen are stored there.
One possibility on why NSC exist is because the plant is stressed. In the case of the pictured plant, purple poppy mallow (Callirhoe involucrata), this native wildflower had originally been planted close to a small tree. Purple poppy mallow likes lots of sunlight, so as the years went by and the tree grew, the plant found itself in shadier conditions. So it is a possibility the plant developed NSC as a stress response.
Another thought on the existence of NSC is that they help plants survive, metabolizing the needed carbohydrates to mount defenses.
NSC are not specific to any one tree or perennial. Nuisance trees can develop NSC if they’ve been cut back numerous times. Mulberry, Siberian elm, and tree-of-heaven are notorious for growing in unexpected places after seeds have blown in or been deposited by birds. Continually mowing or pruning back the seedling trees without dealing with the roots is enough for the plant to respond by developing NSC.
If you’ve been unsuccessful in removing plants by cutting them back, consider digging them out. Size-wise, NSC can get large enough to physically interfere with root growth of surrounding plants.
