If something's lumpy and misshapen, you can describe it as knobbly. Don't throw out those knobbly potatoes from your garden — they're ugly, but they'll taste perfectly fine!
If a camper pitches her tent over a bumpy patch of ground, she'll feel all of those knobbly roots and rocks through her sleeping bag later. This adjective, a variation on knobby, comes from knob, "rounded lump or bump." That gnarled oak tree in your yard is knobbly, and so are your little sister's bony knees.