Adventitious

From PLANTFACTS.OSU.EDU

1. plant parts, such as shoots and roots, produced in an unusual position on a plant or at an unusual time of development.
2. roots or stems arising from unexpected locations.

Adventitious roots on a Coleus plant.
Source: http://botit.botany.wisc.edu/images/130/Root/Adventitious_Roots/Coleus/Adventitious_roots_2_MC.php