We present in situ environmental transmission electron microscopy (ETEM) observation of metallic lithium nucleation, growth and shrinkage in a liquid confining cell, where protrusions are seen to grow from their roots or surfaces, depending on the overpotential. The rate of solid-electrolyte interface (SEI) formation affects root vs. surface growth mode, with the former akin to intermittent volcanic eruptions, giving kinked segments of nearly constant diameter. Upon delithiation, root-grown whiskers are highly unstable, because the segmental shrinkage rate depends on Li+ transport across SEI, which is the greatest around the latest grown segment with the thinnest SEI, and therefore the near-root segment often dissolves first and the rest of the whisker then loses electrical contact. These electrically isolated dead lithium branches are also easily swept away into the bulk electrolyte to become “nano-lithium flotsam” because the hollowed-out SEI tube is very brittle. Our observations are consistent with SEI-obstructed growth by two competing mechanisms; surface growth of dense Eden-like clusters and root growth of whiskers, resulting from the voltage-dependent competition between lithium electrodeposition and SEI formation reactions. Similar phenomena could occur whenever chemical deposition/dissolution competes with irreversible side reactions that form a passivating layer on the evolving surface.