Lagarosiphon majorhas successfully out‐competed native species wherever it has colonised New Zealand lakes in the depth zone 2–6 m—normally occupied by native milfoils(Myriophyllumspp.) and pondweeds(Potamogetonspp.). In this investigation we examined the early growth stages ofL. majorandM. triphyllum. Both species showed an initial lag in shoot growth when grown from vegetative shoot propagules in situ. During this lag in shoot growth, the development of a functional root system was apparent. The length of the lag in shoot growth appeared dependent on the ability to utilise internal nutrient stores and on the water nutrient availability. However, irrespective of nutrient status,L. majoralways grew better thanM. triphyllum. In laboratory experiments investigating this lag phase,L. majornot only produced roots faster, it also grew faster, both in length and biomass, than the nativeM. triphyllum. This suggests thatL. majorhas an early competitive advantage which may explain the dominance this species now has in the littoral zones of many New Zealand lakes.