Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype.