Recent studies have shown that, poorly maintained vertical greenery systems (VGS) have a high fire risk and a tendency to cause very high heat release rates (HRR) in a case of fire. High HRR in materials often results in rapid fire spread in upward orientation. This study used numerical fire simulations with computational fluid dynamics (CFD) to investigate the fire spread along a VGS. The fire simulations were conducted in fire dynamic simulator (FDS) using the pyrolysis model that was specifically developed for vegetation. In FDS, the fire was initiated as a room fire of 1 MW followed by a window ejected flame, which ignited a 12 m tall and 9 m wide VGS. Fire spread along a VGS was modeled with four different moisture contents (MC) and four different packing ratios (PR) of the vegetation fuel bed. The different MCs represented regularly and scarcely watered vegetation, while different PRs represented different bulk densities of the vegetation in the VGSs. The fire spread rate was increased when both the MC and PR were decreased. The rate of fire spread in the upward orientation, which is the most hazardous mode of fire spread, was found to increase by 469.8% when the MC was decreased from 80% to 10%, and by 207.1% when the PR was decreased from 0.5 to 0.1. The results are helpful in understanding the design of a VGS with an optimum level of vegetation density and maintaining the vegetation fresh and alive to minimise the fire hazard.