A zonally averaged photochemical-dynamical model of the middle atmosphere is used to study interactions between the quasi-biennial oscillation (QBO) and extratropical planetary wave activity that controls ascent rates throughout the equatorial stratosphere. We find that the response of equatorial ascent, and QBO period, to varying planetary wave amplitudes depends on the size and location of the imposed changes. In the Southern Hemisphere, where the planetary wave forcing is smaller than in the Northern Hemisphere, increased forcing produces stronger equatorial upwelling and a longer QBO period. In the Northern Hemisphere, increased forcing produces weaker upwelling and a shorter QBO period due to the larger amplitude waves becoming "saturated". Changes in planetary wave amplitudes of ±25% cause 2-3 month changes in QBO period. These differences in the QBO period can alter the decadal variability in model constituents that originates from interaction between the QBO and the annual cycle.