Forcing is a method that is usually used to induce flowering in plants, independent of the natural blooming period. The temperatures required to start blooming in lilac in November are ca. 37°C causing degeneration of flowers. Forcing at 15 °C in November requires 49 days to bloom as compared to 23 days for the standard 37 °C, but gives panicles of much better quality than those forced by standard method (37 °C). In this study, we have investigated the protein content, total protease activity, and cysteine protease activity at different stages of flowering (flower bud whitening and swelling, open flowers, wilted flowers) for lilac flowers (Syringa vulgaris L., fam. Oleaceae) blooming under three different conditions: natural conditions in May and forcing in a greenhouse in November at 37 °C (standard forcing) or at 15 °C (alternative forcing). The protein content was relatively constant during flowering for each of the three sets of conditions. Flowers from 15 °C had a significantly lower total protease and cysteine endoprotease activity than flowers from 37 °C at all stages. Flowers from plants blooming in May had a very time-specific cysteine protease activity, which was dramatically higher for the open flower stage than for the other stages. The partial coding sequence for a KDEL-CysEP was isolated, and its expression was determined by qRT-PCR. The gene expression did not correlate with the cysteine endoprotease activity especially in May natural flowering and November alternative forcing at 15 °C. Alternative forcing method at 15 °C affected the flowering process delaying senescence, presumably due to the low cysteine protease activity. © 2016, The Author(s).