If you breaking any flower using a tool which has the fortune enchantment, then, in addition to the flower itself, there should be a chance for flower seeds to drop.
Each flower would drop it's own seed type. Each flower seed can be placed anywhere the corresponding flower can be placed, producing a seedling. Each seedling turns into a fully grown plant as a result of a single random block tick, although bone meal will also work.
Wither rose seedlings have two additional requirements for becoming an adult wither rose plant: It must be in contact with both a mob who is immune to the Wither Effect, and a mob or player who is withered.