The time it takes for a rose bud to open its petals is largely dependent on the stage of bud development and the variety of rose. Some roses expand slowly and hold their bloom for a long time, while others bloom and drop quickly. It takes about two weeks from the time a bud sets to the time the blossom opens. Sunlight exposure, temperature and other environmental conditions factor into this timing as well.
Each bloom cycle lasts 6 to 8 weeks from the time that roses are cut or finish blooming to the time of the next bloom. The time depends on variety, and many types will complete the cycle sooner than that. Frost can slow down the initial start of a bloom cycle, while hot temperatures can accelerate it. When you remove dead blossoms from bloomed rose buds, a new cycle begins along with new growth of stems and buds. At the start of a bloom cycle, provide nitrogen fertilizer. In a few weeks, rose buds will begin to set. This occurs a few weeks prior to actual blooming.
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From the time buds set to the time they open into bloom lasts about two weeks. Buds are far from fully developed at this time. They are large and beginning to bulge, but are not yet cracked and petals have not emerged yet. Apply fertilizer with higher levels of phosphorus fertilizer and lower levels of nitrogen to encourage blooming. Once buds have set, they prepare to open.
The last two weeks of bud development is when roses begin to bloom. Rose buds that are showing their petals and starting to open may be just two days from full bloom. Standard roses, with petals compact but starting to spread, will be fully open about three to four days later. Sweetheart roses may take just two to three days to spread their petals when in a partially-open state. Fresh roses arriving from flower companies are often delivered as these semi-open buds.
Accelerating Bloom Speed
Certain environmental conditions and rose varieties tend to promote faster bloom times. Full sunshine and warm weather encourage roses to open quicker. Proper fertilization as roses set bud, with extra phosphorus and potassium, encourages better blooming. The black magic rose is a variety with a lengthy blooming period. Growers report that it opens slowly and holds its shape for two to three weeks before losing its petals. Other rose varieties, such as the iceberg rose, may complete the process of blooming and dropping their petals within one week.