Answer: Snip off spent flower blossoms, which is a process called deadheading. It stops the plant from expending energy on seed formation, instead encouraging it to put out more blooms. Also, your plants may need a fertilizer boost. Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorus), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorus helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorus, such as 15-30-15. Bone meal is an organic source of phosphorus.
Finally, there are many types of primrose and you didn't specify what you had, or how long it had been blooming. Plants eventually reach the end of a bloom cycle. Annuals are plants whose life cycle (vegetative growth, bloom, seed set) is completed in a single growing season. The plant dies at the end of the cycle and a new generation begins with seed germination. Perennials are plants that usually grow foliage the first year and bloom the second, although some may bloom the first year. They live for several or many years and as they grow into large clumps can be divided. They tend to have shorter bloom periods and then go into dormancy before blooming again. I hope this info is helpful!
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