Answer: Since they seem to be otherwise healthy, I'd suggest fertilizing them. Use a product for flowering landscape plants.
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, is there any chance the roots are "sneaking" nitrogen from nearby, such as a lawn? Lawn fertilizer is high in nitrogen. Good luck!
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