Answer: Congratulations on your flower beds! I'd wait to apply fertilizer until next year, a few weeks before planting. Then it will be readily available for your plants, rather than leaching through the soil over winter. Miracle-Gro would be a good choice for flowers. You might want to add compost to the soil this fall to improve its organic matter content and help to build a healthy soil structure over time.
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 (phosphorous), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorous 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 phosphorous, such as Miracle-Gro's Plant Food at 15-30-15.
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