University of Illinois Extension
Ferilizing - Bed Preparation and General Maintenance - Stepping Stones to Perennial Garden Design - University of Illinois Extension

Fertilizing

Most Illinois soils have adequate phosphorus and potassium but are deficient in nitrogen. These are the three elements found in a complete fertilizer such as 10-10-10. The bag contains 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium. Read label directions carefully, or call your local Extension Service for help figuring out how much to apply.

The majority of perennials are not traditionally heavy feeders. That is they don't have as high a demand for fertilizer as does your lawn. In fact, high nitrogen often causes lush foliage, fewer flowers and weak stems. Perennials survive very well with one to two pounds of actual nitrogen per 1,000 square feet per year. Remember, at planting time you added organic matter and fertilizer, so you have already provided sufficient nutrients for the first year.

Start a fertilizing program the second year. There are some perennials that are heavy feeders. Astilbe, fall mums, lupines, delphiniums and tall garden phlox all benefit from annual applications of fertilizer. The best gauge is to look at your plants. If plants are not as vigorous and their foliage is light or yellow-green, applying a nitrogen-based fertilizer will be beneficial.

Perennial gardens benefit from spring applications of fertilizer. The plant uses the extra boost for spring growth and stores any excess for later in the season. It is also easier to apply at this time since the plants are small and you can avoid getting fertilizer on the foliage which causes burn.