Question
When do I need to fertilize daffodils and iris, so that they will bloom well?

Answer
If your Soil is nutrient rich, you should not fertilize your Bulbs.  Too much of certain nutrients will block the uptake of others, no matter how badly they are needed.  Fertilizer companies won't tell you this because they want you to buy their fertilizer, but they are creating a need where there is none.

In addition, certain Bacteria thrive on Nitrogen in the Soil; these certain Bacteria cause bulb rot when temps cool down and moisture levels rise.  Unless your drainage is absolutely perfecto, and even if it's not, the Bulbs will turn to mush that way.

Healthy levels of N P and K are always good to have.  But too much of any of them potentially destroys your Bulbs.

THE LONG ISLAND GARDENER