Fertilizing follows no set rule in all locations. One soil may need certain elements and other soils would need different elements. In some parts of Florida phosphate would not be added because it is present in sufficient amounts. The only foolproof way to fertilize is take soil samples, have them tested and add what is needed for daylilies. If testing is too expensive, a good rule of thumb is to use either 10-10-10 or 8-8-8 in the spring and during the growing year. To give a boost to the blooms, a good fertilizer would be one with a high middle number. 0-30-0. In the fall of the year, unless your plants grow all year, a good choice to use would be 5-10-15, a low nitrogen fertilizer. Nitrogen promotes green growth and they should not expose fresh tender green growth with freezing on the way.
During the growing season I use nurserymen's special, a 13-6-6 fertilizer (50 lbs.), with ½ of the first number a time released nitrogen. The numbers represent the parts of active ingredients contained in the mix. (13) 6 ½ pounds of nitrogen, 6 (3) pounds of phosphorus, (6) 3 pounds of potassium with 37 ½ pounds of non active filler materials and minor elements consisting of sulfur, magnesium, boron, copper, iron, manganese, molybdenum and zinc. I pay $14.50 for 50 pounds at Grayco's in Cairo, GA. and apply it about every sixty to ninety days.
No matter what type fertilizer is used, the plants will not take it up and use it unless the PH level is within the correct range. Daylilies will perform well in a growing medium of six to seven on the PH scale but 6.5 should be the target level. Proper PH is essential for maximum healthy growth. To raise the PH lime could be added but the level cannot be changed overnight, it takes several months. Sulfur, peat or other acidic elements could be added to lower the PH and this works faster. Of course, most plants love composted organic material but make sure it is within the correct PH level and not high in salts. I use a Kelway Soil Acidity & Moisture Tester Model HB-2 for testing water, soil and organic materials. (Park Seed $100.95).
Mulching your Daylilies can have many benefits. Among these are retaining moisture and helping to control weeds. My favorite material for mulching is pine straw and oak leaves which eventually compost and become part of the soil.
Good luck with your fertilizing.