Using liquid fertilizer to feed daffodils would appear to be the best way as these plants have simple root systems and by watering in the correct amounts of what your soils need you can get the dilute fertilizer right down to the root tips where it is needed for the plant to pull it up quickly and use the fertilizer before they go dormant in the northern hemisphere.
Here is a handy work sheet for figuring up how much fertilizer to use for various concentrations.
According to this to get about 200 Parts Per Million of potassium in solution out of Char’s 0-0-60 she would need to use about 10 ounces of this particular bag of fertilizer dissolved per 100 gallons of water. This 200 parts per million application is what is sometimes used on a wide variety of greenhouse plants as a “constant feed” for plants grown in containers. This is where constant watering flushes out the salts and soluble fertilizer in the pots. Soils out in flower beds and fields would be catching this soluble fertilizer down deeper where plants could access it over the years.
An inch of rainfall drops approximately .62 gallons of water per square foot. So 10 ounces of 0-0-60 mixed into 100 gallons of water could be used to give an inch of fertilizer water to 161 square feet of daffodil beds. This figures out to spreading out nearly 168 pounds of 0-0-60 fertilizer per ACRE at this rate. OR you would be applying 101 POUNDS of the actual potassium per acre. (I personally use a commercial fertilizer injector that has a rated use of 1-100. I also use some siphon fertilizer injectors that are 16-1). So concentrations used would vary greatly with what types of fertilizer injectors a person wants to use.
Now MY personal soil test on our daffodil fields shows that I STILL need to add 70 pounds of actual potassium per acre just to get to “critical levels needed for plant growth” so I would need to only apply 3/4″ of this fertilizer water per square foot.
BUT without a simple soil test Char cannot be sure as to the exact amount of potassium that her soils need. This amount would NOT be harmful BUT I would only make ONE application right now. Then get a complete soil test done around August. THEN if you are short Boron or any of the other micro/macro nutrients you can also add these into the fertilizer solution and apply them normally in a single mixture. Ideally fertilizer applications should be made in smaller multiple applications as the plants sprout roots, actively grow and bloom and then quit fertilizing well before the plants begin to mature since the fertilizer applied late will NOT get down to the root level in time. Leaving the last fertilizer applications to feed weeds while the daffodils are actually dormant.
Notice the fact sheets actually have some errors on figuring Phosphorus and Potassium as they transposed numbers needed to convert to actual %K and %P. There are other tables to convert to grams and liters for the rest of the worlds’ plant growers:-))
Round-up or Glyphosate (sp) can be used IF and only IF there are no green stems or shoots of the daffodils still above ground. You also do NOT want to spray Round-Up down into the open holes left when the daffodil foliage dies back leaving those open holes right down to the top of the bulb.
Poast is labeled to be used to kill SOME types of grasses that grow among daffodil, day lily and iris foliage. It will not be very effective on Dichondra Turf Grasses that Char has giving her problems. Round-Up used later this summer should wipe out these plants.
Above is the Poast herbicide label. One of the things to read here is the amount and type of nitrogen fertilizer to you can add to the mixture to help speed up the killing of the product or to increase the kill rate of the product thus using less herbicide. This also works with Round-Up allowing you to use less product. I also add in small amounts of Boric Acid to all of our liquid fertilizer and herbicide sprays ONLY because my fields test low for Boron. Boric Acid is about 18% boron. Keith Kridler Mt. Pleasant, Texas Frost warnings out for the next two days.