Generally speaking, cut back daffodils 6-8 weeks after they bloom.
More Info: The actual time to cut back the foliage on the daffodil plant will depend on the weather and growing conditions and the type of bulb planted. Rather than cutting them back at a particular date following blooming, there are signs to watch for that let you know that the foliage is ready to be cut back.
Do Not Remove Live Foliage
Green foliage should never be removed even after the daffodils have faded. The leaves act similar to solar panels that collect the sunlight and then through photosynthesis turn it into food to nourish all parts of the plant including the roots and bulb. It is for this reason that they must remain intact. Bunching and clumping them together can also hinder the photosynthesis process.
If you remove the leaves prematurely because you consider the plant’s lifecycle over for the year, you will find that next year they may have less prolific blooms or none at all. You need to wait until the leaves naturally yellow, wither, and ultimately die on their own prior to removing. This is generally anywhere from 4-8 weeks following blooming.
Give Your Daffodils a Helping Hand
Once the leaves of your daffodil have turned yellow and died back naturally, they no longer provide food for the plant and are ready to be removed. Remove them individually by hand. Do not mow down foliage or cut with a knife or scissors as this can encourage the spread of viruses and disease to the plant.
“Yard and Garden: Daffodils | Iowa State University Extension and Outreach.” Iowa State University Extension and Outreach. N.p., n.d. Web. 21 May 2012. <http://www.extension.iastate.edu/article/yard-and-garden-daffodils>.
Extension, University of Missouri. “G6610 Spring Flowering Bulbs: Daffodils | University of Missouri Extension.” University of Missouri Extension Home. N.p., n.d. Web. 21 May 2012. <http://extension.missouri.edu/p/G6610>.