Answer: When daffodils become too crowded, they may stop blooming. Based on your description, your older daffodils may need to be lifted and divided. Do this in early summer when the foliage is brown and dried up. Dig them up, separate them, and replant.
You can replant them immediately, or you can store them and replant in mid fall. To store, remmove the foliage and shake off the soil. Air dry for several days. Then keep them in a dark place that is airy and somewhat cool such as a garage. Keep them in a single layer on a shelf or in a paper bag or cardboard box. (Do not store enclosed in plastic bags.) Check on them periodically and remove any that begin to shrivel or rot. Replant at normal bulb planting time in October.
Other possible reasons for lack of bloom include lack of direct sun and premature removal of foliage. Daffodils need sun to thrive. Full sun all day is best but they also do well in partial sun. If neighboring trees have grown a lot in the past ten years or so and they are now in deep shade, they may need to be transplanted. As long as you are moving them, take the opportunity to separate them as well, as described above.
The other common cause is removing the foliage too soon. They need to grow and mature their foliage. This process renews the bulbs so they have strength to grow and bloom the following year. Allow the foliage to turn brown and dry up before you remove it. Do not braid it or do anything special to it, just leave it in place until then.
Normally daffodils are planted in areas where they receive some fertilizer either due to being in a flower or shrub bed or near lawn that is fertilized. If you feel they may need fertilizing, you could try using a bulb booster in the fall. Spread it per the label directions in early fall so it moves down through the soil to the bulbs by the time they are actively rooting in mid fall. In early spring just as they begin to come up, apply a granular general purpose fertilizer such as 10-10-10 to the soil around them. This should supply nitrogen and other nutrients to help bulk up the foliage.
Finally, you can deadhead to help the bulbs conserve energy. Remove any flowers as they fade so the bulbs do not devote energy to forming seed. This helps rebuild the bulbs. On the other hand, it also prevents them from increasing by seed which is how many naturalized plantings increase over time to form impressive carpets or fields of daffodils.
I hope this helps you trouble shoot and gives you some ideas to try. If you have a lot of digging and dividing you might want to invite friends to help and share some of the bulbs -- you will be amazed at how many you will find per clump. Replant all of them, large and small, eventually they will all bloom. Good luck with your daffodils!
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