I have both in my mixed borders plus a multitude of other perennials and shrubs. No problem.

I too am faced with a slew of daffodils and the resulting "dead space" problem... Would peony, daylily, hosta, rudbekia, dianthus or iris appreciate a similar environment?

How closely could one interlace with the daffodils?

I am sure they would be fine. I have to confess its not an issue I've ever dealt with personally because I've never grown daffodils, or any bulbs, indeed any type of plant at all, in a dedicated bed. Just mixed beds.

You may want to interplant daffodils with some plants that grow and flower soon after daffies' blooms are done to cover their fading foliage though not completely so that they still get some sun. Bleeding hearts and astilbes are some options. For later blooms in that area Japanese anemones that you inquired about should be fine. I have daffodils in more than one area and grow 'Pamina' and 'Honorine Joubert' near them (each in different area), and 'Old-fashioned' bleeding heart and 'Francee' hosta as well. They are planted about one to two feet or slightly further away from daffodils and work fine together.

Daylilies are also excellent in disguising fading daffodil foliage. And oriental poppies :-) In fact, most perennials coexist with spring flowering bulbs very well!!

I second oriental poppies and daylilies. A neighbor did wonders with the smallish hellstrip in front of their house, interplanting spring-blooming bulbs with daylilies, phloxes, oriental poppies, echinaceas, Japanese anemone. It looks glorious.

I forgot to add a word of caution about J. anemones: you want to be careful in selecting a variety because some are invasive. I've been lucky that 'Honorine Joubert' is not invasive in my area, but some others on this forum report that it spreads aggressively for them. 'Robustissima' is definitely a thug, albeit a beautiful thug :-)

Could I plant on top of the daffodil bulbs? The little bed is filled with bulbs and I'd need to wait till they came up to be able to plant around them if there was even any space

Was reading last night that many of the bulbs don't appreciate extra moisture during the dormant summer period, and that frequent watering of nearby thirsty plants could lead to rot of the bulb.

To me that sounds like either digging and storing each summer, or having super super well drained soil and planting companions that require very little supplementary water.

floral_ul, just to be sure I understand, as long as other plants aren't on TOP of a bulb, spacing requirements are essentially nil?

I can't help you on the water issue since I never need to irrigate. However, we have copious rain which doesn't bother the bulbs.

Yes, you have understood what I meant about not planting right on top of bulbs.

Re: watering. The spring-flowering bulbs should not be watered in summer/fall, but it doesn't have to be an issue if grown with perennials; it depends on what watering system you use. Our soil is tough clay, we water with a hose, and we never had problems with our bulbs when watering nearby perennials. They are spaced at least a foot away, which probably also helps.

But, but, but.... the spring flowering bulbs get watered by nature in my garden. It's not a problem.

Exactly, floral. Sorry for not being clear: my comment was addressing possible concerns about watering nearby perennials in summer. Not an issue in our garden even though our soil is fairly compact clay with so-so drainage, though we do amend it with compost twice a year. Daffies and other bulbs still come back every year, watering nearby plants and all.