As no one has answered this, I will try. I think there are multiple ways to go about this. For me, I generally just try to plant a group of 3 or 4 bulbs in between perennials and small shrubs, where there's space. Sometimes you can get away with placing them fairly close to a perennial as that perennial likely won't be that big/wide in the spring time when the bulb is blooming.

I have heard of people "mixing" perennials and bulbs, depending on the habits of the perennial. For example, planting tulips among lilies. The tulips would be up first in spring, then bloom while the lilies are just coming up. By the time the lilies are a decent size, the tulip foliage is still around but might be dying back for the season.

I often wondered if it might be neat to try planting tulip bulbs among a creeping/spreading perennial such as lamium or silver mound. By the time the tulip foliage dies back, those perennials would increase in width and fill in that spot.

So, I don't know if the bulbs would be planted right underneath a perennials roots, or vice versa, but perhaps it would be more like they are sharing the same area of the flower bed.

This year I am trying a naturalizing effect by planting crocus bulbs underneath some areas of the lawn. I think they'll bloom early enough before the grass even needs cutting and, even then, hopefully the crocus leaves are low enough so that the lawnmower doesn't clip them (so the foliage can stay around long enough to replete their energy after blooming.)

Glen

I have the bulbs planted in their own section. I plant annuals on top of the bulbs after the bulbs are done flowering.

I don't know if this is ideal for the bulbs, however...

I try to let the bulb foliage wither away before cutting the foliage back and planting the annuals, as the foliage needs to be left in place as long as possible.

I have had good success getting things blooming the whole summer but not because I tried. I just planted things in all the spaces, including bulbs and roots like calla lilies, and there always is something blooming. I do try and get things that say they will stay in bloom for several weeks or some things bloom nearly all summer. When things come into bloom write it in a notebook, the date and the date it goes dormant, then you will know how to move things around. However, gardens are not static. They are always getting bigger or something will die off, or get so big you have to move it. But that's part of the fun.

Agree with everyone above. I plant bright annuals (zinnias, nasturtiums) in between the bulbs and perennials. Many of our perennials tend to be spring and early summer bloomers, though daylilies and phlox do bloom mid-July and then, we have some types of black eyed susans that bloom really late (triloba) and also some Autumn Joy sedum and asters. Another really beautiful fall perennial is the Japanese anemone - both white and pink. Another late blooming perennial that I really love is the Blue Lobelia (lobelia syphilitica) - it is blue, which is great in the late summer garden and it is a tall-ish spike - which is a nice contrast to the mums which are "mounds". Hope this helps.

I have had much success planting bulbs under certain low growing spreaders. My low growing tulips, daffodils and hyacinths as well as crocus and muscari grow right up through the rockcress, groundcover phlox and snow in summer. I had planted the bulbs and then over the bulbs I planted to ground covers. It's always nice to have the groundcover when the leaves of the bulbs are dying back because I usually braid them or tie them in a bunch and then just tuck them under the groundcover to hide them.