How about one of the coreopsis cultivars? I have 'Moonbeam' and 'Zagreb' and both are real workhorses, blooming from June through September as long as I shear now and then to remove spent blooms. There are even some new buds forming and opening now. They seem happiest in full sun, lean soil and little supplemental water once established, but are very adaptable to less spartan conditions.

The threadlike leaves give an airy effect, even when a thick clump has formed.

Bonus: The flowers last well in a vase ... if you can bring yourself to cut them!

Another of my favorites for its affinity for spartan conditions, and especially its late bloom (now, here in coastal NJ), is the Montauk Daisy.

Are you looking for a taller perennial? One that I really like is Rudbeckia fulgida var. fulgida...gets to about two feet high. It is still blooming strong. It has a much longer bloom period that standard Rudbeckia fulgida 'Goldstrum' and isn't susceptible to the downy mildew that typically makes the leaves on standard Rudbeckias turn black. I've never had this problem with Rudbeckia fulgida var. fulgida.

A shorter plant that is also wonderful is Coreopsis 'Sunshine Superman.' It blooms non-stop and is still going strong. It starts early in the season and never stops.

I have problems getting yellow into my garden as well. I'm in zone 7 in NC.

Rabbits (eat rudbeckia 'Goldsturm')
Deer (nibble rudbeckia occasionally, nibble helianthus)

After that -- too much rain all summer turned my yarrow 'Moonshine' to black; I lost 1/3 of the coreopsis 'Creme Brulee' over the winter, but my zagreb is now in the 3rd year.

My yellow snapdragons were fine until it got too hot...they prefer a bit cooler temps, but do occasionally overwinter.

If you can use a tall perennial, verbascum might work. I have a peachy color that has bloomed 3 times this summer, but shorter stalks each bloom. Santolina comes to mind as well, though I have no experience with it.

In my cottage garden (away from deer), I have daylily 'Happy Returns' that is a beautiful soft yellow and has bloomed repeatedly all summer. I also have leucanthemum 'Broadway Lights' in the cottage garden that starts out pale yellow and gradually lightens to white.

I bought gaillardia that were labeled as yellow, but turned out to be the burgundy/gold when they bloomed. I may try for those again, but only purchase if I can see the plants in bloom.

I'm moving to the soft apricot colors to see if I have better luck in my outer gardens where the deer and the rabbits play. I've got agastache 'Coronado' and cestrum 'orange peel' so far. Those look good with blues, purples, violet colors.

Cameron

If you want all yellow there is a gallardia that is all yellow as well as those that are a mixture of yellow and red/orange. I find that it does well in lean dry soil.

My coneflowers mildew terribly, my Fireworks goldenrod is 100% carefree and healthy. But give it room, it gets big.

I introduce yellow in my gardens with progression of blooms - daffodils, iris, roses, asiatic lilies, daylilies, dahlias, goldenrod, cannas, toadflax, coreopsis, mums...

Kathy

There are many lanceleaf coreopsis varieties, too (as well as the threadlaf, which I have much more trouble keeping alive0. The lanceleaf coreopsis varieties are very low maintenance. Mine seed politely and I am happy to have more in new locations. They like dry conditions best but in my garden put up with damp and poor drainage surprisingly well. No mildew either, and the wildlife have never eaten them that I could tell.

I think there might be an all-yellow helenium. They do well in a dry area of my bed. Also, I have the Hyperion daylily. Not a rebloomer, but handles the dry spells quite well. Santolina is a great choice. Try the more dwarf variety, "Nana"

Pat

One that has worked well for me is rudbeckia hirta 'Prairie Sun'. It is supposed to naturalize, but so far hasn't. I have it in a new place so we will see for next year.

Here is a link that might be useful: Prairie Sun

Daylilies and Helianthus Angustifolia that are almost bullet proof in my hot/humid summers. There are so many yellow daylilies to choose from that if you haven't checked them out in the last 10 years or so you will be surprised at what the hybridizers have done to spice up the blooms. I'm posting a link to Roycroft daylilies with his selection of yellow daylilies (a reputible supplier from my experience). And daylilies can re-bloom too.

Here is a link that might be useful: Yellow daylilies

Hypericum (St. John's Wort) is a possible option....

I have been impressed with the yellow patrina which sometimes reseeds for me. It does bloom late in the summer and grows quite tall. I use it in bouquets much like baby's breath. Mary

Interesting about the goldenrod saying it's prone to mildew. Mine is in the back of the garden, and hasn't had any problems. I would try it, you'll fall in love.

My goldenrod has never had any problems with mildew either. Solidago 'Fireworks' is a great plant. I also am trying Solidago 'Golden Baby', a shortie goldenrod.

Patrina scabiosifolia is my absolute favorite yellow. Unfortunately, the deer have deemed it a favorite too, so I don't know how much longer I'll be growing it.

Helianthus gigantea 'Sheila's Sunshine' is a tall stately beauty with primrose-yellow blossoms.

For early summer blooms in a well-drained spot, I've been impressed with Oenothera 'Lemon Drop'.

Even though I've only had it one season, I was also impressed by Coreopsis 'Tequila Sunrise', which has yellow-variegated foliage to commend it even when it's not in flower.

Coreopsis 'Zagreb' is as reliable as they come.

Spazzy,
Did you have problems with your 'Tequila Sunrise" reverting back to a normal foliage color? I've planted it twice, and both times the normal green takes over the variegated! Not sure why that's happening.

I still love Gaillardia 'Oranges and Lemons' for a perky, yellow color. I have to divide it every other year, but it's worth it for all season color.

In my yellow and blue border, the very best sources of all-summer yellow color are from shrubs: Spirea Thunbergii "Ogon" (really needs a bit of afternoon sun in our heat), Duranta "Cuban Gold", and Berberis thunbergii "Aurea" (thorny, but low, gorgeous and unstoppable in our hot sun). These yellow leaved shrubs are some of the bones of the border. I also use Tanacetum Isla Gold (again, golden foliage), especially near the front edge.

In early summer I use lots of yellow daylilies. I am very particular about the shades I choose. Cool yellows and warm yellows sometimes clash to my eye. Rudbeckia "Herbstonne" (lovely foliage is a bonus), Helianthus "Lowdown" (tall but never flops), and "Carefree Sunshine" Rose add later blooms than the daylilies.

I also have alot of Zephyranthes candida along the front edge, which is a white rainlily that has a prominent yellow eye. I added zephyranthes reginae to the border this year, and the few blooms I got were a lovely cool yellow. It's very hard to beat rainlilies from July to late fall around here.

In addition, I use a few yellow blooming annuals in that border: Yellowbird Cosmos and yellow marigolds (again, watch the shade).

In combination with several blue salvias, stokesias, Rozanne geranium, etc. it is a very satisfying bed.

It's hard to beat Euryops for ease or drought tolerance in my yard. Bright yellow flowers a good 8 months/year.

hlollar: No, I've not had my C. 'Tequila Sunrise' revert, but I've only had it for one season. I surely hope it doesn't revert for me. It is such an improvement over the C. grandiflora hybrids that I tried and had to pull out because the foliage looked like a cat had chewed it up and spit it out after bloom. (I think it's a cross between C. lanceolota and C. grandiflora).

Early Sunrise Coreopsis! Has a deeper yellow color than the creme brulee. Nice double petals, and loves sunny, dry places.