Q. Is it ok to plant late summer/fall blooming perennials, like chrysanthemums or helenium, over tulips?
Planting a succession grouping to mask the fading tulip leaves is a great idea but you will need to make sure that the health of both plants - tulip and summer perennial - is maintained in the process. The tulips need to continue to receive sunlight on their leaves to gather energy and they will need to re-emerge from between the roots of the other plant next year. A plant that has too dense an above-ground or below-ground structure will be inappropriate. And both plants will be competing for nutrients in the soil so you will need to choose plants with compatible needs.
Chrysanthemums are typically very dense in form and while they will mask the tulip foliage well, they may mask it too well. Depending on the chrysanthemum and the type of tulip (early or late-blooming), you may get covering foliage coming up far too early and heavily and obliterating your tulip display. Pennisetum is too dense and has an above ground element year-round that is incompatible with tulips. While helenium flowers in late summer, its dense foliage and form will also interfere with your tulips.
The best prospect amongst the plants you have ordered is to use your dahlias to cover. These are annuals so you can pull them out at the end of the season and reduce interference with emerging tulips next season. They are also quite late to emerge so while they can be formidable in structure they should accommodate the tulip's bloom period and need for continued light. Do not plant your dahlias directly on top of your tulips but intersperse them amongst the bulbs.
Another common trick to disguise dying bulb greenery is to simply distract the eye by illuminating a separate section of the garden with a brilliant, early summer display. With something else to look at, few people will linger on the spent bulbs.
Courtesy of NYBG Plant Information