A flower that has a scent.
Many modern hybrid flowers have no scent. Take the hybrid rose, lovely, exquisite almost beyond measure, but put one under your nose and you smell no fragrance. A heritage rose, on the other hand, a lovely bouquet.
Mason bees and Leafcutter bees use ultraviolet vision to find the flower from a distance and then smell to locate the nectar and pollen.
Mason Bees especially need an early-blooming flower variety
Mason bees need early blooming flowers, as they are active in March, when very little is blooming, through April, and May. Most modern hybrid-flowers bloom mid-April to late-May, six weeks too late for your mason Bees. Leafcutter bees need flowers that are still blooming in August.
Mason bees and Leafcutter bees need a flower variety that blooms for a month or more.
That eliminates most modern flower varieties, except maybe pansies and pansies, are of no use – they either have no pollen and what little nectar there is bees can’t access it. Pansies have been selectively bred for beauty, this has lengthened their nectar tubes so mason bees and leafcutter bees, all bees actually, can no longer reach this food source.
On the other hand, there are wildflower varieties that have co-evolved with native bees, like the mason bee, and bloom for months.
If you’re like most Mason bee and leafcutter bee enthusiasts you release your bees incrementally, half your cocoons at the beginning of the season and the other half two weeks later. So the flowers that feed your mason bees and leafcutter bees need to last from 8-10 weeks. You can do a couple of planting two weeks apart.
Mason Bees and Leafcutter bees need a flower with copious amounts of pollen AND nectar.
Your mason bees and leafcutter bees need a plentiful amount of pollen and nectar throughout their life span which is from 6-8 weeks. Even in wildflowers, different flowers have differing amounts of nectar and pollen. Some like the California poppy has pollen but no nectar.
I have search out which variety of wildflowers have copious amounts of both, they are a feast to the eyes and a feast for the bees.
Also, they must produce enough blossoms to feed lots of mason bees, a single tulip just doesn’t cut it. A California wildflower (Nemophila menziesii) commonly called “baby blue eyes” has 10 million flowers per acre.