Everyone knows the ox-eye daisy (Chrysanthemum leucanthemum). The flower's bright yellow central disk is surrounded by white petals. It grows almost anywhere brightening hay fields, rural road sides -- even my gravel driveway. It's almost as ubiquitous as the dandelion.

To me, ox-eye daisies are the wildflower that defines the beginning of summer. There are thousands of stems in my hayfield right now, and it blooms through September.

However, the ox-eye daisy is an invasive Old World exotic species. Normally I wage war on exotics. I've battled multiflora rose, autumn olive, house sparrows and starlings for decades.

But ox-eye daisy is just too beautiful to disparage. Its mere presence brightens the landscape, and it volunteers in even the most unforgiving soils. It has no detectable fragrance to my nose, so I just enjoy its visual appeal. Plus it lasts long as a cut flower, so there are always at least a few fresh blooms in the vase on the dining room table.

Ox-eyes and relatives such as dandelions, asters, goldenrod and sunflowers are just a few of more than 22,000 members of the family Asteraceae. Collectively they are often referred to as "composites."

Curiously, the flower head of composites is a cluster of many small flowers. The yellow center is composed of many small disk flowers, each containing male and female reproductive parts. That's where the ox-eye's seeds originate. The white outer petals are ray flowers and are usually sterile.

By observing ox-eyes, sunflowers or other composites over time, you can watch the individual disk flowers develop. The head matures over a period of days with the small disk flowers opening serially in an inward spiral. This ensures that many different pollen donors fertilize the disk flowers, which ultimately form seeds. This is how sunflower seeds are formed. That the Asteraceae is the largest family of flowering plants suggests that this is a very effective reproductive strategy.

Another flower just beginning to bloom is common milkweed (Asclepias syriaca). Though an abundant roadside attraction, common milkweed is native, pleasantly aromatic and an essential monarch butterfly host plant. Add a few blooming milkweed umbels to a vase of ox-eyes for a natural fragrance that can fill a room.

Scott Shalaway is a biologist and author. His other weekly Post-Gazette column, "Wildlife," runs Sundays on the outdoors page in Sports. He can be reached at sshalaway@aol.com or RD 5, Cameron, WV 26033.