If you’ve walked past the sunflower beds on CU Boulder’s Main Campus lately, you might’ve done a double take—some of the blooms aren’t just yellow. They’re red. Really red.

That’s thanks to a multi-year experiment led by CU Boulder horticulturists Jason Wiley and Aaron Shapiro. It’s Jason’s second year growing sunflowers on campus, and last year, most of them turned out the usual bright yellow. But a few had a deeper, reddish tint—and that sparked an idea.

“Genetics are the biggest factor in how red or yellow they’ll be,” Jason explains. “Extremely hot weather can bleach out the color, and we’ve noticed that red veining in the leaves might be linked to red petals.”

So this year, Jason and Aaron decided to lean into the science. They harvested seeds from the reddest flowers from last season and hand-pollinated only the red ones—using watercolor paintbrushes, no less.

“By crossing reds with reds and collecting seed only from those plants, we increased the likelihood of producing red offspring,” Jason says. “We’ll keep doing this each year to see how far we can push the color.”

And it’s working. This year’s patch has noticeably more red throughout the petals, and one plant in the circle planter is so dark it’s nearly black.

For home gardeners hoping to grow red sunflowers, Jason has some advice: “Hand-pollinate only the red flowers with a watercolor paintbrush and save seeds only from those plants. To boost your chances, clip or remove nearby yellow-flowered sunflowers. You can sow seeds densely, then thin them out by pulling the yellow ones as they bloom.”

It’s a slow and steady process, but the results are already turning heads. So next time you’re on Main Campus, stop by the sunflower beds located off the Broadway bike path, just across Broadway from “The Sink Crossing.” You might just catch a glimpse of CU Boulder’s very own red sunflower evolution in progress.

Read more about the University Libraries One Seed project here.