Why are so many flowers yellow?

I’ve been out in the garden this morning, picking some almost-ripe tomatoes and bringing them in to their usual place on the piano, to ripen. As I was doing this, I realized that an awful lot of flowers are yellow. This intrigued me and I started to wonder why that is.

We live in a time of miracle and wonder, as Tom likes to say. Remember the days when we had to go to the library to find stuff like this out? Or to the World Book Encyclopedia? We had a shelf of those that my parents bought in 1964. As a child, whenever I was bored, I would go look through them. Oh gosh, remember boredom? Does that exist anymore?

Heavens, before I croak out something like “well, sonny, back in my day,” let’s move on, shall we? What I meant to say was that you can go to Google Scholar and type in ‘why are so many flowers yellow?’ or ‘color in flowers’ and get to read all kinds of interesting scientific papers, most of which don’t really answer the question, only pose more questions, but that is why we are never bored anymore, right? All hail the Google rabbit hole.

ANYWAY. It’s actually hard to find the percentage of yellow flowers in plants. I did see that early spring plants are often yellow, which is a way to signal early food for pollinators. Wait, let me back up. Why would flowers have color to begin with?

Fossils suggest that early flowers didn’t have much pigment, but rather were a dull yellow or pale green before they evolved over 100 million years ago to produce colors. It is assumed that they evolved that way to attract pollinators. Apparently many plants have evolved colors that match the specific visual systems of different insects or birds.

You might remember that bees (and many other pollinators) have compound eyes. Our own eyes can only detect three colors - red, blue, and green. Bees cannot see red, but they do see blue and green, and also UV light - that means they can see colors we cannot see. Many flowers have ultraviolet nectar guides, a sort of pattern that we cannot see, that are like runways lighting the way down to the inside of a flower.

Bees have awful long-distance sight, so they use scent rather than sight to find nectar, but those compound eyes provide amazing up-close vision, allowing them to see these specific colors and patterns once the smell has lured them to the flower.

Color we see in flowers is the result of reflected light from various plant pigments. These pigments can be anthocyanins, compounds that make autumn leaves red, or blueberries blue. Flavonol pigments make yellow and chlorophyll pigments make green. There are flavanoid pigments that are colorless to us, but also absorb UV light and make colors available to bees and other pollinators.

Therefore, a bee balm that appears red to us might appear white to an insect. Yellow and white flowers (to us) may appear blue to insects. In the course of my research, I also learned about flowers such as borage or fleabane, which turn different colors over a season (like from pink to blue), are doing so to signal to pollinators which flowers are new and have a lot of nectar, and which are too old to produce (thank you to the University of Vermont for much of this post’s information!).

Native bees have evolved to drink from the blossoms of the plants that evolved with them, which is why it’s a good idea to have a percentage of your plants as natives. But they will readily drink from exotic species, and honeybees will forage on almost anything - it is a generalist species. However, there is evidence that bees prefer (what appears to us to be) blue and white flowers. Birds tend to prefer (what appears to us to be) red flowers. Have you ever noticed (I have!) that many red flowers seem to have yellow centers? Could that be those UV light patterns directing bees to the flower, even though it is red?

Evolution is crazy, yo.

I couldn’t find any solid information about yellow flowers in particular and why it seems that nature makes so many flowers yellow (it could just be a coincidence that I’m seeing so much yellow this time of year). But I’ll leave you with this paragraph of an abstract which I found, entitled ‘Pollinator Preferences for yellow, orange, and red flowers of Mimulus verbanaceus and M. cardinalis’ which are both monkey flowers. It is authored by Paul K. Vickery, Jr of the University of Utah.

“Red, orange, and yellow morphs of Mimulus verbenaceus and M. cardinalis were field tested for pollinator preferences. The species are closely similar except that M. verbenaceus flowers have partially reflexed corolla lobes, whereas M. cardinalis flowers have fully reflexed corolla lobes. On the basis of over 6000 bumblebee and hummingbird visits, highly significant (p < .001) patterns emerged. Yellow, which is the mutant color morph in both species and is determined by a single pair of genes, was strongly preferred by bumblebees and strongly eskewed by hummingbirds in both species. Orange and, to a lesser extent, red were strongly preferred by hummingbirds but eskewed by bumblebees in both species. Thus, strong, but partial, reproductive isolation was observed between the yellow mutants and the orange- to red-flowered populations from which they were derived. Color—yellow versus orange and red—appeared more important than shape—partially reflexed versus fully reflexed corolla lobes—in determining the preferences of the guild of pollinators in this particular test environment for Mimulus verbenaceus and M. cardinalis."

So it would seem that native bees prefer yellow, at least on those particular flowers. So interesting!

Other news:

*Tom and I watched three juvenile Cooper’s Hawks fledge over last weekend. They hung about in our yard and on the nearby power lines, together in a group, for quite a while. It was an amazing thing to see.

*Highwire coffee, a local roaster and the place I go to collect coffee chaff for chicken coop and garden purposes, has just been awarded a seal by the Rodale Institute for their organic ‘Conscientious Objector’ line of beans. We think Highwire makes truly delicious coffee and we are happy they are being recognized for the work they do with organic farmers. Their coffee bags are also compostable which is a huge bonus.

*Just another urging to go see The Biggest Little Farm if it is playing near you. Tom and I saw it again this past weekend, and it’s just such a great movie.

*Shoutout to the Merritt Horticulture students from LH1 who came by for a garden tour and talk last night. What an amazing group of people, with some amazing projects and ideas in the works, and many of them already ‘farming’ just like I do here. It was great to hang out with like-minded folks and plant nerds again. I’ve missed the Hort department and was super glad to be a part of it again for a night!

*Did you know I put ‘tags’ at the bottom of each post? This is so if there is a subject that interests you, such as pollinators, you can click on that tag and see everything I’ve ever written about pollinators. It’s not a fail-safe program; the other day I tried to find Tom’s instructions on how to build our garden trellises, and I’m still looking (you should be able to find it under the ‘projects’ tag, but clearly I didn’t tag it that way). You can also go to the Archives and search a word or a term, like ‘cooking,’ and find all the recipes I’ve talked about.

*The harvest has begun. Cucumbers, beans, tomatoes, and some hot peppers are starting to come ripe. This caused me to see that I was not prepared to start preserving the harvest. Here is your reminder to make sure that you have plenty of mason jars, lids, rings on hand; labels, pickling salt, pectin, etc. Here we go!!!