Fava Beans are Full of Surprises

There's always a little thrill to discovering something new and strange about something that seems very familiar. 

Yesterday I was reading The Art of Noticing newsletter by Rob Walker, and he had a link to an Oakland Garden Club newsletter article by Alexis Madrigal. She mentioned a volunteer fava bean plant in her garden that she can't bring herself to pull out. She likes to watch ants climb up to the extrafloral nectaries. 

WHAT? I have grown fava beans for years and years, and had no idea about this. Fava beans are delicious, if labor intensive (you have to shell them AND peel the beans individually, unless you eat them very young). In a mild year, they can overwinter in Seattle, so sometimes I plant them in October. I didn't do that last year, luckily, because they wouldn't have been able to cope with our really cold snap in January, when it got down to 17° F.  I did plant some this spring, though, so I was out the door into the garden and crouched down by the fava patch a moment later. And there they were! 

Fava bean extrafloral nectaries

These little leaf-like structures are called stipules, and are nestled along the main stem where the leaves branch off. The dark dots on each stipule are the extrafloral nectaries. Magnified, they appear a much brighter wine-red color. When I looked at one under the microscope, I couldn't actually see any fluid being exuded. Maybe that one just wasn't ready yet, or was already done? After watching the plants intently for a while, I did see ants (but no other insects) visiting the nectaries. The ant pictured below is just leaving. By the time I get them in focus, they have invariably left the nectaries and started climbing down.

Fava bean extrafloral nectaries

There are different ideas about who natural selection intends to be attracted to the nectar. It could be pollinators like honeybees, who will then also visit the flowers. Ants feed at the nectaries, and then may help defend the plant against insect pests. Ants have a similar symbiosis with acacia trees in Africa, where the trees produce nectar and provide hollow thorns for the ants to live in. In exchange, the ants defend the trees. Parasitic wasps and lacewings can also be attracted.  I know lacewing larvae love sugar. A few years ago, we were eating cake in the garden and one appeared at a smudge of frosting and began devouring.

lacewing larva

Unfortunately, ants also tend black bean aphids on my fava beans. It's too early in the year for them, though, so that is not the attraction for the ants right now. 

Fava beans have lots of other great characteristics. Their flowers are beautiful and smell wonderful. 

fava bean flowers

Like other legumes, they form nodules on their roots that house nitrogen-fixing bacteria. Plants can't take nitrogen out of the atmosphere, but some bacteria can. This symbiosis provides the plant with built-in fertilizer. And, of course, in risotto or pasta, the beans are delicious!


Location: Seattle, WA, USA

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