Backyard Naturalist in Phoenix, Arizona

July 4^th, 2009 There is a Hill of Beans on the ground beneath my Blue Palo Verde (Parkinsonia florida). Thousands of bean pods developed after the flowers finished blooming in April, and now the mature beans are falling, mostly still inside their pods. And my leguminous tree is not alone. All around Phoenix, Tucson, Scottsdale and other Sonoran Desert cities, there are literally tons of mesquite beans, palo verde beans and acacia beans on the ground beneath xeriscape trees awaiting the summer chubascos.

The parent trees have stored plenty of food in the beans so that when rain does come the germinating seeds can quickly establish a strong tap root before all the surface moisture has again evaporated. The food inside the beans is well protected by a very hard seed coat that repels most seed eaters. But seeds dispersed directly in the shadow of the parent are unlikely to survive well after germination (this is also true for human offspring). In evolutionary history the trees have depended on mammals to disperse the seeds. Ungulates, like now extinct camels and mammoths, eat the nutritious pods leaving the seeds to survive the trip through to be deposited far away in a pile of fertilizing dung. Or else, the beans are picked up by rodents and placed into a cache for later consumption. Seed dispersing mammals are now gone from the city so the seeds remain as an annoyance to some gardeners or as a bounty to bruchids.

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July