A Gardening Journal
Today in Key West: Pink-flowered Rain Tree
- Published: March 01 2017
For a Northern gardener, each visit to the tropics is the chance to reconnect with plants that can be in bloom at almost any time of the year, and that flaunt colors and talents no Northern plant would dare. But where such extraordinariness is normal, the real shock can be seeing a familiar Northern plant that, seemingly, is just as comfortable in the tropics. How can any plant that tolerates minus-ten Fahrenheit back home also thrive here, where it never drops below forty?
Take pink-flowered mimosa, which is renowned as far north as coastal Maine for flowering at the height of summer. Is it the same species as the tree in bee-loving bloom here in Key West?
Albizia julibrissin is in particular favor up north as one of the two summer-flowering hardy trees whose blooms are not white. (Can you think of the other? It's Koelreuteria paniculata, whose petals are butter yellow.) The flowers of such mimosa trees thriving from northern Florida to southern Maine seem identical to those of this tree in Key West.
Flowers of both trees are without petals, and flaunt long and numerous pink stamens that emerge from white, conical bases. The blooms are borne in dense round inflorescences; in full flower, a veritable forest of stamens is created, through which bees eagerly scramble to reach the nectaries within the flowers' bases.
Up North in climate zones 6 and 7, there is no hardy species of mimosa other than Albizia julibrissin. I grew its dark-leaved cultivar, Summer Chocolate, for a number of years. But there are scores of Albizia species hardy in zones 9 and warmer. Key West is Zone 11, where frost is unknown and, even at the depth of winter, temperatures in the low fifties Fahrenheit would be rare.
These pink flowers (and pinnate leaves) are definitely those of some species of Albizia—but which? It isn't A. julibrissin. It tolerates climates of increasing mildness only to Zone 9, where winters are always noticeably cooler than the summers, and occasional frosts are the norm. This is the climate of northern Florida, not Key West.
One likely possibility is Albizia saman, one of whose common names is pink-flowered rain tree. The rain reference is due to the response of the pinnate leaves' ranks of leaflets to rain: they close up, which allows the precipitation to fall more quickly through the canopy to the ground. This is thought to be one reason why grass beneath rain trees is more likely to stay green even in a drought.
Albizia saman is native from Mexico to Brazil and, so, might also be native to—or at least readily establishable in—Key West. It is a titanic tree, growing larger than even the most wide-spreading live oak, and with a short trunk as thick as that of a sequoia. Here's an immense specimen in a park in Venezuela:
Even the largest beech tree is diminutive in comparison, while any Albizia julibrissin would seem to be just a sapling: hardy mimosa rarely grows higher than fifteen to twenty feet, with trunks rarely thicker than a foot.
Given the extreme hurricane susceptibility of Key West—let alone the small properties in the densely settled Old Town section of the island where "my" Albizia samon is growing—the species will never grow as large here. Even if there were room for this exceptional spreading growth, hurricane winds would shear off such incredibly cantilevering limbs.
Here's a profile of the purple-leaved cultivar of hardy mimosa, Albizia julibrissin 'Summer Chocolate'. It demands perfect drainage, especially in winter, which my dead-level garden doesn't provide. Instead, I kept mine in a container year-round; I overwintered it in the greenhouse.