In the Garden: A Pretty Tree, But…

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Albizia julibrissin

Can a weed be pretty?

The answer might depend on whom you ask. But picture for a moment a small, umbrella-shaped tree with delicate ferny foliage, topped with powder-puff pink flowers, looking like it might have come off the African savannah. Sounds kind of pretty to me. As always, there’s two sides to a story.

The mimosa tree (Albizia julibrissin) is native to Asia, ranging from Iran eastward to Korea.  A member of the legume family (the Fabaceae), it’s one of some 150 members of its genus.  As often happens with common names, this tree is not a true Mimosa, which belongs in its own genus. Mimosas, albizias, acacias, etc. are all related trees, comprising hundreds of species, native to much of the world’s tropical and subtropical regions. Most of us can probably conjure up the iconic thorn tree of the African savanna, as depicted on PBS’s Nature series opening shot.

If you’re shopping for a mimosa, you’re not likely to find the straight species for sale at most garden centers, although it is available through online retailers. A cheaper alternative: find a tree somewhere and collect its seeds in the fall, then sprinkle onto a bare patch in your yard.  Hmm. “Comes up easily from seed.” Are we noticing a possibly undesirable trait here?

People are more likely to be selling one of the mimosa cultivars, such as ‘Ishii Weeping.’  This tree will never get very tall, instead perpetually drooping over. A color breakthrough, ‘Summer Chocolate’ came onto the market a few years ago and stands out for maroon-purple leaf coloration that holds well in summer heat. To my casual observation, it does not flower as profusely as the species. The latest mimosa release is Chocolate FountainTM, a purple-leaved weeper with the foliage color of ‘Summer Chocolate.’

So our “mimosa” is indeed a pretty tree with some interesting cultivars, and looks like nothing else that grows in our area. So, what could possibly go wrong?

To start with, it’s a weed. I dropped a generous hint on that when I advised you to obtain your very own mimosa merely by plucking a seedpod. You’ll have no trouble finding one of those, owing to the mimosa’s ubiquity along roadsides and other disturbed areas. By the way, my definition of a “weed” has nothing to do with a plant’s appearance. I know some people look at a perennial or wildflower and declare, “It looks like a weed!” if it’s more than two feet tall and isn’t currently flowering.

Another issue with mimosas is their susceptibility to problems. The fall webworm will weave its webs on the leaves, and the caterpillars start munching. By mid- to late summer, the tree will look awful, even though there are no long-term effects. A more serious issue is vascular wilt disease, caused by a fungus that invades the plant and ultimately blocks the transport of nutrients. It will eventually kill a mimosa to the ground, although the tree may re-sprout from the roots. Although sales literature may tout mimosas as “trouble-free” and “disease- and insect-resistant,” this doesn’t seem to be the case in our part of the world.

Do you want this plant? If you’re adventuresome, you could try one of the Chocolate varieties, but be prepared for possible problems. Otherwise, enjoy it along the Virginia roadsides. And thanks to a reader for suggesting mimosas as a topic; suggestions are always welcome.

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On a somewhat related note, the other day I noticed caterpillars attacking one of my baptisias. The relationship to mimosas? Baptisia is yet another legume, one of 19,000 species in this family.

The caterpillar in question is that of the Genista Broom Moth, Uresiphita reversalis. (Genista is the scientific name of a genus of leguminous shrubs often known as brooms.) The adult moth is brownish, with a wingspan of a little over an inch, not a raving beauty. The adult caterpillar, about an inch long and typically yellowish green with black and white dots, prefers legumes, so the baptisia is easy prey.

The caterpillars first appear as ¼” long critters in webs where they hide and feed. Gray-brown areas on the leaves indicate where they’ve dined, although the tiny caterpillars are hard to spot. As they mature they become solitary and start carving out semi-circular chunks of leaves.  Like many caterpillars, they hang out on leaf undersides, making them difficult to see. After they’ve been at work for a while, their presence is all too obvious.

If left unchecked, the caterpillars will reduce a baptisia to a brown mound in a few days.  What to do?

For now, I am using “manual controls”, i.e., smushing the varmints. But they’re not always easy to find until they’ve done some damage, and if I relax my vigil for a couple of days I’m afraid the population could explode. To complicate the situation further, I’ll be out of town for the first part of August.

I could just let nature take its course. The caterpillars will reduce the baptisia to rubble, and I’ll cut the whole mess down and bag it up, caterpillars and all. The plant should recover eventually, but probably not until next spring.

Or I could resort to a biological control. You may have heard of Bt or Bacillus thuringiensis, a naturally occurring bacteria that attacks caterpillars. Reportedly, it is not very effective against the adult caterpillars. They could still be feeding, turning into moths ready to start the whole cycle again.

Bt is considered an organic control for caterpillars, so it’s totally cool, right? Unfortunately, it’s not selective. It kills all caterpillars, including monarchs. It’s used extensively in farming and has been introduced into genetically modified crops. I suspect that by now I’ve raised the hackles of some readers.

I’m going to leave you hanging.  What will I do about those caterpillars?

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