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Sunday, January 31, 2016

An attractive menace

On July 14, I wrote about doubting I could keep my potentially invasive silk tree (Albizia julibrissin, also known in the US as mimosa) from spreading. Now that looks even more unlikely.

    With last weekend’s snowfall mostly melted, I notice many silk tree seed pods lying on the grass. Some are empty, and some still contain their brown seeds.

    Silk tree is a native of Asia that was imported to North America in 1745. It's a pretty tree with delicate, feathery foliage that makes pink puffs of flowers in July in my yard. 

This is a photo from Texas. Mine don't flower this densely

Silk tree spreads especially in disturbed ground and is now invasive in the southeast US. It has a number of characteristics that enable invasiveness: it produces a lot of seeds, it’s a legume, so it can extract nitrogen from low-nutrient soil, and if reproducing by seeds fails, it can also spread vegetatively from root fragments. 

     Its seeds require scarification (breaking through the seed coat mechanically, chemically or with heat or cold) to sprout and without this can stay dormant for many years before germinating, which allows them to seize growth opportunities when they arise. The seeds are efficiently dispersed, initially by gravity, secondarily by wind, water and animals. 

    That may explain why I’m finding the pods farther and farther from the tree, some at least 50 yards away on the other side of the house. More ominously, I see some have left our property and made it to neighbors’ lawns. I’ve found clusters near tree trunks, so it may be that squirrels or birds are carrying the pods away to eat the seeds and are dropping pods on the ground under the branches. 

    In this weekend’s thaw I’m picking up as many of the pods as I can and throwing them in the trash—I won’t compost them, because that would probably just distribute viable seeds wherever I spread compost. 

     I’ll be sorry to lose this pretty tree, but next spring I think I’ll have to cut it down.

Beautiful but damned

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