QUESTION: There’s a weed spreading through my perennial bed, under the trees and even growing out of a pile of stepping stones in my yard. It has small, two-petal blue flowers and long, pointed leaves. What is it? And more importantly, how can I get rid of it?
It sound like you are describing Asiatic dayflower, a weedy annual that grows and spreads quickly and will consume anything in its path if you allow it to proceed. It blooms summer into fall, opening its flowers in the morning (it actually has three petals; two blue and one smaller petal that’s smaller and more pale) and closing up tight by late afternoon. It reproduces from seeds dropped each year, but it also spreads vegetatively, rooting at the leaf nodes on moist soil. Common Asiatic dayflower is in the same family as spiderwort.
Dayflower grows in a variety of conditions, but really spreads aggressively in moist, shady spots. As explained in the Illinois Wildflowers database site, “At favorable sites, the Asiatic Dayflower forms colonies that can exclude other species of plants.”
So yes, you may want to try to eliminate dayflower from your garden. It won’t be easy. Glyphosates (Roundup and other non-selective herbicides) don’t faze it, apparently. Gardeners who have struggled with it say that it’s easy to pull out of the ground, but even if you think you’ve gotten every single leaf, stem and root, it likely will come back. It’s easiest to pull up when it’s young and small. Diligence is advised.
Here are some comments I read in a forum at Dave’s Garden as I researched Commelina communis:
“It seems to grow several inches over night and can overwhelm an area in little time.”
“Invasive as they get — one of my biggest pests!”
“This plant is a superweed!”
But let’s end with a couple of good things to know about Asiatic dayflower and its blue blooms: Usually, flowers we call “blue” are closer to violet or purple. The flowers of Asiatic dayflower are truly blue, a color that is found in few other plants. And a commenter at the Dave’s Garden forum says this: “It is extremely invasive; but at least it is edible. The young leaves and stems can be added to salads or boiled for 10 minutes and served with butter.”