The weed everyone's asking about

Four times in the last week, people have asked me to identify a particular weed. By the third time, I knew the answer before seeing the plant. So, I figured I would share the plant here, because there are certainly others who are wondering about this little botanical invader and maybe this post will help.

When today's email, with photos, came in, I decided I had better make a few images of the offending plant of my own. So I waltzed on out the door of my office building and into this slightly weedy flowerbed twenty feet away.

Within seconds, I had found examples of the weed du jour: hairy bittercress, Cardamine hirsuta. It's not a big thing - the little basal rosette of leaves isn't even as big around as your palm, and the flowering stems only rise six inches or so. While not on the size scale of kudzu or bush honeysuckle, hairy bittercress can be prolific and sometimes dominates yards and gardens. Thus, it becomes offensive to those who wish a manicured landscape.

Hairy bittercress is a member of the mustard family (Brassicaceae), and some of the mustards are quite adept at being weedy. Indeed, members of this family may be among the first plants to jump the garden fence and establish themselves as weeds back in the Old World. This plant, like so many others in our nonnative weed flora, are of Eurasian origin, and have long been adapted to landscapes highly disturbed by people. When hairy bittercress and others of its ilk made their way to the New World, they found a ready niche in our disturbed habitats - like your yard, or flowerbed.

This species is a winter annual, and those little rosettes of leaves are often evident from late fall on through the winter. This growth habit enables the plants to pop forth early in the spring, and burst into flower and fruit with vigor before most other plants have reached the reproductive stage.

Tiny white flowers are borne at the summit of stems that support only a few cauline (stem) leaves. The leaves are pinnately dissected, meaning the leaf is divided into smaller segments that branch from either side.

This is a typical hairy bittercress flowering stem, showing flowers and fruit at the summit, and two cauline leaves adorning a largely naked stem. Close inspection would reveal scattered hairs on the leaves, especially around their bases, which helps distinguish this species from one or two similar but non-invasive species.

The tubular pods that project nearly straight upwards are the fruit, which are termed siliques in mustard-speak. Hairy bittercress is a prolific fruiter, and each silique contains many tiny seeds. One plant can carpet bomb its immediate environment with its embryonic spawn. These seeds seem to have a high rate of fertility, thus a few plants can quickly turn into many.

Hairy bittercress has been around ever since I began botanical field work around 1990. But it surely has become much more abundant since then. The first Franklin County, Ohio collection dates to 1898, but back then it would have been a seldom-seen oddity. By 1991, the mustard had been collected in at least 29 Ohio counties, but definitely was scarcer than today, and I'm confident that it is easily found now in all 88 of our counties. No one collected this plant in Michigan until 1976, but I'm sure it isn't hard to find up there now.

Why is hairy bittercress becoming so prolific? In a word, nurseries. Many of the first collections in the "wild" come from greenhouses and nurseries, where hairy bittercress would pop up as a weed. Such a growing environment sets the stage for an eventual massive escape. The tiny, prolific seeds easily adhere to feet, digging implements, and anything else they come into contact with. Thus, the soil that nursery stock is planted and transported in eventually becomes infested with hairy bittercress seed.

As localized infestations become more severe, the plants are more easily spread, not only in the local yardscape but also to new sites. Before long a giant mustardish snowball is rolling downhill and out of control, casting seeds far and wide. And now, we are at the point where there is such a proliferation of hairy bittercress that nearly everyone notices the plants.

Fortunately, because of their diminutive stature and propensity for growing in open, already highly disturbed environments, Cardamine hirsuta does not cause much harm to most natural areas. That's not to say that it can't; infestation of a habitat SUCH AS THIS could be very bad for some of our highly specialized native mustards.

I'm afraid hairy bittercress will be the bane of many a gardener for some time to come.