What’s a Weed?

Taraxacum officinale Dandilion @DBG LAH 001

A weed is…

I subscribe to a variety of online gardening groups, mostly on Facebook. Lately there has been a lot of ranting discussion about whether or not dandelions are weeds. On the one hand, various gardeners are asking for help controlling dandelions in their lawns and gardens, often to please a landlord. On the other hand, various opinionated persons climb onto their soapboxes and extol the virtues of dandelion leaves, wine, and bee-friendly flowers, chastising anyone who would dare to disagree.

@BoulderCO 3may07 LAH 027Of course, both groups are correct. Dandelions do have beneficial uses. But we can’t just let them grow anywhere they want to—and the way they disperse their seeds, that is exactly what happens. If you want to dine on dandelion greens, by all means encourage the plants to grow—on your own property. If you want to avoid a HOA penalty, or just prefer your lawn to be green, then murder the suckers!

Dandelions point out an important distinction when it comes to defining the word “weed.” It’s a subjective term. There is no botanical definition; it all depends on the circumstances.

Parsley_home-EPC-CO_LAH_7336When we call a plant a weed, however, we typically have a list of characteristics in mind. For one, where the plant is growing matters. I grow a lot of parsley in my garden beds. Two years ago, I didn’t get a chance to deadhead the flowers, and now I’m weeding parsley out of the lawn, the paths, and my perennial borders. Oops.

They are plants we didn’t want. They may be natives or garden escapees, but we didn’t plant them and we aren’t interested in keeping them.

Then there’s the tendency for weeds to out-compete the plants we do want. Species we label weeds often germinate in early spring, before our flowers or vegetables, and then grow much faster, choking out neighboring seedlings. If we don’t keep after them, or use mulch to suppress them, we’ll eventually have all weeds and nothing else.

We’ve all heard that “nature abhors a vacuum.” The speed with which a vigorous crop of weeds invades bare ground proves the saying.  Weeds typically are the first plants to move into bare dirt. Their seeds, which are already present in the soil, are primed to germinate when they’re exposed to the light. Removing any covering plants provides an ideal seedbed.

@home 23aug05 LAH002We saw this in action in the open space next to our yard. When we first looked at the lot, it was just below a hillside covered with Rocky Mountain penstemon. The blue flowers were gorgeous, and I couldn’t wait to live right next door. However, in preparing to build, the developer cut down the hillside, presumably to reduce erosion or the risk of a mudslide. In the process, they destroyed both vegetation and topsoil.

The next spring, the landscapers dropped off rolls of straw mesh impregnated with seeds, designed to cover the bare hillside. However, the straw was packaged in clear plastic, and it sat in the hot sun until August, when it was finally unrolled on to the dirt. Yes, you guessed—the sun plus plastic effectively solarized the seeds, killing them all. On top of that, fall is our driest time of year, and no rain fell to encourage any sprouting.

Horseweed_COS_LAH004

Finally, spring arrived. Instead of grass and wildflowers next door, we had a hillside thickly covered with primarily horseweed (a European species that’s now widespread in the USA), plus mullein and assorted knapweeds (aka tumbleweeds, several  knapweeds are on the Colorado noxious weed list). We’ve been fighting the invaders ever since.

There is one group of weeds that go beyond mere annoyance. Labeled noxious weeds, this is a list of species so invasive that landowners are required by law to control or eradicate them. Noxious weeds are 1) non-native, 2) invasive and hard to kill or control, and 3) harmful to livestock or their forage. (Yes, there’s an economic aspect to this.) To learn more, see my post on noxious weeds from 2010.


Weeds, from top: Dandelions; dandelions again; leafy spurge*, common mallow, bindweed*; parsley; mullein*, ragweed, black medic; redroot pigweed; horseweed. Those with (*) are on the Colorado noxious weed list.

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