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Weeds That Hurt Us

Weeds can be more than cosmetic nuisances that tarnish the look of a tended landscape. Some of them can be downright harmful and even deadly when ingested.

Weeds graduate from annoyance to threat in four ways:

1. Poisonous

These are the worst – ones that make people sick and even kill when ingested, such as the famed poison hemlock that killed Socrates.

Poison hemlock still grows wild throughout North America and harms people every year who sample the leaves or get its sap in their eyes or nose after touching these hollow-stemmed weeds.

The Weed Science Society of America also lists these among its “rogue’s gallery” of most poisonous weeds:

  • water hemlock (a carrot-family perennial usually found in wetlands)
  • bittersweet nightshade (a tomato relative with dark berries)
  • common pokeweed (a tree-like weed with pea-sized dark-purple berries)
  • meadow deathcamas (an onion-like weed that grows in forests and meadows)
  • jimsonweed (another tomato relative with toothy leaves and pink tubular flowers)
Poison Ivy

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2. Skin rashes

Some weeds defend themselves by producing oil or sap that causes contact dermatitis – a painful, itchy rash and red, oozing skin that can result in scarring.

Poison ivy is that ubiquitous viny weed often seen climbing trees. It's the most commonly known poisonous weed but it’s just one of many weeds capable of causing rashes.

Poison oak is a shrubby cousin that grows in the same three-leaf clusters as poison ivy but produces yellow flowers. 

Poison sumac is a third rash-causing woody weed that’s identified by its red stems, opposite leaves, and green-yellow flowers.

Two of the worst, though, are giant hogweed and wild parsnips.

Giant hogweed is a perennial capable of growing upwards of 10 feet tall and can be recognized by its umbrella-like white flower clusters. Its rash can be even more painful than poison ivy, especially when skin is exposed to light after contact.

Wild parsnips are upright biennials that produce umbrella-like yellow clusters. Their sap can cause severe blistering.

Poison Rash

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3. Thorny/jaggy

Another weedy defense is foliage, stems, and seed heads with jaggy structures that range from tiny piercing hairs to stiff, skin-piercing thorns.

Almost anyone who’s walked the woods has encountered “jagger bushes,” which are usually various forms of wild brambles (raspberries, blackberries, wineberries, etc.) that have thorns all along their arching stems.

These days, the multiflora rose – a once-sold pink-blooming hedge rose gone invasive – is almost as common from untended parts of yards to woodland edges.

Smaller but nearly as pain-inducing are:

  • nettles (perennials with skin-piercing hairs)
  • the mile-a-minute weed (a fast-growing vine with triangular leaves and “prickers” along its stems)
  • cocklebur (a summer annual with egg-shaped, spiny seed heads)
  • teasel (an upright with cone-shaped prickly seed heads at the top of the stems)
  • prickly sida (an upright perennial with heart-shaped leaves and a spine at the base of each leaf).

In the lawn, burweed is a common creeper best known for producing little round “stickers” that latch onto socks, shoes, and pant legs when walking through a patch.

4. Sneeze-inducing

The fourth category of trouble comes from weeds that produce pollen that causes sneezing, runny noses, and red, itchy eyes in allergenic people.

Ragweed is arguably the worst culprit. This pale-yellow bloomer causes most of its trouble in late summer around the same flowering time as goldenrod, a showier native perennial that gets undeserved blame for the sneezing.

Plantain, mugwort, lamb’s quarter, and pigweed are four other of the most common sneeze-inducing yard weeds, while bermudagrass and orchard grass are two of America’s most allergenic grassy weeds.

What to do?

The most important defense against harmful weeds is to never eat a wild plant without knowing exactly what it is. Some poisonous plants look a lot like their non-toxic relatives.

Even with rash-causers and sneeze-inducers, it helps to know the enemy so you don’t inadvertently try to pull a problem weed bare-handed or overlook what could be the main but less-obvious source of your hay fever.

Images and descriptions of over 110+ weeds can be found in Preen’s gallery of weeds as well as in numerous books. Phone apps are also available to help with weed and plant ID.

Weed Scanning

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Existing weeds will have to be cut down, pulled out, or sprayed with herbicides. Be sure to cover all skin before touching potentially rash-causing weeds, and never use a weed-whacker on them, which can spew the trouble-causing oils and saps onto skin, into eyes, or into the air.

New or follow-up outbreaks can often be prevented by stopping seed germination with twice-a-year applications of a weed-preventer such as Preen.

Mulching and covering as much bare soil as possible with desirable plants are two other weed-defense tools.

Read more about how to stop new weeds from germinating

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