Brittlestem hempnettle (Galeopsis tetrahit)

Synonyms: Ladanum tetrahit, Lamium tetrahit

Common Names: Common hempnettle, common hemp-nettle, hemp nettle, brittle-stemmed hemp-nettle

Description:

Brittlestem hempnettle (Galeopsis tetrahit) is a bristly annual in the mint family, native to Europe and northwestern Asia [1][2]. It was introduced to North America and is now a widespread weed of roadsides, fields, pastures, and open woods, where it can form dense single-species patches [2][3]. MISIN tracks it because it spreads readily by seed, crowds out other plants, and is a weed of crops and gardens [3].

Habit:

Brittlestem hempnettle is an annual that grows 1 to 3 feet tall on square stems ? a mint-family trait you can feel by rolling the stem between your fingers [2][3]. Two features set it apart: the stems are swollen and thickened just below each leaf joint, and they are covered in stiff, downward-pointing bristly hairs [2][3]. The leaves grow in opposite pairs, are egg-shaped to oval with coarsely toothed edges, and are hairy on both sides [2][3]. From summer into early fall it bears small, two-lipped, snapdragon-like flowers in dense whorls where the upper leaves meet the stem; the flowers are pink to purple or white, often with markings on the lower lip [2][3]. It is easiest to spot in mid- to late summer when in flower. Each flower leaves behind four small nutlets [3].

At-a-glance cues:

1. Square stems, swollen just below each leaf joint, with stiff downward-pointing bristly hairs [2][3]
2. Opposite, egg-shaped, coarsely toothed leaves, hairy on both sides [2][3]
3. Small, two-lipped, snapdragon-like flowers (pink, purple, or white) in dense whorls at the upper leaf joints [2][3]
4. An annual, 1 to 3 feet tall, on disturbed or open ground [2][3]

Leaves:

1. Arrangement: opposite (two leaves per node) [2]
2. Shape: egg-shaped to oval (elliptic), roughly 1 to 4 inches long [2]
3. Margin: coarsely toothed [2][3]
4. Surface: hairy on both the upper and lower sides [2][3]

Stems:

1. Habit: erect annual, 1 to 3 feet tall [3]
2. Shape: square in cross-section (a mint-family trait), and noticeably swollen just below each leaf joint [2][3]
3. Surface: covered in stiff, mostly downward-pointing bristly hairs, mixed with some gland-tipped hairs [2][3]

Flowers:

1. Color: pink to purple or white, often with markings on the lower lip [2][3]
2. Shape: small, two-lipped, snapdragon-like flowers (typical of the mint family) [2][3]
3. Arrangement: in dense whorls where the upper leaves meet the stem [2][3]
4. Bloom time: summer into early fall (about June to September) [3]

Fruit and seeds:

1. Fruit: each flower produces four small nutlets, each holding one seed [3]
2. Spread: by seed only ? it is an annual with no creeping roots [2][3]
3. Seeds move in soil, on equipment, and as a contaminant in crop seed [3]

Habitat:

Brittlestem hempnettle is native to Europe and northwestern Asia [1][2]. It is not native to North America; it was introduced and has naturalized across the northern United States and Canada [2][4]. It is a pioneer of disturbed and open ground, invading roadsides, fields, pastures, gardens, open woods, and waste places, and it does best in moist, fertile soils [3].

Reproduction:

Brittlestem hempnettle is an annual ? it lives a single year and spreads only by seed, with no creeping roots [2][3]. Each plant can produce many small nutlets that drop near the parent and also travel in moved soil, on tools and equipment, and mixed in with crop or grass seed, which is how it reaches new fields and yards [3]. Seeds sprout in spring, plants flower through summer, and seed ripens from late summer into fall [3].

For control timing: pull or cut plants before the flowers fade and set seed in late summer ? stopping seed is the whole job for an annual [3].

Impact and Damage:

Brittlestem hempnettle's bristly stems can irritate the skin of some people, so it is worth wearing gloves when handling it [3].

It is mainly a weed of fields, gardens, and disturbed ground. It can form dense, single-species patches that crowd out other low plants, and it is a troublesome agricultural weed that grazing animals tend to avoid [2][3]. It can also harbor crop pests and diseases, serving as a host for a potato fungus and for several nematodes (microscopic soil roundworms) [3]. Because it spreads only by seed, stopping seed production is the key to keeping it from building up [3].

Similar species:

Brittlestem hempnettle can be hard to tell from its close relatives in the same group.

Splitlip hempnettle (Galeopsis bifida) is a very similar introduced hemp-nettle that shares the square, swollen-jointed, bristly stems and two-lipped flowers. The two are separated mainly by small details of the flower's lower lip, so confirming which one you have usually takes a close look with a botanical key [2].

Monitoring and rapid response:

Signs to watch for:

1. Bristly, square-stemmed annual plants 1 to 3 feet tall, with stems swollen just below the leaf joints, on roadsides, fields, gardens, or disturbed ground [2][3]
2. Dense whorls of small, two-lipped pink-to-purple or white flowers at the upper leaf joints from summer into early fall [2][3]
3. New patches in cultivated ground, often where soil or crop seed has been moved [3]

What you can do:

1. For a small patch, hand-pull or hoe out the plants ? including the taproot ? while they are still in bud or early flower, before any seed sets [3]
2. Wear gloves and long sleeves, since the stiff bristly stems can irritate skin [3]
3. Bag and trash plants that already carry flowers or seed rather than composting them, to avoid spreading seed [3]
4. Clean soil and seed off tools, boots, and equipment before leaving an infested area [3]
5. Because it is an annual that spreads only by seed, repeating the removal for a few years to exhaust the seed in the soil usually clears a patch [3]
6. Natural area and farm crews can prioritize small, new outbreaks and cut or pull them before seed set across a site [3]

When to call a professional:

1. Large or well-established infestations in crop fields or pastures where hand methods can't keep up [3]
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Galeopsis tetrahit L. powo.science.kew.org
2. Go Botany, Native Plant Trust. Galeopsis tetrahit (brittle-stemmed hemp-nettle). gobotany.nativeplanttrust.org
3. Wisconsin Department of Natural Resources. Hemp nettle (Galeopsis tetrahit). dnr.wisconsin.gov
4. Invasive Plant Atlas of the United States (UGA Center for Invasive Species and Ecosystem Health / Bugwood). Common hempnettle (Galeopsis tetrahit). invasiveplantatlas.org

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