Weeds to Watch Out For: June 2017

Enjoying the recent spell of sunny weather? You’re not the only one: Across King County, a number of noxious weeds are already bolting, flowering, or going to seed, and our Weed Specialists are working hard to keep them under control.

Let’s start with those weeds flowering or seeding right now: garlic mustard (Alliaria petiolata), Dalmatian toadflax (Linaria dalmatica ssp. dalmatica), orange and yellow hawkweed (Hieracium aurantiacum and Hieracium caespitosum), poison-hemlock (Conium maculatum), and multiflora rose (Rosa multiflora).

Class A garlic mustard (Alliaria petiolata) is a highly variable weed that’s tough to identify, but many individuals are going to seed right now regardless of shape and size. You can distinguish this biennial or winter annual from its look-alikes by its:

  • leaves that don’t feel fuzzy or hairy
  • always white flowers and long, skinny seed pods
  • roots with a distinct “S” or “L” shape
  • recognizable garlic smell when crushed

Garlic mustard can grow in a wide variety of conditions, including diverse soil types, sun and moisture levels. It can even take over relatively undisturbed forest understories, and has done so throughout much of the Northeast and Midwest, and is starting to really take off in the Pacific Northwest. It’s also self-seeding, with one plant producing more than 60,000 seeds. It’s one dangerous weed!

For more information on garlic mustard identification and control, visit this page. If you see garlic mustard in King County, please report it to us on this page.

Meanwhile, Class B Dalmatian toadflax (Linaria dalmatica spp. dalmatica) is in full bloom. This perennial grows to three feet tall with yellow and orange-tinged snapdragon-like flowers that produce up to 500,000 seeds.

It also reproduces through lateral roots, which can be up to ten feet long and will sprout new stems when disturbed. Dalmatian toadflax thrives in disturbed soils, especially areas with sparse vegetation like roadsides, pastures, forest clearings, and industrial areas. It resembles the much more common yellow toadflax (Linaria vulgaris) but has larger, wider, heart-shaped leaves that clasp the stem.

For more information on Dalmatian toadflax identification and control, visit this page.

Orange and yellow hawkweed (Hieracium aurantiacum and Hieracium caespitosum) are smaller perennials (one to three feet tall) that are flowering now too. Like Dalmatian toadflax, these plants flourish in disturbed soils and other open areas. They prefer full sun but can handle partial shade. They readily hybridize with native and non-native species, but can be identified by:

  • yellow or orange flower heads on top of hairy, almost leafless stems
  • buds that are black and ball-shaped in tight clusters
  • hairy leaves that are not lobed on the edges
  • dandelion-like seed heads
  • milky juice inside all plant parts
  • fuzzy white stolons when flowering

Both plants reproduce via seeds as well as well as stolons.

For more information on orange and yellow hawkweed, visit this page.

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Poison-hemlock blooming.

Poison-hemlock (Conium maculatum) is the first of two non-regulated weeds blooming in full force right now. We’ve mentioned this plant in a few recent posts. A biennial highly toxic to people and animals, this plant can be identified by its:

  • hollow, hairless, ridged stems with red or purple splotches
  • lacy fern-like leaves with a distinct musty smell
  • numerous umbrella-shaped clusters of small, white, five-petaled flowers when blooming

First-year plants grow in small rosettes that resemble wild carrots. Second-year plants grow eight to ten feet tall and produce up to 40,000 seeds. Poison-hemlock prefers areas with moist soil and sun, such as riparian areas, ravines, vacant lots and the borders of community gardens. Because poison-hemlock is widely distributed in King County, control isn’t required, but it’s definitely a weed to be careful around!

For more information on poison-hemlock identification and control, visit this page.

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Multiflora rose both climbs and forms thickets.

Finally, you might have noticed multiflora rose (Rosa multiflora) blooming along freeways or streams in your part of King County. This rambling or climbing invasive rose species grows five to 12 feet tall with:

  • upright, arching, bright green or reddish stems, often marked by curved thorns
  • showy and fragrant white or pink flowers with notched petals in clusters
  • clusters of numerous small, red, leathery rose hips

Multiflora rose resembles blackberry and other similar plants, but can be distinguished by its clusters of red rose hips and its ability to climb in addition to forming thickets.

For more information on multiflora rose identification and control, visit this page.

Just behind the above-mentioned plants, three regulated noxious weeds are bolting right now and getting ready to bloom: giant hogweed (Heracleum mantegazzianum), policeman’s helmet (Impatiens glandulifera), and spotted knapweed (Centaurea stoebe).

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Giant hogweed bolting, May 16th, 2017.

Giant hogweed (Heracleum mantegazzianum) is a Class A noxious weed that’s of special concern because of the clear, watery sap in its stems. If your skin touches the sap and then is exposed to sunlight, you might develop severe burns and even scars.

As its name implies, giant hogweed is also massive, growing up to 15 feet tall with sharply incised compound leaves up to five feet wide. A perennial, it has hollow stems with red or purple bumps and stiff white hairs. From mid-May-July in its third year (or later), the plant produces a 2.5-foot-wide umbrella-shaped cluster of numerous small white flowers.

Giant hogweed resembles the native plant cow parsnip (Heracleum maximum) but grows much larger, with more steeply cut leaves, bumpier reddish blotches, and shorter hairs on the undersides of its leaves. Giant hogweed thrives in moist areas with full or partial sun. It was first introduced as an ornamental, and now is often found in yards, parks, ravines, roadsides, and woods.

For more information on giant hogweed identification and control, visit this page. Please be sure to tell us if you see giant hogweed in King County by reporting it on this page.

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Policeman’s helmet as of May 30th, 2017.

Class B policeman’s helmet (Impatiens glandulifera) is also bolting and getting ready to bloom. Though it grows up to ten feet tall, this plant is an annual that germinates February-March and flowers June-October. You can identify policeman’s helmet by its:

  • hollow, upright, juicy, purple- or reddish-tinged stems
  • oblong to egg-shaped leaves with serrated edges, arranged opposite on the stem
  • white, pink, or purple flowers that look like a traditional English policeman’s helmet, or might remind you of an orchid

Policeman’s helmet thrives in moist areas such as streambanks and ditches. Each plant’s 800 seeds shoot up to 20 feet from their pods when touched and can even germinate underwater.

For more information on policeman’s helmet identification and control, visit this page.

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Spotted knapweed bolting May 25th, 2017.

Class B spotted knapweed (Centaurea stoebe) is also nearly set to bloom, and has the distinction of being our June “Weed of the Month.” This biennial or short-lived perennial forms a rosette its first year and later bolts up to five feet tall. It can be identified by its:

  • upright branched stems
  • small, oval flower heads with pinkish or light purple flowers
  • bracts with black spots above vertical lines
  • sparse, deeply lobed green leaves tinged with silvery-gray
  • stout taproot

Spotted knapweed prefers well-drained soils in full sun, and in King County often grows in disturbed areas such as vacant lots, industrial sites, pastures, and roadsides. Each plant produces many seeds that remain viable for up to eight years, making infestations difficult to remove.

For more information on spotted knapweed identification and control, visit this page.

Think you’ve found a regulated weed growing in King County? Let us know! Feel free to fill out this form, email us, or call 206-477-WEED (206-477-9333). Wondering about that weed taking over your yard, neighborhood, or pasture? Send us a photo and we’ll get back to you!



Categories: Tips, Weed Identification

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