What’s Up With Weeds: December 2014

Deck the Halls!

Holly Collecting

Boys collecting holly and mistletoe to sell (1959)

December is the month when our busy lives get busier. The December mad dash of shopping, baking, wrapping, and mailing, ultimately gives way to the holidays spent surrounded by our loved ones. December should be a time of enjoying each others company, not concerning ourselves with weeds. After all, the cold weather of winter has sent most of our weeds underground or into slow motion as they wait for the warmer days of spring ahead.

Invaders in December

Yet surprisingly, invasive weeds actually play a key role in December. In fact, this is the month when we when many of us welcome invasive weeds into our homes.

Invasive species first creep their way into our holiday season through our Christmas trees. Here in Oregon we take our Christmas trees seriously! (They are important to our local economy and we work to support conservation with many growers.) Many of us trod our way through the rainy (or snowy) wet weather and mud to our favorite u-cut Christmas tree farm. With our families in tow we sift through the thousands of trees to find that one perfect Christmas tree for the season.

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Early detection is the key

Pests can be exported on plants

But Oregon Christmas trees are also shipped throughout the county and across the globe. This global trade has actually allowed us to export our own invasive pests and diseases to other areas as our local insects and plant pests hitch a ride via Christmas tree to lands far away. (This can occur with any exported plant materials, not just Christmas trees.)

Holly isn’t noxious but it is invasive

December is also the month when we “Deck our halls with boughs of holly”. Although its a very old tradition, many holly branches are still hung today, and even more are coiled into ornate wreaths for our front doors. To many folks holly is synonymous with the holiday season. So you may be surprised to learn that this species is a non-native. As European settlers immigrated to North America, they brought their traditions along as well, and holly is an example of this migration.

Because of the cultural significance of holly, it is actively cultivated in our region. Although holly is not a state listed noxious weed, it is widely recognized as an invasive plant throughout the Pacific Northwest. (Noxious weeds are a particular subset of weeds classified by the Oregon State Weed Board to be “injurious to public health, agriculture, recreation, wildlife, or any public or private property”. This regulatory designation limits the sale and distribution of a plant and as such very few culturally and economically significant species receive the noxious weed designation.)

Holly infestation

Holly infestation dominating a forest understory in Clackamas County

Although holly is not a state listed noxious weed, it is naturalizing and becoming more widespread throughout our area, particularly in the conifer forest understory were it can become the dominant forest understory vegetation. When holly invades it competes with other understory plants and can suppress natural regeneration of forest trees over time. Natural resource managers across the Pacific Northwest are spending considerable resources to treat and control holly in an effort to preserve native conifer forests.

To combat this species, some folks have taken an innovative approach to controlling their holly, by replacing their traditional conifer Christmas tree for a wild caught holly tree — an innovative and economical approach to control a problem invader. But watch out because those holly trees will resprout with relative ease. Check out one of our weed wrenches from our tool library free of charge to and pull the entire plant up.

So don’t forget to “Deck the Halls” this holiday season. Or if that doesn’t work you can always consider an invasive species as the perfect accompaniment to your holiday feast.

Merry Christmas and Happy Pulling!

So what is up with Weeds in December?

Grass-like plants

  • Jubata grass (Cortaderia jubata) is an annual shrub that follows the same timeline as Pampas grass. Plants have seeded. Dig up the entire plant.
  • Common reed grass (Phragmites australis) is a grassy perennial. Plants that were emerging in April, May, and June are seeding, cut and bag seed heads and dispose in trash.
  • False brome (Brachypodium sylvaticum) is a perennial grass. Plants have seeded and going dormant. Please [contactweedwise] if you see false brome!

Herbaceous flowering plants

  • Russian knapweed (Acroptilon repens) bolts in May with flowers appearing in June, July, and August. Control can be through handpulling, tilling, mowing, or digging. Root fragments resprout so it’s important to get all of the plant out of the ground.
  • Garlic mustard (Alliaria petiolata)was forming rosettes through the winter and flowers in April and May. Seeds may be present and viable from June through September! You can hand pull plants, although root fragments can resprout. Please [contactweedwise] if you see garlic mustard!
  • Spotted knapweed (Centaurea biebersteinii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
  • Purple starthistle (Centaurea calcitrapa) sprout from seed in winter and develop into rosettes in spring. Starthistle plants flower from July to Spetember. Please [contactweedwise] if you see purple starthistle in your area.
  • Meadow knapweed (Centaurea debeauzii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
  • Diffuse knapweed (Centaurea diffusa) is a herbaceous perennial. In May, plants bolt. Flowers appear in May, June, and July. Dig up the entire plant (easier when soil is moist in the spring).
  • Yellow starthistle (Centaurea solstittialis) sprout from seed in winter and develop into rosettes in spring. Starthistle plants flower from May to July. Please [contactweedwise] if you see yellow starthistle in your area.
  • Rush skeletonweed (Chondrilla juncea) bolts in April, May, and June. When it flowers in July, it sets seed quickly!
  • Paterson’s curse (Echium plantagineum) emerges in March and April, flowering all summer.
  • Orange hawkweed (Hieracium aurantiacum) can be dug up in March and April. Be sure to get the roots and runners. Flowering occurs in May and June, and can extend later, with flowers and seeds occurring from July through September. More information is available on the Oregon Department of Agriculture’s page about Orange hawkweed. Please [contactweedwise] if you see Orange hawkweed!
  • Yellow-flag iris (Iris pseudacorus) is a herbaceous perennial that emerges in April and flowers in May and June. Seeds are set from July through September
  • Yellow archangel (Lamiastrum galeobdolon) is a herbaceous perennial the grows throughout the winter and flowers from April to June. Seeds are set in July and August.
  • Kudzu (Pueraria lobata) is a vining perennial. Leaves emerge in April, May, and June. Flowers emerge in July. Please [contactweedwise] immediately if you think you have spotted Kudzu in Clackamas County!
  • Lesser celandine (Ranunculus ficaria) is a herbaceous perennial. Rosettes form in January and February, and the plants flower in March. In April, seeds are set. Remove all of the bulblets and tubers.
  • Tansy ragwort (Senecio jacobaea) is a herbaceous biennial. Rosettes form in March and April, and flowers in May, June, and July. If soil is moist, dig up the rosettes. If you remove the plant during flowering, bag the plant so seeds can’t spread.

Shrubby plants

  • Blessed milk thistle (Silybum marianum) overwinter as rosettes that bolt in March, April, and May. The plants flower in June and July. Mow or hand pull before flowers fully develop.
  • Butterfly bush (Buddleia davidii), a tall deciduous shrub, displays new leaves in April, May, and June, but doesn’t generally flower until July and August. Control by digging up the entire plant. Our Weed Wrench might work!
  • Giant hogweed (Heracleum mantegazzianum) is a tall herbaceous perennial. Plants emerge in March and April from winter rosettes, and accelerate in growth in April and May. When the plants flower in June and July, remove the heads and cut off the plant at the base…but be very careful because the sap of this plant is caustic. ODA says this about Giant hogweed: “This plant is a public health hazard. Do not expose bare human skin to the plant or breathe the smoke from fires if it is being burned. The plant exudes a clear watery sap which sensitizes the skin to ultraviolet radiation. Humans often develop severe burns to the affected areas resulting in blistering and painful dermatitis. Blisters can later develop into purplish or blackened scars.” Please [contactweedwise] immediately if you spot this plant!
  • Gorse (Ulex europaeus) is a single to multi-stemmed spiny shrub that buds in February and flowers in March, April, and May. It goes to seed in June and July. Dig up small plants. Please [contactweedwise] if you see gorse!
  • Policeman’s helmet (Impatiens glandulifera) is a herbaceous annual the begins to emerge in April and grows through May. Flowers appear in June and July.
  • Purple loosestrife (Lythrum salicaria) is a herbaceous perennial that emerges in May and June. Flowers begin to appear in July.
  • Scotch thistle (Onopordum acanthium) is a herbaceous biennial that overwinters as rosettes. Flower stalks emerge in June and flowering is in July and August.
  • Spurge laurel (Daphne laureola) is an annual shrub that emerges in March and flowers in April, May, and June. Berries appear in June and July. Dig up as much of the plant and root system as possible. Please [contactweedwise] if you see spurge laurel!
  • Knotweeds include Japanese knotweed (Polygonum cuspidatum), Giant knotweed (Polygonum sachalinense), Himalayan knotweed (Polygonum polystachyum), and hydrid knotweeds began emerging in April. With warmer temperatures, growth accelerates throughout May and June. Flowers emerge in July and August. Mechanical control requires frequent, persistent effort: cut twice each month from April through August. In the fall, additional steps are needed, including cutting and covering the plants. Knotweeds are so persistent that you need to do this repeatedly for five years to achieve control.

Aquatic plants

  • Parrot’s feather (Myriophyllum aquaticum) is a perennial with underground rhizomes that overwinter. Stems emerge in April and May. Flower stalks appear in June and July. Removing the entire plant, including root fragments, is necessary.

Report weeds!

Visit our page on reporting weeds to file a report.

Report Weeds!
Early detection is the key

The images on this page are licensed under creative commons. Photos courtesy of:

Joseph M. DiTomaso, University of California – Davis, Bugwood.org

Geoff Charles Collection at the National Library of Wales

Samuel Leininger, Clackamas SWCD

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Clackamas SWCD