Orange hawkweed flowers
April showers bring May flowers! May in the Willamette Valley is a time of lush green colors spiced up with various flowers in bloom. Some of those flowers are weeds. So what’s up with weeds in the month of May?
(Originally published on May 1, 2012. Republished May 2013 and May 2014.)
Grass-like plants
- Jubata grass (Cortaderia jubata) is an annual shrub that follows the same timeline as Pampas grass, flowering in May and June. Dig up the entire plant.
- Common reed grass (Phragmites australis) is a grassy perennial. Plants emerge in April, May, and June.
Knapweeds
- 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).
- Meadow knapweed (Centaurea debeauzii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
- 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.
- Spotted knapweed (Centaurea biebersteinii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
Knotweeds
Knotweed plants are so invasive and pervasive they deserve their own heading! 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. 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.
Shrubby plants
- Blessed milk thistle (Silybum marianum) overwinter as rosettes that bolt in March, April, and May. 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 mid-summer. 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, remove the heads and cut off the plant at the base.
- Gorse (Ulex europaeus) is a single to multi-stemmed spiny shrub that buds in February and flowers in March, April, and May. Dig up small plants. Please [contactweedwise] if you see gorse!
- Parrot’s feather (Myriophyllum aquaticum) is a perennial with underground rhizomes that overwinter. Stems emerge in April and May. Removing the entire plant, including root fragments, is necessary.
- Policeman’s helmet (Impatiens glandulifera) is a herbaceous annual the begins to emerge in April and grows through May. Flowers begin to appear in June.
- 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. Dig up as much of the plant and root system as possible.
Other herbaceous plants
- Garlic mustard (Alliaria petiolata)was forming rosettes through the winter and flowers in April and May. Catch it before it sets seed in June! You can hand pull plants, although root fragments can resprout.
- Kudzu (Pueraria lobata) is a vining perennial. Leaves emerge in April, May, and June. Please [contactweedwise] immediately if you find 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.
- 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. More information is available on the Oregon Department of Agriculture’s page about Orange hawkweed.
- Paterson’s curse (Echium plantagineum) emerges in March and April, flowering all summer.
- Rush skeletonweed (Chondrilla juncea) is bolts in April, May, and June. When it flowers in the summer, it sets seed quickly!
- 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.
- Yellow archangel (Lamiastrum galeobdolon) is a herbaceous perennial the grows throughout the winter and flowers from April to June.
- Yellow-flag iris (Iris pseudacorus) is a herbaceous perennial that emerges in April and flowers in May and June.
Report weeds!
Visit our page on reporting weeds to file a report.
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