What’s Up With Weeds: April

Garlic mustard flowers

April in the Willamette Valley is a spectacular time as winter gives way to sunshine and new plant growth. So what’s up with weeds in the month of April?

Grass-like plants


  • Diffuse knapweed (Centaurea diffusa) is a herbaceous perennial that forms rosettes in April. Dig up the entire plant (easier when soil is moist in the spring).
  • Meadow knapweed (Centaurea debeauzii) is forming rosettes in April. Dig up the entire plant (easier when soil is moist in the spring).
  • Russian knapweed (Acroptilon repens) is forming rosettes. 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) is still forming rosettes in April and begins flowering in May. Dig up the entire plant (easier when soil is moist in the spring).


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 are emerging starting in April. 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 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. 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.
  • Purple loosestrife (Lythrum salicaria) is a herbaceous perennial that begins emerging next month in May.
  • 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 begins flowering in April. 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 begins flowering in April. Before it sets seed, you can hand pull plants, although root fragments can resprout.
  • Kudzu (Pueraria lobata) is a vining perennial. Leaves emerge in April. 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 begins in May.
  • Paterson’s curse (Echium plantagineum) emerges in March and April, flowering all summer.
  • Rush skeletonweed (Chondrilla juncea) is bolting in April. When it flowers in the summer, it seeds seed quickly!
  • Tansy ragwort (Senecio jacobaea) is a herbaceous biennial. Rosettes form in March and April. 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 begins flowering in May.

Report weeds!

Visit our page on reporting weeds to file a report.

The featured image on this page is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Source: https://en.wikipedia.org/wiki/File:Garlic_Mustard_close_800.jpg


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Clackamas SWCD