What’s Up With Weeds: September

What’s coming up?

We are preparing for a busy fall season. Our WeedWise program will be focused on Knotweed control. Work is commencing along the Sandy River mainstem, as well as in the Salmon River and Hackett Creek subbasins of the Sandy. We are also targeting the greater Eagle Creek subbasin of the Clackamas, and the Milk Creek subbasin on the Molalla.

We are also looking at fall False brome treatments, including at a newly discovered infestation in a portion of the county where it was not previously known.

Remember September with a need to weed!

Japanese knotweed plants turn a golden yellow and orange before dying back to below ground roots each winter

September may be the best month of the year to target perennial weeds. As the hot days of summer shift toward the cooler days of autumn, many of our toughest weeds take notice. Invasive plants like Japanese knotweed start reallocating their above ground growth down into below ground roots and rhizomes as they prepare for winter.

As one of the 100 worst global invaders Japanese knotweed is highly adept at recycling nutrients each season. In its native range it can be found growing on lava flows high on mountain slopes where it is one of the first plants to colonize. This efficiency also allows Japanese knotweed to displace with many of our native tree and shrub species particularly along streams and rivers where it becomes dominant.

The leaves of Japanese knotweed start turning from green to yellow by mid to late September. By November nearly all of the above ground nutrients are safely stored in its robust underground root system. The knotweed plants stay safely tucked away underground until re-emerging the following spring. The difficulty in fighting perennials such as Japanese knotweed is that these robust roots make it incredibly resistant to cutting, digging, and pulling. Even herbicides used at the wrong time of year do little to harm the robust root system below ground.

But September gives a vigilant landowner a fighting chance. As knotweed starts reallocating its above ground growth down into its roots system it becomes more susceptible to the effects of herbicides. As such a carefully timed herbicide application can provide superior control to any other control methods. A landowner guide to controlling knotweed is available to learn more about these strategies.

Target knotweed control just as plants begin to yellow each fall.

What weeds do you have that are best controlled in September?

Grass-like plants

• Jubata grass (Cortaderia jubata) is an annual shrub that follows the same timeline as Pampas grass. Plants are seeding, cut and bag seed heads and dispose in trash. 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 that emerged this winter are beginning to flower. Please [contactweedwise] if you see false brome!

Herbaceous flowering plants

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.

Photo and illustration courtesy of Samuel Leininger, Clackamas County Soil and Water Conservation District