Pollinator plantings can certainly add both natural beauty and valuable wildlife habitat on or near the cranberry marsh, but occasionally unwanted species can begin to creep in.  As our pollinator gardens become fully established on participating cranberry marshes, we have noticed several weed species, i.e., spotted knapweed (Centaurea stoebe), horseweed (Conyza canadensis), sedges (Carex spp.), common mullein (Verbacsum thaspus) and ragweeds (Ambrosia spp.), commonly making their way into the plantings. Of these weeds, spotted knapweed is of the most concern. An invasive species from eastern Europe, spotted knapweed roots grow quickly and release growth inhibiting chemicals into the soil. Spotted knapweed is also distasteful to grazing wildlife and thrives in disturbed soil, making it a strong competitor to native plant species.

Interestingly, spotted knapweed, common mullein, and ragweeds are attractive to bees, particularly honey bees and the many bumble bee species we have in Wisconsin. Spotted knapweed is prized by apiarists as a “honey plant”, as it provides ample pollen and nectar resources in mid to late summer, when other nectar sources large enough to support numerous honey bee colonies are difficult to find. Common mullein likewise blooms in midsummer, and the towering yellow flower spikes (sometimes 5-10 feet tall!) are often found covered in bees. Ragweeds are also valuable sources for pollen and nectar in the late summer/early fall. Despite their apparent usefulness, it is recommended that those managing pollinator plantings remove these species to promote the growth of desirable wildflower species.

These unwanted species, typically biennials or perennials with similar growing requirements as the desired plant species, can be extremely difficult to control if they become established and widespread, and could potentially take over your pollinator garden. The first line of defense against weeds in a pollinator planting is yearly mowing/burning, typically between mid-April and mid-May. Mowing or burning at this point removes new weedy growth at a time when most of the wildflower species in pollinator plantings are still dormant. Careful scouting and early intervention are critical in maintaining a desirable plant community. In established pollinator plantings during the growing season, the best strategy for coping with weeds is to hand-pull them as soon as you see them. This prevents weeds from becoming well-established, producing seeds or spreading by vegetative tissues. Keep in mind before handling them that some of these species may pose human health hazards and it is recommended to take appropriate precautions.  For example, spotted knapweed can act as a skin irritant for some people, and ragweeds are common culprits for seasonal pollen allergies.

Unfortunately, most herbicides just aren’t selective enough to control one unwanted species without adversely impacting the desirables in a mixed-species planting.  Thus, early intervention by mowing/burning and hand-pulling the early invaders is very important.  In addition, some species such as horseweed may be resistant to several herbicides.

The only herbicide option to control broadleaf weeds in a pollinator planting would be to use the non-selective herbicide glyphosate (sold as Roundup and many other trade names; look for a label that includes Habitat Management as an application site) as a spot application to suppress or control unwanted species.  In general terms with many perennial unwanted plants in particular, this can be an effective time of year for such spot applications as the herbicide will be translocated to the root system as the plants “pack it in” for winter from the flower bud growth stage until the first hard frost.  As always, be sure to read and follow the herbicide label and keep in mind that glyphosate is a non-selective herbicide – any plant that you contact with it will likely be injured or killed.

Happy harvest!

This article was posted in Cranberry, Insects and tagged Christelle Guédot, Cranberries, cranberry, Jed Colquhoun, Nolan Amon, pollinator gardens, pollinators, Weed Management, Weeds.