We've been working with the Village of Shorewood Hills for a couple years now on rehabilitating a slope that they have set aside as a stormwater control area within the village. By the time we got there in the fall of 2010 it was an area of weedy trees and invasive brush, dominated by a canopy of black locust, with mostly bare soil on the ground. The ground layer plants that were present were mostly invasive species such as burdock, canada thistle, reed canary grass, garlic mustard and dames rocket, but a few scattered patches of native plants remained from the original seeding done in 2000. One thing we did not see when we first started working on the site was American pokeweed (Phytolacca americana).
We've done some regular but limited management work on this slope in the past few years. A prescribed burn, some brush removal, annual control of garlic mustard, dames rocket and other weeds, and a couple rounds of interseeding. Overall the site looks a lot better today, but it's still a work in progress.
The one plant that has responded best to all this treatment is the American pokeweed. It has gone from being absent, or perhaps a small, trivial population, to covering more area on the slope than any other species. It clearly dominates the entire lower 1/3rd of the slope.
This leaves us to wonder, is it healthy to have this much pokeweed? Should we begin to manage pokeweed alongside summer weeds like bull thistle and burdock at this site? I personally have always admired pokeweed as a resilient pioneer species. Its large size, colorful stem and tropical appearance make it an interesting find on any site. But land-managers around southern Wisconsin are becoming concerned about the rapid increase in abundance of this species in their natural areas. I decided to do a little more research on this species to develop an informed decision on how we should approach managing it... as a weed or a native warrior, and I found out some interesting things.
Poison, meal, medicine and dye
It turns out pokeweed is a poison, a medicine, and an edible wild plant, all in one.
The roots, stems, and leaves of this plant contain high amounts of toxic compounds such as phytolaccine, phytolaccatoxin formic acid, tannin, and resin acid. The berries also contain trace amounts of these compounds, which cause diarrhea, colic, bloody stools, possible anemia, and occasionally death. The young leaves contain less toxin and were used as food by Native Americans and early settlers.
Some people still eat this "poke sallet" today by choosing leaves from the plant when the stems are still green (before they turn purple) and boiling them twice, dumping out the water from the first boiling to remove the toxic compounds. The stems can also be cut, covered in cornmeal, and fried like okra (I assume after being boiled once first). According to the site, Eat the Weeds, "Nutritionally, pokeweed is a powerhouse: A half cup of the greens provides 35 calories (10 from fat), no cholesterol, three grams dietary fiber, and 90% of your daily need for vitamin A, 60% of vitamin C, 8% calcium, and 6% of iron." Eat the Weeds has a really good write up about the edibility and cultural impacts of pokeweed consumption so the above link is a worthwhile side trip.
Pokeweed was even sold as a canned vegetable in the southern US until recently. And the popular song from the 1950's, Poke Salad Annie, was written about a poor girl who gathered and ate this plant.
The compounds produced by this plant don't just make the plant difficult to eat, they also have potential use as medicines. Native Americans discovered the medicinal effects of this plant early on and used it to stimulate the heart and to treat cancer, rheumatism, itching, and syphillis. It was also effective as a laxative and to induce vomiting. Compounds found in the pokeweed plant are being examined by modern medical science; particularly the 'pokeweed antiviral protein' (PAP) is being explored for the medicinal value at treating cancer, herpes and HIV. Other compounds in this plant may enhance the immune system and have additional anti-cancer effects. See cancer.org for more fascinating medical info.
On top of all of that, the juice from the berries was once used to make ink and dye, and is still used today to make red food coloring. This pokeweed ink was used by Thomas Jefferson to scribe the Declaration of Independence.
An Ecologically Valuable Plant? Just ask the birds.
The brightly colored berries are an important food source for many songbirds including eastern bluebirds, rose-breasted grosebeaks, cedar waxwings, northern cardinals, hermit thrush and many other birds which typically consume berries, (including, but not limited to these listed on Illinois Wildflowers) as well as small mammals. We noted a pair of cardinals and woodpeckers feeding on the pokeweed at our last visit to the slope.
So What's the Prognosis?
Pokeweed is considered by some to be just a big weed. But its clear that the plant has value feeding wildlife, perhaps on our own dinner plates, and in our medicine. However, it can get overpopulated at times and shade out other more conservative native speices and may need control work to balance the system. It can be particularly overwhelming in small areas. On the other hand, considering the alternatives in such sites are often invasive weeds such as garlic mustard, dames rocket, burdock and thistles, the pokeweed is often a better option than the alternative.
I think the key to this issue in southern Wisconsin is the below image:
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| Image courtesy: Kartesz, J.T., The Biota of North America Program (BONAP). 2013. Taxonomic Data Center. (http://www.bonap.net/tdc). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2013. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)] |
Southern Wisconsin is on the very northern edge of the range of this plant, it thrives in warmer, more moist climates to the south and east of here. It's only in the past decade or so that we've been finding it in abundance in southern Wisconsin. It is likely that this insurgence of pokeweed is a result of global warming altering our local weather patterns in such a way that benefits pokeweed. Consider also that the species responds positively to disturbance, and you have a species that we can expect to be common on natural sites into the future. Perhaps there are various parasites and pests that curb the growth and abundance of pokeweed where it is more common further south that will slowly move their way north to slow this species down in our area. Or perhaps due to habitat fragmentation, these potential control agents won't be able to make the trip.
As a point of comparison, American pokeweed has become an invasive plant in some areas of China. According to this study, it is a sucessful invader in forests composed of black locust. Black locust is an invasive plant in Wisconsin. Though native to the southern US, it must have been imported to China for its timber value. It is interesting to note that our work site also has a canopy of black locust, so there appears to be some association between these species. Perhaps pokeweed is immune to the allelopoathic toxins that black locust releases? This would make sense since they evolved in the same environment. The other interesting thing about this study is that they found that where indigo bush, a native leguminous shrub, was found it was able to exclude the pokeweed, though it's not clear why.
So my advice would be to consider doing control work on pokeweed carefully. If you have a lot of other weeds in the area, it appears to be very effective at suppressing plants of shorter stature. There is no garlic mustard under the dense pokeweed patches on our slope in Shorewood. There's nothing else under there either though. So if you have it taking up a lot of space in a habitat surrounded by a lot of more conservative species, then you might want to control it, to speed the process of ecological sucession up a little bit. However, I suspect that a careful inspection of most sites where pokeweed occurs will reveal that pokeweed is the best plant among a list of awful alternatives. I'd speculate that pokeweed wouldn't compete well in more intact habitats against more conservative natives.
My other thought is that if you have a smallish garden, whether it be vegetable, perennial, or both, this species could be a bit too much for your landscape, so removing some or all of the plants makes sense.
If it Must be Done
If you're going to try to control this species, you'll need to know that it forms a large, deep taproot from which it can resprout if cut or only the upper portion of the root is removed. However, if removed when they are still small plants then it's possible to dig them out. The other organic option for control is mowing. However, studies have shown that mowing will knock it back for a growing season, but it will resprout at full strength the next year. So repeated mowing 2-3 times per year for 2-3 years would probably be needed for permanent control.
Glyphosate, the active ingredient in Round-Up® and other common herbicides, has been shown to be effective at controlling this species. But because of the large size of pokeweed it will have to be applied with caution. For a foliar application, the best bet is to get it while the plant is still relatively small in spring to avoid overspray. The other option would be to use a "stump treatment" method, similar to what is used on brush where a 50/50 mix of the glyphosate concentrate and water are mixed and applied directly to a stump immediately after it is cut. This is time consuming but will nearly eliminate any potential for collateral damage. See the label of the herbicide bottle for specific instructions.
