What Is Hemp Dogbane: How To Get Rid Of Dogbane Weeds
By: Bonnie L. Grant, Certified Urban Agriculturist
Hemp dogbane weed is also known as Indian hemp (Apocynum cannabinum). Both names refer to its one-time use as a fiber plant. What is hemp dogbane and why do we want to get rid of it? The plant is poisonous to animals with a toxic sap and has roots that can burrow 6 feet into the earth. It has become an agricultural pest which makes dogbane control important, especially in commercial garden regions.
What is Hemp Dogbane?
In a perfect world, all life would have its place on earth. However, sometimes plants are in the wrong space for human cultivation and they need to be removed. Hemp dogbane is a good example of a plant that is not beneficial when growing in cropland and can do more harm than good.
It will crowd out the intended crops and establish itself as a creeping perennial that is difficult to mechanically remove. Studies in Nebraska show its presence is responsible for crop losses of 15% in corn, 32% in sorghum and 37% in soybean production.
Today, it is a crop weed but the plant was once used by American native people for fiber used to make rope and clothing. The fiber was crushed out of the stems and roots of the plant. The woody bark became material for baskets. More modern applications show it harvested in fall for string and cordage.
Ancient medicine used it as a sedative and treatment for syphilis, worms, fever, rheumatism and more. The woody herb is a spreading threat in agricultural situations today and a common topic is how to get rid of dogbane.
Hemp Dogbane Description
The plant is an herbaceous perennial which grows in tilled or untilled fields, ditches, roadsides and even the landscaped garden. It has a woody stem with stiff green oval leaves arranged opposite along the purplish stem. The plant exudes latex-like sap when broken or cut, which may irritate skin.
It produces small whitish green flowers that become characteristic slender seed pods. The pods are reddish brown, sickle shaped and 4 to 8 inches (10-20 cm.) long with slightly hairy flat, brown seeds inside. This is an important feature to note about the hemp dogbane description, as it differentiates the plant from milkweed and other similar looking weeds.
The deep taproot and creeping peripheral root system enables hemp dogbane weed patches to double in size in one season.
How to Get Rid of Hemp Dogbane
Mechanical control has limited effectiveness but can reduce the plant’s presence the next season. Tilling will control seedlings if used within 6 weeks of their appearance.
Chemical control has the greatest chances of success, especially on established stands of the weed, except in soybeans where there is no acceptable herbicide control. Apply to the plant before flowering occurs and follow application rates and methods. In studies, high concentrations of glyphosate and 2,4D have been shown to give as much as 90% control. These need to be applied after crops are harvested in cropland situations but will then only give 70-80% dogbane control.
Note: Chemical control should only be used as a last resort, as organic approaches are safer and much more environmentally friendly.
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Weeds — hemp dogbane
Let us speak for a moment of hemp dogbane.
Do a search for it online. “Hemp dogbane: Apocynum canabinum.” The United States Department of Agriculture’s Agricultural Research Service — “Normally, animals avoid hemp dogbane because of its bitter, sticky, milk-white juice. Sheep are more frequently affected than other animals, as they will eat large quantities of hemp dogbane leaves and tops . . .” and “Death from poisoning generally occurs 6 to 12 hours after animals eat the plant. Both dried and green plants are toxic. A lethal dose for most animals is reported to be about 0.5 gm/kg body weight, but as little as 15 gm of green leaves have been reported to cause death of some cows. ” Or the Virginia Cooperative Extension, which informs us “this weed may be poisonous ether green or dry, and only 15-30 grams of green leaves are required to kill one horse or cow.” Further investigation will turn up many other similar reports, all sufficiently similar to seem to indicate a single parent source.
Now, let’s look at a Penn State bulletin from 2016 (thank you, Penn State!), also treating of Aponcynum canabinum, commonly known as hemp dogbane: “Much of the literature on hemp dogbane claims that it is poisonous to livestock, but these claims were based on an early investigation in which oleander (Nerium oleander) was mistaken for hemp dogbane.” (As little as a single leaf of oleander can kill an adult human.) It also adds, “Animals find fresh hemp dogbane distasteful, but can eat it in hay without suffering ill effects.”
Now, we’ll go ask the cows, who eat it first and voraciously when they are turned into a new paddock. Distasteful? It’s a favorite forage. Ill effects? None.
What is the agribusiness industry recommending? Well, the USDA says it “may be controlled by repeated treatment of 2,4-D (italics ours)” while the Vermont extension suggests killing it with Accent, Beacon, Banvel, or Roundup Ultra herbicides, alone or in combination. Poison the poison, in other words. Only, it seems this particular plant isn’t poisonous after all. We are to douse the fields with toxic chemicals, without even knowing the enemy.
And this is science, people, the god we’re never supposed to question.
Allergy sufferers will be quick to accept the classification of ragweed plants as "noxious weeds." Both common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida) are major contributors to hay fever.
Ragweed season is dreaded by young and old alike. Parents should not jump to the conclusion that a kid's runny nose in fall is the result of a virus picked up at school: It's possible that ragweed is to blame.
Once you learn what ragweed looks like, you can at least eliminate it from your own yard and thereby perhaps limit your exposure to the source of your hay-fever problems.
Here is a quick summary of common herbicide options:
- 2, 4-D ester 4E (1/2 to 1.5 pt/A) - 2, 4-D is a systemic herbicide that controls annual, biennial, and perennial broadleaves. The ester formulation is slightly more active than the amine and should not be used post--emergence if temperatures are greater than 80°F. 2, 4--D is commonly tank mixed with other herbicides (e.g., dicamba) to improve control and broaden weed spectrum. This is a good, inexpensive herbicide with limitations. It tends to be weak on a number of weed species including wild carrot, dock species, bedstraw, horsenettle, hemp, dogbane, common milkweed, pokeweed, brambles and most woody perennials. It has a 7 day grazing and 30 day haying restriction. Check product labels as some restrictions/uses vary.
- Clarity 4S or Banvel 4S (0.5 to 4 pt/A) - Clarity/Banvel (dicamba) is a systemic herbicide that controls many annual and biennial broadleaf weeds and provides suppression or control of numerous perennials. Clarity/Banvel is commonly tank mixed with other herbicides (e.g., 2, 4-D) to improve control and broaden weed spectrum.
- Overdrive 70WDG also contains dicamba in addition to diflufenzapyr (a synergist) and can be used in established grass stands (not seedlings) for control of numerous broadleaf weeds. The Overdrive use rate is 4 to 8 oz/A and can be tank mixed with numerous herbicides. Dicamba is fairly broad spectrum but tends to be weak on wild carrot, buttercup species, dandelion, milkweed, and bedstraw to name a few. At 1 to 2 pints/acre, dicamba has a 21 day grazing and 51 day haying restriction. Overdrive does not have any grazing or haying restrictions.
- Cimarron 60DF (0.1 to 1 oz/A) - Cimarron (metsulfuron-methyl) is an ALS - inhibitor herbicide that controls many annual, biennial, and some perennial broadleaf weeds, depending on the rate used. It can be used in established warm or cool season grass stands. For most grass species, do not apply until one year after establishment (minimum of 6 months) timothy and fescue require a longer period. It is often tank mixed with 2, 4-D or dicamba to increase activity and weed control spectrum. This combination provides good control of weeds like Canada thistle, bull, musk, and plumeless thistle, and multiflora rose to name a few. COC or NIS must be included in the spray solution. Cimarron does not have any grazing or haying restrictions.
- Milestone 2L (3 to 7 fl oz/A) - Milestone (aminopyralid) is a newer active ingredient labeled for grass hay and pasture. Milestone controls many annual, biennial, and perennial broadleaf weeds and is effective on thistles (Canada, bull, musk, plumeless), burdock, dock species, bedstraw, horsenettle, knapweed, sowthistle, ironweed and others. It is less effective on wild carrot, hemp dogbane, common milkweed, and most brush species to name a few. Milestone can be tank-mixed with other herbicides and the addition of NIS is recommended to enhance activity. Milestone is non-volatile. ForeFront R&P 3L (1.5 to 2.6 pt/A) is a premix of aminopyralid plus 2,4-D that can also be used in grass hay and pasture to broaden the spectrum of activity. Milestone has no grazing or haying restrictions, while Forefront has a 7 day haying restriction. For both Milestone and ForeFront, special manure handling precautions are recommended to prevent injury to sensitive broadleaf plants (see label guidelines).
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SPECIES: Apocynum cannabinum
- IMMEDIATE FIRE EFFECT ON PLANT
- DISCUSSION AND QUALIFICATION OF FIRE EFFECT
- PLANT RESPONSE TO FIRE
- DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
- FIRE MANAGEMENT CONSIDERATIONS
Fire likely top-kills Indianhemp. It is probably resistant to fire-induced mortality because of its deep root and rhizome system.
DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
The development of new Indianhemp plants 100 days following severe fire in Ontario, on a site previously dominated by white spruce and quaking aspen, resulted from rhizome and root crown sprouts .
PLANT RESPONSE TO FIRE:
There are conflicting results as to how Indianhemp responds to fire. Johnson and Knapp  stated that populations of Indianhemp increased with increased fire frequency on tallgrass prairie in Kansas . However, Tester  found a nonsignificant negative effect of burn frequency on Indianhemp in an oak savanna of east-central Minnesota .
Indianhemp was found on all 5 burned study plots 100 days after fire in a white spruce-quaking aspen site in Ontario . Indianhemp also produced new spring growth within days following a prescribed burn in a tallgrass prairie in Kansas .
Prescribed burns were implemented in consecutive years in an oak savanna in east-central Illinois. The 1st fire resulted in a "hot, intense" fire, and the fire the following year was not as hot or intense. Indianhemp increased on burned sites but only in the 2nd postfire year . It increased in percent cover on both control and burn plots studied after a prescribed fire on a mid-elevation wetland in southeastern Arizona .
Conversely, the findings of Bowles and others  indicate that Indianhemp populations did not survive after 8 dormant-season prescribed burns on graminoid fens in Cook County, Illinois. Indianhemp plants were recorded on the study plots before burning, but were not present after 5 years . On study plots dominated by post oak, winged elm (Ulmus alata), and white ash (Fraxinus americana) in southern Illinois, Indianhemp was observed during prefire sampling and not found in postfire months 5 or 6 (burns characterized as "moderate, at best") . In a sedge-beaked spikerush-Kentucky bluegrass (Carex spp.-Eleocharis rostellata-Poa pratensis) wetland near Tucson, Arizona, Indianhemp density increased more on control plots than burned plots. Indianhemp increased on high-frequency repeat spring burns (every 2-3 years), medium-frequency repeat spring burns (every 5-7 years), and unburned control plots . However, analyses of variance failed to demonstrate a significant (p=0.70) effect of burning on Indianhemp cover.
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
No additional information is available on this topic.
FIRE MANAGEMENT CONSIDERATIONS:
The current body of research provides no clear direction for using fire as a management tool for Indianhemp populations. The results of studies done to date, 2006, are conflicting. Further research is needed on the fire ecology of Indianhemp.