Distinguishing Harmful Weed Species
Some weeds can cause rashes or burns if you touch them or get their sap on your skin. A few of these can cause significant harm, while others are much less problematic. In our area, most harmful weeds are either from the cashew family (poison ivy) or the carrot family (giant hogweed, wild parsnip, etc.). The carrot family species can be hard to tell apart; we hope these tools can help.
Of these species, the most harmful (causing damage if you touch them or get their sap on your skin) are poison ivy, giant hogweed, and wild parsnip. When handling these plants, it’s a good idea to wear long sleeves and pants and thick or multi-layered gloves to protect yourself. When you are done, wash the clothes in hot water and wipe down tools with alcohol (for poison ivy) or scrub them with strong soap (for any of them). The University of Michigan has suggestions for removing poison ivy oil; so does the FDA. New York’s Department of Environmental Conservation (DEC) has advice on handling and cleaning up from giant hogweed exposure. The guidance for giant hogweed works for wild parsnip as well.
Identify Flowering Plants
It’s easiest to identify plants when they are in flower. If your plants don’t have flowers, skip down to the “vegetative” key below.
Identification key for flowering plants
1a. Upright plants………………………………………………………………………………………..2
1b. Vine with thee leaflets/leaf……………………………………………………………..Poison ivy
2a. White flowers………………………………………………………………………………………………..3
2b. Yellow flowers………………………………………………………………………………….Wild parsnip
3a. Hairy stems………………………..………..……………………………………………………………………….4
3b. Hairless stems……………………..……..…………………………………………………………………………5
4a. Ends of leaf segments have straight margins towards a point….……………………………….6
4b. Ends of leaf segments curve towards a point………………………………………….Wild carrot
5a. Toothed leaflets…………….…………………………………..Spotted water hemlock
5b. Deeply dissected leaf leaflets…………………………………………………………….Poison hemlock
6a. Green stems with purple splotches………………………………………………..Giant hogweed
6b. Green stems with no purple splotches………………………………………………..Cow parsnip
Cow parsnip stems with no purple splotches.
Giant hogweed stems with purple splotches.
Identify Mature Plants Without Flowers (Vegetative Plants)
During the vegetative stage (after the seedling stage and before the plant begins to flower), plants can be distinguished mainly by their leaves and stems.
Vegetative plant key
1a. Rosette or upright plants……………………………………………………………………………………………2
1b. Non-flowering, vine plant………………………………….……………………………….Poison ivy
2a. Green stems with no splotches……………………………..……………………………………….3
2b. Green stems with purple splotches…..…….……………………………………………………….5
3a. Stems with hair………….…………………………………………………………..….…………….4
3b. Stems with no hair……………………………………………………………………….Wild parsnip
4a. Leaves up to 24 inches long…………………………………………………………..Cow parsnip
4b. Leaves 8 or less inches long…………………………………………………………….Wild carrot
5a. Hairless stems……………………..……………………………………………….…………………6
5b. Hairy stems…………………………………………………………………….………Giant hogweed
6a. Deeply-lobed leaf margins………………………………………………………….Poison hemlock
6b. Serrated leaf margins…………………………………………………..…..Spotted water hemlock
Poison hemlock leaves
Spotted water hemlock leaves
5a. Leaves up to 6 inches long……………………………………………………………………………….6
5b. Leaves up to 24 inches long……………………………………………………………Cow parsnip
6a. Deep lobes on leaf margins……………………………………………..……………….Wild carrot
6b. Shallow lobes on leaf margins………………………………………..………………….Poison ivy
Wild carrot leaves
Cow parsnip leaves
Identify Seedlings
This is the hardest stage to distinguish weeds, but it’s when management is easiest. It is particularly difficult to distinguish giant hogweed seedlings from cow parsnip seedlings and poison hemlock seedlings from spotted water hemlock seedlings. Giant hogweed is significantly more harmful than cow parsnip; poison hemlock and spotted water hemlock have similar levels of toxicity and similar management. Since giant hogweed is a noxious invasive in New York and free control is offered by NYS Department of Conservation (DEC), a good option would be to contact DEC if you’re not sure about whether your seedling is giant hogweed.
Seedling key
1a. Shallow lobes on young leaves/mostly entire leaves.…………………………………………….3
1b. Deeply lobed and narrow young leaves.…..……………………………………………………….2
2a. Grayish-green young leaves………………………………….………………………..…Wild carrot
2b. Light green leaves.………………….……………………Poison hemlock/Spotted water hemlock
Poison hemlock leaves
3a. Dull leaf surface.…………………………………………….……………………..……Wild parsnip
3b. Glossy leaf surface……………………………………….……………………………………………….4
4a. Young leaves do not taper at the end………………………..Giant hogweed/Cow parsnip
4b. Young leaves taper at the end…………………………………………………………………Poison ivy
References
Uva R H, Neal J C, DiTomaso J M. 1997. Weeds of the Northeast. Book published by Cornell University, Ithaca NY. The go-to for weed ID in the Northeast; look for a new edition sometime in 2019.
Cornell University Cooperative Extension and Horticulture Center of Demonstration and Community Gardens at East Meadow Farm, 2003. Home Grounds Fact Sheet-Poison Ivy and Poison Oak.
University of Missouri’s Integrated Pest Management page for Poison Ivy. Contains helpful descriptions for identification of the poison ivy plant, comparisons with similar plants, information regarding the plant’s toxin, and weed management.
University of Maryland Extension’s Home and Garden Information Center page for Identifying Poison Ivy.
University of Georgia Extension’s webpage for Controlling Poison Ivy in the Landscape
pixy.org. High quality free stock images.
University of Florida’s IFAS Extension page on identification descriptions for common poisonous plants, including poison ivy. Addition information on similar plants and helpful images.
Identification sheet on poison ivy by The Pennsylvania State University College of Agriculture. Includes seedling characteristics, biology, and similar species.
Peck, G M and I A Merwin. A Grower’s Guide to Organic Apples. Covers organic weed control methods for organic apple orchards.
Breth, D I and E Tee. 2016. Herbicide AI by Weed Species. This tool allows you to look up the efficacy of an herbicide active ingredient on a particular weed species.
Cornell University Turfgrass and Landscape Weed ID- Weeds and Human Health: The Severe Seven http://turfweeds.cals.cornell.edu/weeds-and-human-health
Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2021.
King County Noxious Weed Control Program, Bet Management Practices- Giant hogweed https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/hogweed-control.pdf
NYS Department of Environmental Conservation, Giant Hogweed https://www.dec.ny.gov/animals/39809.html#Identify
New York Sea Grant (NYSG) Invasive Species Factsheet Series: 07-1 Giant Hogweed (Heracleum mantegazzianum) – Poisonous Invader of the Northeast by Charles R. O’Neil, Jr. http://www.nyis.info/wp-content/uploads/files/Giant_Hogweed_FS_Print_Rev081009.pdf
Ohioline, Ohio State University Extension- Giant Hogweed (Heracleum mantegazzianum) by David L. Marrison and David J. Goerig https://ohioline.osu.edu/factsheet/anr-35
Cornell University’s Cornell Fruit Resources: Berries. New and Emerging Pest Issues in Berries- Wild Parsnip https://blogs.cornell.edu/berries/ipm/pest-alerts/wild-parsnip/
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed. http://turfweeds.cals.cornell.edu/plant/control/150
Cornell University’s Turfgrass and Landscape Weed ID- Weeds and Human Health: The Severe Seven http://turfweeds.cals.cornell.edu/weeds-and-human-health
Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2021.
Management of invasive plant in Wisconsin-Wild Parsnip by Brendon Panke, Ryan deReginer, and Mark Renz. University of Wisconsin-Madison. 2012. https://portage.extension.wisc.edu/files/2018/04/Wild-Parsnip-Management-A3924-15.pdf
New York Invasive Species (IS) Information- Wild Parsnip http://nyis.info/invasive_species/wild-parsnip/
New York State Department of Environmental Conservation- Wild Parsnip https://www.dec.ny.gov/animals/105364.html
University of Minnesota Extension- Wild parsnip: problem plant in hay and pastures https://extension.umn.edu/horse-pastures-and-facilities/wild-parsnip
Wild Parsnip (Pastinaca sativa): A Major Threat to Wisconsin’s Prairies, Fields and Roadsides by Lia Johnson and David J. Eagan. University of Wisconsin-Madison. 2007. https://portage.extension.wisc.edu/files/2018/04/Wild-Parsnip-Management-A3924-15.pdf
Cornell University, College of Agriculture and Life Sciences, Department of Animal Sciences- Plants Poisonous to Livestock: Piperidine Alkaloids. https://poisonousplants.ansci.cornell.edu/toxicagents/piperidi.html
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed. http://turfweeds.cals.cornell.edu/plant/control/147
Cornell University’s Turfgrass and Landscape Weed ID- Weeds and Human Health: The Severe Seven. http://turfweeds.cals.cornell.edu/weeds-and-human-health
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed. http://turfweeds.cals.cornell.edu/plant/identify/150
Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2021.
The Ohio State University- Ohio Perennial and Biennial Weed Guide: Poison-Hemlock. https://www.oardc.ohio-state.edu/weedguide/single_weed.php?id=114
University of California Agriculture & Natural Resources -Statewide Integrated Pest Management Program, Pests in Garden and Landscape: Poison Hemlock. http://ipm.ucanr.edu/PMG/PESTNOTES/pn74162.html
Washington State Noxious Weed Control Board: Poison Hemlock. https://www.nwcb.wa.gov/weeds/poison-hemlock
Management of spotted water hemlock developed by the Texas A&M Agrilife Extension. https://aquaplant.tamu.edu/management-options/how-to-control-spotted-water-hemlock/
University of Florida Center for Aquatic and Invasive Plants – Spotted water hemlock. https://plants.ifas.ufl.edu/plant-directory/cicuta-maculata/
The Ohio State University’s Ohio Perennial and Biennial Weed Guide – Poison hemlock. https://www.oardc.ohio-state.edu/weedguide/single_weed.php?id=114
North Carolina Cooperative Extension formed between N.C. A&T State University and NC State University – Cicuta maculata. https://plants.ces.ncsu.edu/plants/cicuta-maculata/
USDA’s Field Guide for Managing Poison Hemlock in the Southwest. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5410121.pdf
Environment and Natural Resources Trust Fund contributed by Minnesota Environment and Natural Resources Trust – Cicuta maculata. https://www.minnesotawildflowers.info/flower/water-hemlock
Identification description for water hemlock. https://www.illinoiswildflowers.info/wetland/plants/water_hemlock.htm
Cornell University Turfgrass and Landscape Weed ID- Weeds and Human Health: The Severe Seven http://turfweeds.cals.cornell.edu/weeds-and-human-health
Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2021.
Golden Gate National Parks Conservancy – Cow Parsnip https://www.parksconservancy.org/conservation/cow-parsnip
New York State Department of Environmental Conservation- Cow Parsnip https://www.dec.ny.gov/animals/105388.html
New York State Department of Transportation- Cow Parsnip (Heracleum maximum) https://www.dot.ny.gov/dangerous-plants/cow-parsnip
North Carolina State Extension – Heracleum maximum https://plants.ces.ncsu.edu/plants/heracleum-maximum/
Minnesota Wildflowers: Heracleum maximum (Common Cow Parsnip) https://www.minnesotawildflowers.info/flower/common-cow-parsnip
West Virginia Extension Service- Wild Parsnips https://extension.wvu.edu/lawn-gardening-pests/weeds/wild-parsnips
Minnesota Department of Agriculture – Wild carrot. https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/wildcarrot#:~:text=You%20must%20use%20extra%20caution,then%20is%20exposed%20to%20sunlight.
Washington State’s Noxious Weed Control Board – Wild carrot. https://www.nwcb.wa.gov/weeds/wild-carrot
A Pacific Northwest Extension Publication contributed by Oregon State University, University of Idaho, and Washington State University – Wild carrot. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/pnw447.pdf
Ohio State University’s Ohio Perennial and Biennial Weed Guide – Wild carrot. https://www.oardc.ohio-state.edu/weedguide/single_weed.php?id=21
Weed Control in Natural Areas in the Western United States by University of California’s Weed Research and Information Center – Weed Report on Wild Carrot. https://wric.ucdavis.edu/information/crop/natural%20areas/wr_D/Daucus.pdf