BYGL Weekly News for April 12, 2021 – Planting Dates and Poison Hemlock

The following articles were compiled during the last 7 days by members of the Buckeye Environmental Horticulture Team to benefit those who are managing a commercial nursery, garden center, or landscape business or someone who just wants to keep their yard looking good all summer.  Access the BYGL website for additional information on other seasonal topics at:

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Don’t Let This Warm Weather FOOL YOU!

Authors Carri Jagger

Published on April 8, 2021

This has been a long, cold winter.  Thank goodness spring is in sight, Saturday, March 20 marked the first day of spring.  With this being said, it’s time to start thinking about planning flower and vegetable gardens.  If starting a new garden, soil testing the site where the garden will go is a good idea.  If it is an existing garden and the soil has never been tested, now would be a good time to think about testing it.  Your local OSU Extension office can help with soil testing.

Another gardening task to be thinking about is seed starting.  Growing plants from seed is a lot of fun and now is the time to be doing this.  Below is a chart from The Old Farmer’s Almanac that will help determine when to start seeds indoors, transplant seedlings outdoors, and when to start seeds outdoors.


Crop      Start Seeds Indoors      Transplant Seedlings           Start Seeds Outdoors
Beans                 May 17 – June 7
Beets                 April 26 – May 17
Broccoli      March 29 – April 12          April 19 – May 10  
Brussel Sprouts      March 29 – April 12          April 12 – May 3  
Cabbage      March 15 – 29          April 12 – 26  
Cantaloupe      April 12 – 19          May 24 – June 14              May 24 – June 1
Carrots                  April 5 – 19
Cauliflower      March 29 – April 12          April 12 – 26  
Collards      March 29 – April 12         April 12 – May 3  
Corn                  May 10 – 24
Cucumbers       April 12 – 19          May 24 – June 14              May 24 – June 1
Eggplants       Feb. 28 – March 15          May 24 – June 14  
Kale       March 29 – April 12          April 12 – May 3  
Lettuce       March 29 – April 12          April 26 – May 24  
Onions                   April 12 – May 3
Peas                  March 29 – April 19
Peppers        Feb. 28 – March 15           May 24 – June 14  
Potatoes                  May 3 – 24
Pumpkins         April 12 – 26            May 24 – June 14              May 24 – June 1
Radishes                  March 15 – April 5
Spinach                  March 29 – April 19
Sweet Potatoes          April 12 – 19            May 24 – June 14              May 24 – June 1
Squash          April 12 – 26            May 24 – June 14              May 24 – June 1
Swiss Chard          March 29 – April 12            April 19 – 26              May 24 – June 1
Tomatoes          March 15 – 29            May 17 – June 7  
Turnips                  April 12 – May 3
Watermelons          April 12 – 19            May 24 – June 14              May 24 – June 1


Taking a look at the chart above notice that some of the vegetable crops we like to plant in the garden can handle cooler temperatures and those are recognized as cool season crops.  Some of those include:

  • Cole crops (or brassicas) which are an amazingly large and varied family, whose edible portions span from   leaves to flowers to roots. This includes broccoli, cauliflower, kale, cabbage, radishes, turnips, kohlrabi, arugula, Asian greens, and mustard greens (Brussels sprouts, a brassica, are planted in the cool season but take many months to mature).
  • Peas (both edible-podded and shelling) are another familiar cool-season crop.
  • Lettuce is yet another group that has a huge number of varieties.
  • Spinach is also included the cool season assembly.


Now that we have talked about testing the garden soil, starting seeds and cool season crops. We need to think about the frost free date in your county.  According to the Old Farmer’s Almanac the frost free date is May 10th for Morrow County where I live.  However, I caution folks of following this date.  I like to use Memorial day as a frost free date in Central Ohio because the last several years have presented us with a frost and or freeze near Mother’s Day.  Mother’s Day has always been a good rule of thumb for safely planting vegetables and flowers outside, but I caution folks to watch the weather and think about planting around Memorial Day, all threat of frost should be gone by then.


I know the temptation is always there to start earlier especially if we are experiencing 65 and 70 degree days.  That is why it is important to follow the planting guide above.  If you have raised beds or micro climates under cold frames the soil might warm up quicker allowing you to start a little earlier.  Ideally cool season crops would like soil temperatures to be above 40 degrees and warm season crops would like soil temperatures to be at or above 55 degrees.


If you do jump the gun and plant before memorial day your crops can potentially be protected from frost with old blankets, cardboard and row covers.


Whether you are starting transplants from seed or purchasing them, watch the weather forecast to ensure your little plant babies are protected.  Have a fun and successful spring.


If you have questions call your local OSU Extension Office.


Poison Hemlock and Wild Parsnip: Control Them Now!

Authors Joe Boggs

Published on April 6, 2021

Poison hemlock (Conium maculatum L.) and wild parsnip (Pastinaca sativa L.) are combined in this report because these invasive non-native weeds are increasingly found growing together in Ohio.  However, the defense chemicals of these weeds are very different and have vastly different modes of action.  This is important to understand relative to management options as well as medical treatments for exposure to these highly dangerous weeds.

Life as a Biennial

Poison hemlock and wild parsnip belong to the so-called carrot family, Apiaceae (= Umbelliferae).  They superficially share floral characteristics with other members of the carrot family such as Queen Anne’s lace (Daucus carota); however, this non-native biennial blooms much later in the season.

Poison hemlock has a biennial life cycle. The first year is spent in the “vegetative stage” as a low-growing basal rosette; the stage that is currently very apparent.  Plants “bolt” during the second year “reproductive stage” to produce erect multi-branched stems topped with umbrella-like flowers.

Wild parsnip is also reported to have a biennial life cycle.  However, it may occasionally behave as a monocarpic perennial spending more than a year in the vegetative stage before flowering once and then dying.

Mature poison hemlock plants can measure 6 – 10 ft. tall.  Mature wild parsnip plants are shorter in stature but still impressive at up to 4 – 5 ft. tall.  Both are prolific seed producers with seeds remaining viable for 4 – 6 years for poison hemlock and around 4 years for wild parsnip.


Poison Hemlock

Poison hemlock was imported into the U.S. as an ornamental in the late 1800s from Europe, West Asia, and North Africa.  Rogue plants remained relatively rare until around 30 years ago.  Since that time, poison hemlock has elevated its profile from an uncommon oddity to a common threat.


This non-native is one of the deadliest plants found in North America.  It is the plant used to kill Socrates as well as the Greek statemen Theramenes and Phocion. Poison hemlock plants contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals.


All parts of the plant are poisonous: leaves, stems, seeds, and roots.  However, the toxins must be ingested or enter through the eyes or nasal passages to induce poisoning.  The toxins do not cause skin rashes or blistering.  Regardless, this plant should not be handled because sap on the skin can be rubbed into the eyes or accidentally ingested while handling food.  Immediate emergency medical attention should be sought if an accidental poisoning from this plant is suspected.


All stages of the poison hemlock plant have bluish-green leaves that are 3-4 times pinnately compound.  The deeply cut parsley-like leaflets have sharp points.  Flowering plants have hairless, light-green to bluish-green stems that are covered with obvious purplish blotches; Maculatum means ‘spotted’.  Clusters of tiny white flowers are borne on structures called umbels that look like upside-down umbrellas.


Wild Parsnip

Wild parsnip sap contains psoralen which is a naturally occurring phytochemical grouped in a family of organic compounds known as linear furanocoumarins.  Psoralen acts as a photosensitizing compound by inhibiting DNA synthesis in epidermal cells which kills these light-shielding cells responsible for protecting us from long-wave ultraviolet radiation (LWUVR) bombarding us in sunlight.


Severe blistering occurs when the affected skin is exposed to LWUVR. The synergistic effect is called phytophotodermatitis (a.k.a. Berloque dermatitis) and the burn-like symptoms, as well as skin discoloration, may last for several months.


Connecting skin blistering to exposure to wild parsnip sap can be a challenge.  It takes around 24 hours for symptoms to first appear after exposure to LWURV and severe blistering typically doesn’t peak until 48 -72 hours.  The time required for symptoms to appear after exposure to the sap means the effect may be disconnected from the cause.


Psoralens are also found in several other members of the Apiaceae family including the notorious giant hogweed (Heracleum mantegazzianum) which has captured national attention in the past.  However, giant hogweed has only been confirmed in Ohio growing in the extreme northeast part of the state primarily in and around Ashtabula County.  Wild parsnip is found throughout the state and is equally damaging.  Of course, giant hogweed has a more threatening sounding common name while wild parsnip sounds like a vegetable gone wild; which it actually is!


Parsnips have been cultivated as a root crop in Europe for centuries, perhaps millennia. The “L.” in the scientific name Pastinaca sativa L. means Linnaeus first described the species.  Both the cultivated and wild types share the same scientific name; however, it is clear that there are significant differences in toxic biochemical properties between the two types.


It is theorized that the wild parsnip plants in Ohio represent “escapes” from cultivated types brought to North American from Europe and a “reversion” back to a wild type.  The wild genes were always there but remained suppressed until revealed through natural selection.


Wild parsnip rosettes have celery-like leaves confined to growing from a short stem near the ground.  While in this stage, the plant produces a long, thick taproot.


Flower stalks that eventually arise from rosettes have leaves that are alternate, pinnately compound, branched, and have saw-toothed edges.  Each leaf has 5 -15 ovate to oblong leaflets with variable toothed edges and deep lobes.  The mature flowering plants have a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with hundreds of clusters of the yellow umbellate flowers.



Unfortunately, poison hemlock and wild parsnip are becoming more common throughout Ohio and many other states in the upper Midwest as well as states in the eastern U.S.  Worse, owing to the lack of awareness (e.g., identification) or poor management practices, or both, these dangerous non-native weeds are increasingly being found growing in close proximity to people which increases their risks to human health.


Additionally, it is not unusual to find poison hemlock and wild parsnip growing together which can create misinterpretations of exposure symptomology.  This may account for some online resources incorrectly attributing skin blistering to contact with poison hemlock.


Mechanical management of poison hemlock can be used if it is certain that no wild parsnip is lurking within the poison hemlock.  Still, personal protection equipment is strongly recommended particularly eye protection, gloves, and clothing to cover arms and legs to prevent sap from entering through the eyes or skin wounds.  Hand-pulling and tilling are effective options if the area is immediately overseeded with grasses or other competitive plants to help suppress poison hemlock re-establishment from seeds germinating this fall.


Mowing can also be used; however, given that a sizable percentage of the current low-growing rosettes may escape the blade, it’s best to delay mowing to target bolting plants.  String trimmers are also effective but present an even greater risk of flinging sap compared to mowing.  All mechanical control options should be applied before plants begin to flower!  Waiting until after plants flower, or worse after seeds are produced, can increase an infestation by removing canopy competition.


Given the extreme risk of phytophotodermatitis from wild parsnip sap, mechanical control is problematic.  Hand-pulling is a high-risk endeavor and not recommended.  Likewise, tilling could release a huge amount of harmful sap.  There have been reports of sap spattered by mowers and string trimmers producing phytophotodermatitis on exposed arms and legs of equipment operators.


The safest approach to controlling this invasive weed as well as poison hemlock is to use herbicides.  Of course, as always, read and follow label directions paying close attention to application sites, recommended rates, warnings against making applications close to desired plants (e.g. trees) or near water, and whether surfactants are recommended to enhance herbicide efficacy.


Both poison hemlock and wild parsnip are susceptible to several selective and non-selective postemergent herbicides.  However, keep in mind that non-selective herbicides such as glyphosate (e.g. Roundup) can also illuminate plants that compete with these weeds.  Herbicidal openings produced by non-selective herbicides provide perfect opportunities for more wild parsnip and poison hemlock to spring forth from previously deposited seed.  Thus, it’s important to have a plan for establishing competitive plants such as over-seeding with grasses.


Selective post-emergent herbicides will preserve competitive plants.  Herbicides effective against wild parsnip and poison hemlock include clopyralid (e.g. Transline), triclopyr (e.g. Pathfinder II), metsulfuron (e.g. Escort XP), and combination products such as 2,4-D + triclopyr (e.g. Crossbow), or 2,4-D + mecoprop + dichlorprop (e.g. Triamine).  Applications made now and before plants start to flower can significantly reduce infestations of both wild parsnip and poison hemlock.






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