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OSU Extension

College of Food, Agricultural, and Environmental Sciences

April 7, 2020 - 8:00am --

Every spring I notice pasture and hayfields that have been invaded by poison hemlock.  All parts of this plant are poisonous, and those toxic properties are not reduced when the plant gets incorporated into a hay bale or silage. Mark Loux, OSU Extension weed specialist, provides the following information and recommendations regarding control of poison hemlock.

“Poison hemlock remains one of the more persistent and prevalent poisonous weeds that we deal with in Ohio.  It's most typically a biennial plant (sometimes perennial), emerging from seed in year one and developing into a low-growing rosette by late fall.  The rosette overwinters and then resumes growth in the spring of year two.  Stem elongation initiates sooner in spring than many other biennials, and this is followed by continued growth and development into the often very tall plant with substantial overall size.  Flowering and seed production occur in summer. 

Failure to control poison hemlock occurs partly because, while it often grows in edges and fencerows around crop fields, no one really pays much attention to it until it does reach this large size when it’s less susceptible to herbicides.  And everyone is busy getting crops planted in spring anyway so control of hemlock gets low priority.  Stages in the poison hemlock life cycle when it is most susceptible to control with herbicides are:  1) fall, when in the low-growing rosette stage; and 2) early spring before stem elongation occurs.  It’s most easily controlled in fall, but several products can work well in spring.  Herbicide effectiveness ratings for poison hemlock can be found in Table 21 of the current Weed Control Guide for Ohio, Indiana, and Illinois.  Herbicides rated 8 or 9 on poison hemlock include the following:  9 – Crossbow, Remedy Ultra; 8 – Cimarron Max, Curtail, dicamba, glyphosate.  Mixing glyphosate and dicamba can improve control compared with either applied alone. 

Evaluate Alfalfa Stand Health

A good time to evaluate alfalfa stands for winter injury and overall stand health related to yield potential is after plants have greened up and have put on 2 to 4 inches of growth. 

One of the primary concerns is the possibility of heaving damage.  Tap rooted crops such as alfalfa and red clover are particularly susceptible to heaving damage.  Conditions that increase the likelihood of heaving are wet, saturated clay soils with high shrink/swell potential, exposed to rapid freeze/thaw cycles.  During these conditions plants can be physically lifted (heaved) out of the soil exposing the crown of the plant to possible low temperature damage and/or physical injury from harvest operations.  In severe cases the plant can be heaved several inches or more out of the soil, breaking the taproot and killing the plant.

Forage stand health evaluation includes stem counts and digging plant roots.  Select random sites throughout the field and evaluate the plants in a one-foot square area.  Check at least one site for every 5-10 acres.  Increasing the number of random samples provides a more accurate assessment.  Begin your stand health evaluation by counting the number of stems per crown.  Do this evaluation in at least 4-5 random locations for every 20-25 acres.  Stem density counts provide an indication of the yield potential of the stand.  The following table is taken from University of Wisconsin Extension publication A 3620; “Alfalfa Stand Assessment: Is this stand good enough to keep?” (


Stem number/square foot

Expected result or action

Over 55

Stem density not limiting yield


Some yield reduction expected

Less than 39

Consider stand replacement


While you are counting stems, take note of where growth is taking place.  Healthy plants have symmetrical, even growth on both sides of the crown.  Damaged plants often have more stems on one side of the plant than the other.

Evaluation of crown and root tissue provides the best indication of how the plant will hold up to stresses in the coming growing season.   Dig up five to six plants in each of 4 to 5 random locations per 20-25 acres.  Split the plant open.  A healthy root will have a creamy white color and no to very little discoloration in the crown and taproot. 

Discolored crowns and roots indicate a plant health problem.  They are a darker white, tending towards a tan color.  There may be obvious areas of root rot and crown rot that are dark brown to black in color.  There may be streaks of brown running down the root.  Generally, these plants green up in the spring of the year and appear productive, but because of their compromised root system, they may not survive the entire production year, especially if we have a hot, dry year. 

In general, if more than 30% of the split roots have brown streaks running down the root and/or there are black areas of root/crown rot that cover greater than 30 to 50% of the root’s diameter, then yield potential is significantly reduced.  In this situation consider rotating the stand to another crop.


Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.


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