Vegetable plants with signs of herbicide damage are not safe for consumption

"Fielding Questions" columnist Don Kinzler also answers questions about caring for rhubarb and what may be damaging a reader's marigolds.

Herbicide injury on cucumber and potato plants.jpg
A reader asks Kinzler what may be causing damage to her cucumber, potato, and tomato plants. Special to The Forum

Q: Here are pictures of my cucumber, potato, and tomato plants. We planted the garden about May 22 and the photos were taken just yesterday. The plants have barely grown. We put composted manure on the garden beds last fall. Could the plant damage be from that? Or is everything just heat-stressed this year? – Julie V.

A: The plants are showing symptoms compatible with herbicide damage, with the curling, cupping leaves and distorted growth. When garden plants show these symptoms, one of the first suspected sources of damage is compost or manure that has been added to the soil. A second possible source of damage is herbicide spray that might have been applied to surrounding lawns or fields and has affected the garden vegetables.

You mentioned that composted manure was put on the gardens last fall. Herbicides that are used on pastures, hay and other feed can pass through cattle’s digestive system and persist in the manure, even after composting. Some herbicides have long residuals and can persist for years in manure or compost. When materials tainted with residual herbicide are added to garden or flowerbed soil, plants can show symptoms of damage.

Although all plants can potentially be affected, among the most sensitive are tomatoes, potatoes, beans and cucumbers. Unfortunately, once plants have absorbed these herbicides, there is little that can be done to reverse the damage. Depending on the herbicide and its potency, some plants will outgrow the damage, but others won't.

Vegetable plants that have been affected by herbicides are unfortunately not considered safe for consumption, even if they seem to recover, because there isn't a practical test to determine if levels of herbicide still remain within the plant’s tissue or its fruit or other structures.


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Q: Our rhubarb plants have very thin stalks. The first picking of the season is pretty normal in size, not overly thick, but after that the stalks are very thin. Is there something I can do to get the nice thick stalks again? – Michelle O.

A: Rhubarb responds well to fertilizer, and is actually a “heavy feeder,” as described in many gardening books. Rhubarb also enjoys cool weather and cool soil temperatures, which is why the earlier stalks are sometimes the nicest. As summer progresses, and soil becomes warmer, production and stalk size often diminish, especially on the reddest varieties, which tend to have thinner stalks than the large green types.

Rhubarb can deceive us as being easy-to-grow, because it’s so commonplace in the Upper Midwest, but providing it with some amenities can increase production and success. Add organic material to the soil around the plants, such as peat moss or compost that is known to be free of herbicide residue. A three-inch thick layer of mulch will help keep soil cool and moist.

To satisfy rhubarb’s hunger for fertilizer, apply a granular fertilizer such as all-purpose 10-10-10 in spring, following label quantities. Incorporate into the soil and water well to activate. During the growing season, water-soluble fertilizer like Miracle Gro can be applied every two weeks to supplement the granular application.

Discontinue fertilizing by July 4, which is also the time main harvesting of rhubarb stalks should stop for the season. Allowing stalks and leaves to remain after early July gives the rhubarb plant a chance to replenish its energy for the following spring. A light harvest can be done in early fall, but harvesting throughout summer runs the risk of diminishing a rhubarb plant’s strength, especially if the upcoming winter is severe.

Q: Something’s pulling the flower heads from my marigolds, but I don’t see any critters attacking them. Could it be squirrels? – Ron M.

A: Although squirrels are always suspect because they’re tricky and quick, birds are frequent predators on marigold flowers. They often go unnoticed because some of their best work is in the early dawn hours.


Certain birds find marigold flowers attractive because as the blossoms fade, a large, capsule filled with dry seed forms at the flower’s base. Faded marigold flowers pop off the stems very easily, making easy picking for some bird species.

Preventing birds from attacking marigolds isn’t easy. Bird repellent products are sold, but results are certainly not consistent or predictable.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at or call 701-241-5707. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler
Don Kinzler, Growing Together and Fielding Questions columnist. The Forum
Contributed / Special to The Forum

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at
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