Any gardener who has watched their tomatoes shrivel, wither and die knows the heartbreak of losing plants.
But figuring out just what sickened those tomato plants, zucchini squash or bush beans, sometimes is a mystery worthy of Sherlock Holmes.
There are harmful insects such as aphids and squash bugs that seem to suck the life out of their hosts. There are diseases spread by bugs, such as the curly top virus that beet leafhoppers carry from plant to plant. There are a handful of soil-borne viruses.
There also is a little-known and somewhat unusual problem that can devastate tomatoes and other garden plants: herbicides that are meant to kill weeds in grasses but get onto crops inadvertently.
Cheryl Kent, a Bernalillo County Extension horticulture agent, sees at least 20 cases of accidental herbicide exposure a year.
“That’s very common,” she said. “We don’t really know the extent of the problem. A lot probably goes undiagnosed.”
Broadleaf herbicides, such as 2,4-D and dicamba, are used on pastures, roadsides, golf courses and other turf grass to eliminate weeds, including toxic plants that can sicken animals that graze on them.
There are a few ways the herbicides can end up on home gardens, including in residue in compost or mulch and when herbicides volatilize and drift on the wind.
North Carolina State University describes it this way in a paper: “Many farmers and home gardeners have reported damage to vegetable and flower crops after applying horse or livestock manure, compost, hay and grass clippings to the soil. … These symptoms can be caused by other factors, including diseases, insects and herbicide drift. Another possibility for the source of these crop injuries should also be considered: the presence of herbicides in the manure, compost, hay or grass clippings applied to the soil.”
When horses, for example, eat hay that has been treated with herbicide, the chemical passes through their digestive tract and is excreted in manure, remaining active even after composting.
Horse manure sometimes is used by backyard composters. Herbicides also can remain active in hay or straw that home gardeners use as mulch in their vegetable beds.
The broadleaf killers, known as auxinic herbicides, “also are very prone to volatilization,” Kent said.
In a typical scenario, the herbicide is applied and then a day or so later the weather warms up to 80 degrees or more, causing the herbicide to vaporize and float around, sometimes drifting a long distance in an erratic pattern.
“It’s not always easy to determine how you got exposed,” Kent said.
Rick Hobson, garden radio show host and owner of Jericho Nursery, is skeptical about just how much herbicides are damaging home gardens.
He acknowledges it’s a problem, but says herbicide damage is rare.
“It’s just so obscure,” he said, adding that other problems are much more likely to be the cause of problems in backyard vegetable gardens.
In his own tomato garden, for example, the fungus summer blight took out most of the plants for the last two years.
Most problems in this area, Hobson added, are due to lack of water.
And most cases of herbicide damage are self-inflicted by gardeners who apply weed killers incorrectly.
“Some think a little is good, so more is better,” Hobson said.
The symptoms of herbicide exposure are very consistent, Kent said, and include badly deformed leaves, cupping of the entire leaf, fringed edges on leaves, spindly and small leaves and plants, and strange-looking flowers and fruit.
Bottom line: herbicides are “doing some weird stuff to those plants,” she said.
Plants with a color change such as yellowed or mottled leaves, are more likely to be suffering from a different problem.
Tomatoes, grapes, peppers, sycamore trees and beans are particularly sensitive to herbicides. Other plants are not as susceptible and might not show signs of exposure.
Vegetable gardeners who are motivated in part by a desire to grow organic produce have another concern. Plants that have been affected by herbicides not only suffer damage, they also might not be safe to eat.
There are a few things gardeners can do to prevent problems.
When buying compost or mulch, ask questions. Find out about the origin of manure and whether the animals ate grass hay and what any grass was treated with. Because alfalfa is a broadleaf plant, it’s safe to assume it hasn’t been sprayed and, therefore, is safe to use.
“I prefer as much as possible and recommend to people as much as possible to first use plant-based compost, stuff that’s not made with manure,” Kent said. “That eliminates in part that herbicide issue.”
Of course, if you’ve sprayed herbicide or a “weed and feed” product on your lawn, don’t add the grass clippings to your compost pile.
“Have some respect for these broadleaf herbicides,” she said.
Gardeners who are unsure about whether compost or mulch is safe or who have had damage from herbicides can test their soil, compost or mulch with a bioassay.
To do that, fill three to six small pots with a one-to-one mix of the suspect soil or compost and a commercial potting mix.
To test mulch, fill pots with commercial potting mix and top with a thick layer of the mulch. As a control test, fill a few pots with only the uncovered commercial potting mix.
Plant beans, peas or tomatoes and let them grow for two to three weeks, making sure that drainage water from the pots does not mix.
If plants grow normally, it is reasonable to assume the material is fine, according to North Carolina State University.