How to Protect Your Garden From Persistent Herbicides

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As you’d guess, weed kill products cause major problems when they come in contact with plants you want to keep alive. Sometimes, that can happen through herbicide drift, or when an herbicide such as glyphosate drifts in the wind onto non-target plants.

Since a lot of vegetable plants are extremely sensitive to herbicides, it only takes a tiny amount to severely injure them. Even when the spraying is done many yards away (like at your neighbor’s place), it can still damage your crop if wind is moving.

The second, increasingly common way herbicides can wreck folks’ gardens is through the use of compost and soil amendments contaminated with persistent herbicides. These nasties belong to a different group of chemicals than your typical household weed killers, and the consequences can be much worse when they end up somewhere they aren’t supposed to be.

Persistent herbicides may be lurking in store-bought soil or compost, grazing livestock manure, or mulch materials like straw and hay. In other words, many of the things organic gardeners might look for to improve our soil or put in our home compost bins. They’re called “persistent” because they take a long time to break down compared to other herbicides. And y’all, that’s bad news if they find their way into your garden.

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What are Persistent Herbicides?

Persistent herbicides belong to a group of chemicals designed to target broadleaf weeds without affecting surrounding grasses. They are often used in livestock grazing fields, grain production fields, and straw and hay fields, and they work by disrupting the target plants’ growth at the cellular level (makes you wonder what they do to our cells, am I right?).

While there are many plant-specific herbicides in use in commercial operations, the “persistent” kind known to poison garden soil are those in the pyridine carboxylic acid family. Some examples include aminopyralid, clopyralid, and picloram.

Knowing those chemicals by name isn’t super important when it comes to avoiding them, and I’ll explain why shortly.

Why are Persistent Herbicides So Scary?

Despite glyphosate’s dark reputation for being a human carcinogen and a scourge to the environment, it doesn’t pose a long-term risk in gardens because it degrades so rapidly. Once it’s sprayed, it becomes inactive within days. This means there’s not much chance of carryover into compost and other amendments.

Persistent herbicides are a totally different story. These long-lived chemicals can withstand hot composting, can pass through a grazing animal’s digestive tract unaffected, and can remain destructive for years when they end up in your garden soil.

Once contaminated, your soil won’t be safe for planting again until the herbicide has broken down. That might take a year, it might take three years, and sometimes, it can take even longer.

Vulnerable Plants and Symptoms of Exposure

Among garden plants, those that are severely affected by persistent herbicides include:

• Flowers in the aster, daisy, and sunflower family
• Legumes such as all varieties of peas and beans
• All nightshades, ex. tomatoes, peppers, eggplant, potatoes
• Cucurbits, ex. squash, melons, pumpkins, cucumbers
• Beets, leafy greens, and lettuce

Sensitive plants will show major symptoms when they’re exposed to herbicides. The most telltale sign of herbicide injury is cupping, curling, and fern-like, badly deformed new growth. The plants may be stunted and probably won’t set fruit. Direct sown seeds will sometimes fail to grow at all, or won’t grow beyond the first seedling leaves. Look at some photo examples of affected tomato plants here.

While other issues like pests and nutrient deficiencies can cause deformed leaves on your plants, broadleaf herbicide injury has a striking appearance that’s hard to confuse with anything else. The most likely problem that might be mistaken for herbicide damage are viral diseases such as Beet Curly Top Virus.

Is it viral, or herbicide damage? Viruses tend to infect only a few plants at once, while others stay healthy (much like viruses in humans). If all the plants in a specific area or bed have symptoms at the same time, you can suspect herbicide residue is the culprit.   

How much herbicide residue does it take to cause damage? According to the US Composting Council, sensitive plants can be damaged by persistent herbicides at concentrations of one part per billion, which is “equivalent to ½ teaspoon of product in an Olympic-sized swimming pool.” Yikes.

How to Avoid Introducing Herbicides to Your Garden Soil

The first step is knowing the potential sources of contamination. They include:

• Mulch materials like straw and hay
• Manure from grazing animals such as cows or horses
• Compost that contains manure or plant material (bagged, bulk, or municipal)
• Garden soil products containing compost or manure (bagged or bulk)

When it comes to all those things on the list, never assume anything is safe, even if it’s a well-known brand name. Most of those big box companies selling bagged soil and compost at home improvement stores aren’t checking out their sources, because they know there won’t be consequences.

Now, I know this sounds like fearmongering, but persistent herbicide damage is happening way more often than people realize. I have no doubt it’s to blame for many mysteriously flopped gardens in which the gardener wasn’t familiar with the signs. If you don’t know what to look for, you’ll be left wondering what went wrong.

What if the bag of soil/compost says “organic?” Does that mean it’s free of herbicides?

That’s a big, capitalized NO.

Bagged soil products will often slap the word “organic” on the packaging, which is misleading. It’s not the same thing as the “organic” label you’d see on organic food items from the grocery store. While organic food is certified as such through guidelines and inspections, regulations are simply nonexistent for soil products. “Organic” just means the soil mix consists of only naturally occurring ingredients, which may or may not have been contaminated with herbicides.

I’m pretty sure there’s no such thing as a bag of “certified organic” soil or compost (and if there is, I’m sure it’s well outside of my budget!).

What about getting your straw and manure from local farmers?

Sadly, you can’t assume anything you get from friends and neighbors is safe, either. I personally know several people in our small, rural community whose gardens were wiped out by contaminated cow manure within the last few years. In each of those situations, the well-intentioned local farmers who supplied that manure didn’t understand how it happened. They hadn’t sprayed their fields.

Where did the herbicide come from, then? Sometimes, farmers stall-feed their animals with hay brought in from elsewhere over the winter. If they unknowingly bought hay from a sprayed field, any manure collected during that time will contain herbicide residue. Also, it’s likely some farmers simply don’t know that certain herbicides can carry over in animal manure or remain active on aged straw.  

So, how can you make sure soil amendments are clean?

Remember how I said it’s not too important to memorize the names of those herbicides? That’s because it’s usually impossible to trace the supply chains of every ingredient used in soil amendments you buy.

Purchased compost will include waste materials from dozens of sources, and most companies who sell it don’t test their product for herbicide residue. For that reason, avoiding herbicide contamination by asking the seller or researching isn’t usually a worthwhile strategy (unless you can verify the company does test their product for herbicides before selling it).

In most cases, you’ll need to take a more hands-on approach to protect your garden from persistent herbicides. Here are a few ways to do that:

Consider Safer Mulch Options

Anything natural and compostable can be used for mulch, including wood chips, grass clippings, shredded fall leaves, and pine needles. And there’s a good chance you can get these things for free! For example, since tree companies usually shred up downed trees and dispose of the chips after a tree removal job, they’re often willing to dump them in your yard without charge.

Even if you need to gather those natural mulch options from somewhere other than your own property, the risk of herbicide contamination is low, since persistent herbicides are mainly just used in commercial fields.

Make Your Own Compost

With the issue of persistent herbicides on the rise, one of the best things you can do for your garden is to start your own compost pile. This way, you have control over what’s going in it. But if you have a large garden, it’s hard to make all the compost you need at home.

I’m not a compost guru or anything, but I’m committed, and it still takes me at least three or four months to get a few wheelbarrows’ worth of compost. I need to supplement that in order to put a good layer on all my garden beds in the fall. If that applies to you like it does to many of us, keep reading.

Use Lower-Risk Manure

Try to source manure from animals that aren’t pasture-raised, such as rabbits, chickens, or guinea pigs. If the animal in question eats a plant-based diet, the manure is a good addition to your soil.

Source Manure from Certified Organic Farms

By law, certified organic farms aren’t supposed to use herbicides or herbicide-treated products in the raising of their livestock. Getting manure from such an operation isn’t always a practical option, though, since a lot of us don’t have one close by. You can also buy certified organic straw for mulching, but it can be hard to get and ridiculously pricey.

Do a Bioassay Test on Your Amendments

Since it’s hard to get straight answers when asking about the ingredients of an amendment you want to use, testing it yourself is the best way to ensure you’re not introducing herbicides to your soil. You don’t need any special equipment to do it, but you do need to plan ahead, since it’s going to take three to four weeks. See below for detailed instructions.

Bioassay Testing for Herbicide Residue

To conduct a bioassay test, you’ll sow some herbicide-sensitive plant seeds in a sample of the compost or other amendment you’d like to check for herbicide residue. If the sample is contaminated, the seedlings will show symptoms of exposure (or may not sprout at all). If they grow normally, you’ll know it’s safe for your garden.

While you can use seeds from any sensitive plant species for the test, it’s common practice to go with legumes like beans, since they sprout and grow quickly.

Supplies Needed:

Preparing Your Test:

Step 1: Label your pots. Three will be labeled “control” and three will be labeled “test.” If you’re testing more than one amendment, make sure you have at least three pots for each test.

Step 2: Fill the control pots with the soilless seed-starting mix. For the test pots, mix a blend of 1/3 soilless mix and 2/3 compost or soil to be tested. If your test compost is dense or has large pieces of “stuff” in it, I suggest lowering the ratio to 1/2, since seeds might struggle to germinate in rough soil.

If you’re testing straw or another mulch material, you’ll fill the test pots with the soilless mix as well (see note* at the bottom of this section). Be sure to keep the control pots and test pots in separate trays so you don’t cross-contaminate when watering.

Step 3: Plant the seeds and water them in well. I like to plant two per pot in case I have a few duds. While you wait for them to grow, give them the same good care you’d give any other seedlings- appropriate light, water, and airflow. You don’t want any non-herbicide-related health issues to affect your test.

Step 4: Watch and wait. As the seedlings develop, keep an eye out for anything weird going on in the test pots, such as deformed leaves and stunting.

Note that the cotyledons (the very first leaves that sprout) might look a little frumpy sometimes, particularly with beans. This can happen when they get stuck in the seed coat, and it’s not related to herbicide residue. Let the plants grow until they have at least three sets of true leaves. At this point, if they all look healthy and are growing at the same rate as the control seedlings, you’ll know your amendment is free of herbicides.

Anything that does show signs of herbicide contamination shouldn’t be used in your garden. If it’s a bagged product you bought from a store, take it back. If you got a bulk delivery, you may be able to convince the seller to remove and dispose of it for you.  

*Note on bioassay testing straw or hay: I’ve done a lot of research on this topic, and it seems like there are two ways to do it.

Method one, just lay the straw to be tested on top of your soilless medium after the seeds are planted, and water in as usual. Method two, soak the straw in a bucket overnight, strain the water, and use it to water in your test seeds. I prefer the second method, because steeping the straw will produce a higher concentration of any herbicides that may be present.

What if your test seeds didn’t sprout?

Compost-heavy soil can sometimes trap air pockets, which might cause seeds to dry up and not sprout. But a total lack of germination in bean seeds is a reason for concern, since they’re pretty vigorous and have good germination rates.

If the seeds fail to sprout in both the test pots and the control pots, your seeds were probably just old. But if it’s ONLY in the test pots, it could be a sign of herbicide contamination. Try lowering the amount of compost, and wait to see if they germinate.

If the seeds sprout on the second try, carry on and watch for symptoms of herbicide exposure. If not, it’s safe to assume something isn’t right with whatever your testing. In other words, when in doubt, throw it out.

What if Your Garden Has Already Been Affected by Herbicides?

You’re not alone, if this is the case. While there really are no quick fixes for herbicide-contaminated soil, there are a few steps you can take to help the herbicide degrade as quickly as possible.

Support Your Soil’s Microbiome

To increase the soil life that will speed up the breakdown of herbicide residue, keep the soil well-watered and fed. Add amendments like herbicide-free compost and manure, add organic fertilizers, and mulch deeply to retain moisture.

Planting cover crops might help, too; herbicide-sensitive crops may accumulate some of the herbicide and can then be disposed of (don’t compost them).

Use Biochar or Charcoal

There’s evidence that adding biochar or activated carbon (charcoal) can help restore soil health more quickly after herbicide contamination.

Be aware that both biochar and charcoal can drastically raise the pH of soil when applied heavily. If your soil is already bordering on being too alkaline (high pH), you might have trouble getting the pH back under control after treatment. Many types of plants don’t like overly alkaline soil.

Test Your Soil Regularly

To know when your soil is safe to plant in again, take regular bioassay tests of the affected area. Test at least a couple times a year until they come back negative. Once you have a clean test, it means the herbicide is gone, and you can go back to gardening as usual in that spot.

Stay Positive and Adapt

Whatever you do, don’t let a situation like this kill your motivation to grow things. In the meantime, you have several options to work around it and keep the harvests coming. If you have the room, you could break ground in a different part of your yard and work on building up the soil there; then, when your original spot is usable again, you’ll have expanded your gardening space.

You could also build some raised beds away from the herbicide-affected area. Even if you prefer to grow in-ground, you’ll find that a few raised beds make a good permanent addition to any garden. One of the biggest benefits is the ability to customize the soil blend and grow things that might not thrive in your native soil. I like to grow sage, rosemary, and tropical chilis like ghost peppers in my raised beds, since they don’t love the clay-heavy soil in my main gardens.

If you don’t have room to expand your garden or install raised beds, no problem. Get into container gardening, which has all the benefits of raised beds but requires less space and less upfront cost.

You can grow a ton of different crops in five-gallon buckets (which you can often find for free at delis and bakeries). Fabric grow bags are another cost-effective option. Plenty of folks with limited yard space grow huge amounts of produce this way, right on their back deck.

Finding out your soil has been affected by persistent herbicides is one of the most disheartening situations a gardener can face these days. But remember it is temporary, just like every setback we encounter when gardening. The good news is that you’ve identified the problem, and you now know how to prevent it in the future.

Final Thoughts

If you’ve never dealt with herbicide damage, don’t be next in line. Use non-risky mulch and amendments when possible, and do those bioassay tests on anything questionable!

Bioassay testing is super easy, and it’s well worth the time it takes if you have ANY uncertainty about a soil amendment you’re planning to use in your garden. An ounce of prevention is worth a pound of cure. Or, in this case, a few weeks of waiting for a bioassay test beats a year or more of waiting for your soil to be clear of herbicides.