18. Weed Control (part two)

Weeds are always seeking to invade the healthy soil of a garden.  They can easily become intimidating and overwhelming, and it may seem like you can never win.  But it’s not true! There are many good options for weed control, and I’ve already written about the design factors and mechanical controls for weeds.  Now let’s take a look at herbicides:

Herbicides

Part of my motivation in writing this is to answer a very common question: how bad are those weed killers and should I use them?  As with a lot of things, the short answer is “it depends” - on the specifics of your garden situation and the specific herbicide in question.  Herbicides can be generally defined as substances toxic to plants, typically coming in spray or granular form.  They are divided into 2 categories according to when they are effective:  pre-emergent and post-emergent, and there are natural and chemical versions of each.

Pre-emergent Herbicides

Pre-emergent herbicides are applied before weeds emerge from the ground, preventing seeds from sprouting or killing seedlings as they sprout.  They effect all seeds in the ground that are trying to sprout, and from what I’ve read, the chemical ones supposedly last for 6 to 8 weeks.  The advantage is that they don’t effect plants that are already there. If you’re planting a meadow of pre-grown native grasses, you can use a granular chemical pre-emergent to block the sprouting of weeds that will compete with the grasses.  Pre-emergents are also commonly applied for weed control in lawns.

The key is in the timing of the application, which must occur one or two weeks before a certain weed is going to sprout.  This limits their effectiveness and also increases the chances for environmental damage.  If applied at the wrong time you are simply putting chemicals in the ground with no positive effect and many negative ones.  This is especially true of lawn ‘weed and feed’ products – sounds like an efficient combination, but you don’t need to apply fertilizers and herbicides at the same times or frequencies, so you are most likely applying excessive chemicals which then move into the larger environmental systems, especially contaminating our water – creeks, rivers, and the Bay.

The chemicals used in pre-emergents are said to have similar hazards as those used in post-emergents, which I talk more about below.  The main natural alternative is Corn Gluten, a byproduct of corn processing.  This is spread as a powder or granule and then wet down to cover the ground.  The one direct experience I have with this product was pretty good for weed control, but we were surprised by the smell of it when applied, a sort of compost-y garbage-y smell, so be sure to test it out on a small area first.  The timing issues with pre-emergent herbicides make these less popular than the post-emergent ones.

Post-Emergent Herbicides

Post-emergent herbicides are usually sprayed directly on the weed you want to get rid of.  Most of them are indiscriminate, meaning that any plant they come into contact with will feel the effects, so you have to be careful in a garden with established plants.

The more environmentally safe post-emergent herbicides are typically called “contact” herbicides.  This is because they will kill any plant parts that they come into contact with, but leave other parts alone.  Common contact herbicides have active ingredients such as Vinegar (acetic acid), Clove Oil, Citrus Oil, or a type of soap, referred to as Herbicidal Soap.  These are all relatively benign ingredients that won’t linger and harm people or animals. They do, however, leave the underground parts of the plant there, so many weeds will tend to grow back.  This obviously makes the job harder, and some argue that contact herbicides are a waste of money and hand pulling or sheet mulching is far more effective.  Others argue that because of this drawback, the only solution is to use a chemical herbicide.

Chemical herbicides enter the plant’s systems and kill the entire plant by disrupting biochemical processes.  The most common retail weed killers of this type rely on glyphosate, a chemical introduced by Monsanto Corporation in the 70’s as Roundup.  Chemical herbicides can be seen in many ways as the “nuclear” option – very effective, but with many lasting and toxic side effects.  If used, they must be carefully and judiciously applied, following all the directions precisely.  Especially important is avoiding their use in vegetable gardens, or areas where you can’t keep children, pregnant women, and pets out of for 24 hours after application.  The reason for concern is that these are chemicals that are designed to cross into the internal systems of a plant and disrupt them.

Update (Summer 2021)

There have been important new developments on glyphosate based weed killers that I wanted to include in this discussion.  Recently several legal actions have led to substantial judgments in favor of people who got cancer from use of these weed killers, specifically Roundup.  More and more evidence is pointing to gylphosate based weed killers as cancer-causing, especially to people with long term exposure, such as those in the field of landscape maintenance.  In late July 2021 the San Francisco Chronicle reported that Monsanto is removing Roundup from U.S. store shelves in response to the lawsuits.  Clearly avoiding its use entirely is the safest thing to do, I've added 3 new reference articles at the bottom of the list below.

Overall, Mechanical Methods are Better

Based on what I’ve read, and my personal experience, I would have to say that any herbicide is of limited use in the home garden.  The chemical ones are just too environmentally damaging to use except in extreme cases in very controlled conditions, and the natural ones are not usually as effective as mechanical means.  I encourage you to go out there and try things yourself to see what works for you, just be sure to read the label and be very careful with those chemicals.  Happy weeding!

References:

Golden Gate Gardening, by Pam Pierce

Bay Friendly Gardening Guide, https://rescapeca.org/wp-content/uploads/2016/01/Bay-Friendly-Gardening-Guide.pdf

The New Sunset Western Garden Book, published by Sunset

Peaceful Valley Garden Supply, www.groworganic.com

Roundup: Birth Defects Caused By World's Top-Selling Weedkiller, Scientists Say, by Lucia Graves, Huffington Post, August 24, 2011  https://www.huffingtonpost.com/2011/06/24/roundup-scientists-birth-defects_n_883578.html

Weed Killer Kills Human Cells. Study Intensifies Debate Over 'Inert' Ingredients, by Crystal Gammon, Environmental Health News/Scientific American, June 22, 2009  https://www.scientificamerican.com/article/weed-whacking-herbicide-p/

How Toxic is the World's Most Popular Herbicide Roundup?, by Katrina Zimmer, the Scientist, February 7, 2018  https://www.the-scientist.com/news-opinion/how-toxic-is-the-worlds-most-popular-herbicide-roundup-30308

Monsanto Parent Bayer said to propose $8 billion settlement over Roundup Claims, by Aimee Piccii, CBS News, August 9, 2019   https://www.cbsnews.com/news/monsanto-parent-bayer-said-to-propose-8-billion-settlement-over-glyphosate-claims/

Herbicide Roundup to Be Pulled From US Store Shelves in Response to Lawsuits, by Bob Egelko, San Francisco Chronicle (online), July 29, 2021  https://www.sfchronicle.com/bayarea/article/Herbicide-Roundup-to-be-pulled-from-U-S-store-16350403.php