How to Identify a Weed

Weeds are plants that grow where they’re not wanted, consuming sun and space that should go to desirable crops. They can spread disease that ravages those crops, harming humans and animals who depend on them. They can smother and choke out native species, threatening biodiversity. They may interfere with the mycorrhiza (symbiosis between fungus and plant roots), weakening the roots and depriving them of nutrients; and they can produce substances that inhibit the germination or growth of other plants, a phenomenon known as allelopathy.

Proper identification of weeds is the first step in determining what to do with them. It’s best to try and identify them when they are flowering, since that is when they display the greatest number of defining characteristics, but this can be challenging. If you can’t wait, use a field guide or manual on weeds in your area, especially one that lists the major weeds by family and provides detailed photos and descriptions of each.

Another option is to use a dichotomous key, starting with the most readily recognizable plant characteristics and gradually narrowing the possibilities until you’ve arrived at a short list of likely IDs. If this approach seems confusing, you can also attempt to identify a weed by direct comparison with similar-looking specimens in the manual or guide. To help you do this, it’s helpful to familiarize yourself with the jargon used in these references. For example, a basic illustrated glossary of plant identification jargon can be found in many of the guides and manuals mentioned above.

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