Ralph Waldo Emerson, called a weed a plant whose virtues that have not yet been discovered. I disagree, and yet generally speaking he is correct, what we call a weed is in the eye of the beholder, a plant that it is not helpful, useful or wanted.

An invasive plant by definition in Executive Order 13112 (Feb. 3, 1999) is a plant whose introduction will likely cause economic or environmental harm, or endangers human health. The same EO also defines alien weed. Generally speaking this is the best agreed upon definition for invasive weeds (causing economic, environmental or human harm). Not everyone uses or knows about this definition, and I hope I am consistent in using it too.

Noxious weeds are generally those that are very likely to spread and cause significant economic, environmental or human harm and/or are difficult to control and have been designated as noxious by an agency. They are generally worse than our regular invasive weeds because someone took the time and effort to request an agency to list that species, and the agency agreed to list it (but see CA Noxious weed definition pasted below). If you want to get even more complicated, in California and many other states we rank our noxious weeds (with “A” generally being the most problematic and with limited distribution, to “D" being of minor concern).

A noxious weed is defined more specifically than given in this post:

(https://www.cdfa.ca.gov/plant/ipc/encycloweedia/winfo_weedratings.html

with species listed here:

https://www.cdfa.ca.gov/plant/IPC/encycloweedia/weedinfo/winfo_table-sciname.html)

A colonizer is any species (native, alien or invasive) that will spread in disturbed areas. These generally are weeds because they will colonize someplace outside of a disturbed area, such as your garden. However, life can be tricky and some colonizers persist terribly outside of disturbed areas. 

Transformer species are those, that like the name says, will transform the system you are working in (grassland to shrubland, forest to grassland, chaparral to grassland, etc.). These are also invasive species (by overlapping definitions) and sometimes listed as noxious and are usually exotic, but can be native.

The definition of a pest can vary depending on the context. In California, a pest is an organism that is detrimental, causes damage or economic loss, or can transmit disease. A pest can be an insect, a weed, or a disease or more. A pesticide in California generally covers any product used to control, destroy, attract or repel a pest, not just insecticides but herbicides too (see Ca Dept. of Pesticide Regulation for exact definitions and exceptions). If your weed is causing damage or loss, it could also be a pest.

As far as muddling these definitions, people do it all the time, including professionals. The plant definition police are few and far between, and often people are speaking colloquially about weeds.

Invasive gets tossed around loosely to signify this weed is more important (i.e. more bad) than that weed, whether harm is proven or not. And then we get into the definition of “harm," which is the dividing line between plain old exotic weed and invasive weed. Who judges very minor or moderate harm? Noxious tends to be used more correctly since a weed has to be listed by an agency to officially receive the noxious designation. But noxious can also be used in the sense of being poisonous, although oftentimes (but not always) that species will be described as toxic or poisonous instead of noxious.

Yes, there are native weeds too. When a native plant gets in the way of what you want to do (too many shrubs or trees in a rangeland, sycamores growing over your house, etc.) it can be called a weed. It does not matter if its a native or exotic weed, its a weed, its unwanted. Many lawns can be covered with Cardamine oligosperma (bittercress, toothwort) in the winter, because its rosette grows too low to become chopped by the blades of a lawnmower. This species is native to California, but for most people it is a weed, although I allow mine to grow outside the garden, so its not a weed in part of my yard, but it is in other parts of my yard.

If you've made it this far, to sum up, all plants can be weeds, as long as they are unwanted in some way, the ones that were brought here from far away by people are exotic or alien, if they cause a loss they are a pest, those causing harm are invasive and those that cause harm and have been found to be pernicious by an agency are noxious.

And just to throw you for one final loop, remember all species produce more offspring than needed to replace their parents (otherwise there would be no population growth). So any species when provided the right environmental conditions can become invasive.

Thanks for reading and keep your weeds to yourself.

Chris

Disclaimer: This post has been prepared for general informational purposes only. It is not intended to provide legal advice or specific definitions of pests or weeds or guidance on any specific pest problem. Readers requiring detailed, legal or specific information should consult relevant laws and regulations and/or a lawyer.

CaDPR

http://www.cdpr.ca.gov

CaDPR pesticide fact sheet

http://www.cdpr.ca.gov/docs/dept/factshts/what2.pdf

California Food and Agriculture Code (FAC) Sec. 5004 - “Noxious weed” means any species of plant that is, or is liable to be, troublesome, aggressive, intrusive, detrimental, or destructive to agriculture, silviculture, or important native species, and difficult to control or eradicate, which the director, by regulation, designates to be a noxious weed. In determining whether or not a species shall be designated a noxious weed for the purposes of protecting silviculture or important native plant species, the director shall not make that designation if the designation will be detrimental to agriculture.