To answer that question, we must first ask what a weed is, and that is best addressed after looking at two other questions.
What is a non-native species?
A non-native species is a plant or animal living in a place where it was not historically found. Many plants popular among traditional gardeners are non-native, being European or Asian in origin. Even lawn grass is non-native! North America has over one thousand native species of grass, but none of them will put up with continual mowing. For this reason, when the developers of the early suburbs imported the idea of turf grass from Europe, they also imported the grass.
Unfortunately, European grass does not like the North American climate, which is why it tends to turn brown in the summer. This dormant state is natural and healthy for the grass, but some people find it unattractive. For this reason, some homeowners choose to water their grass to keep it green throughout the growing season.
What is an invasive species?
An invasive species is a non-native that tends to spread in its adoptive home. For example, the European species garlic mustard is considered invasive in North America. In its home range it is a harmless plant that has been used as a cooking herb for centuries, but elsewhere it tends to spread rapidly and kill other plants by poisoning fungi the plants rely on.
What is a weed?
A weed is simply a plant that a human observer does not like. It is a subjective designation, not a biological class.
Many gardeners consider dandelions to be weeds. Mowed, pulled, or herbicided, they just won’t go away!
Dandelions are a pioneer species. This means they are able to colonize disturbed areas where other plants cannot thrive. The dandelions, with their deep tap roots, are able to access nutrients other plants can’t reach. In the course of absorbing these nutrients, dandelions improve the soil, creating favorable conditions for other plants. In the absence of further disturbance, new plants will move into the area, quickly crowding out the dandelions, which have lost their competitive advantage.
Some species of dandelions are native to North America, while others were brought here by European settlers. The settlers purposely grew dandelions for their edible and medicinal qualities. The flowers are also an important early-spring food source for native pollinators.
Why does that yard have weeds in it?
In short, what one person sees as a weed, another person sees as a beautiful or useful plant!