Taraxacum...

...or dandelion.

A very underrated plant, and yet it is bright and cheery and has other interesting forms.
It is thought to have evolved in Eurasia about 30 million years ago, and has been used as food and a herb, as well as lots of other uses.
It's a good companion to other plants, it adds minerals and nitrogen, attracts pollinating insects and helps fruit ripen.
As a herbal medicine it can be used to treat infections and bile and liver problems, as well as a diuretic.
Why is it a weed?