Why is this the answer?

The answer is thistle because this clever riddle paints a secret picture of a very spiky wild plant you might see growing on the side of the road. Let us be nature detectives and look at the clues one by one to solve this plant mystery. The first clue is: "He stands beside the road." Thistles are very common wildflowers that are tough and strong. They do not need a fancy garden and often pop up all by themselves right next to sidewalks, fences, or dirt paths. They are truly travelers of the roadside, just like the riddle says. Sometimes people call them weeds because they grow everywhere and can be hard to get rid of, which is another reason they are often found standing alone by the road. The next clues are the best ones because they describe the thistle's special outfit. The riddle says the plant is "In a purple cap at tattered green cloak." A thistle has a beautiful, fuzzy, bright purple flower right at the top. This flower is exactly what the riddle means by its "purple cap." The rest of the plant, the stem and the leaves, are a pale, dusty green. This color covers the plant just like a long "green cloak." The word "tattered" is a hint too, because thistle leaves are not smooth and soft. Instead, they are very jagged, prickly, and almost look like they have been torn or ripped, making the cloak appear old and tattered. The final clue tells you exactly how people feel about this plant: "Those who touch him, curse him." This is the easiest clue of all. Thistles are famous for being covered in very sharp prickles or spines all over their stems and leaves. If you accidentally bump into a thistle or try to grab one, it will poke your fingers and hurt quite a bit. It is such a surprising and painful sting that a person might make a loud, unhappy sound or say an unhappy word, which the riddle calls a "curse." You learn very quickly to look but never touch a thistle. All of these clever clues together perfectly describe the thistle.