The Plant Edit
Marigolds are an annual flower native to Mexico and Central America. It has a strong scent and produce yellow, orange, or red-yellow flowers. The plants bloom from mid-summer until the first frost. It has become a popular garden plant in North America. 
Marigolds are a great companion plant. They deter deer and other pests and can also help repel mosquitos. Marigolds are used in traditional Hindu celebrations and are an important part of the Oaxacan dye tradition. 
How to Harvest Edit
With marigolds, the more flowers you pick, the more flowers the plant produces. It is beneficial to pick flowers often throughout the growing season. The flower heads are gathered and are easily broken from the stalks. For the cleanest colors, only the flowers should be used, not the leaves or stalks. Marigolds can be used fresh or dried for later use. Trudy Van Stralen recommends weighing them as soon as you pick them and using the weight of the freshly picked flowers opposed to the weight of dried plant materials when calculating your dye bath. The fresh weight can be recorded on a paper bag, which is then used to store the dried plant material. 
How to Extract Color EditTrudy Can Stralen recommends pre-mordanting fibers when using all flower-based yellows to ensure colorfastness. Soak flowers overnight, then bring the water to a boil and simmer for one hour. Strain the plant material from the dyebath, add your pre-mordanted fiber, and simmer for another hour.
If the flower heads are gathered before the flowers are fully open, they will produce greenish yellows and lemon-limes. If picked in full bloom, they will produce yellows, golds, and oranges.