Marigolds are easy to grow and love the full sun. The marigolds I used to dye with were grown in my garden this summer with hardly any help from me at all. They are often planted as companion plants in the vegetable garden, especially tomatoes, to repel beasties.
Do not hesitate to harvest the flowers - the more you pick, the more will blossom (but always make sure to leave some for the bees & butterflies please!).
I picked mine in the early mornings throughout the summer and dried them in a room in my house that was generally warm and had pretty good airflow. You can dye with fresh or dried marigolds, but I chose to use some of the ones I had dried earlier this summer.
If you plan to dye with Marigold, make sure to pre-treat your fiber in an alum or soda ash bath before dyeing: (https://www.sashabotanica.com/post/pre-treating-fiber-before-dyeing)
1.) Add your marigold flowers to a pot of water - make sure you have enough water to allow your fiber to float freely once the flowers have been strained out.
2.) Let the flowers steep for about an hour or two on a medium/high heat - do not boil!
3.) After the steeping, the color should be a rich yellow/orange. Strain out the flowers and return the pot to a medium/low heat.
4.) Add your fiber (make sure you wet the fiber completely in warm water before adding! This allows the color to dye evenly), stirring occasionally to make sure there are no trapped air bubbles, allowing the fiber to dye evenly.
5.) Keep in the dye bath for about an hour, or longer for a deeper color.
6.) Once the fiber has reached the color you like, rinse with lukewarm water and hang to dry out of direct sunlight.