Calendula, sometimes called pot marigold or English marigold, refers to about 20 species of edible flowers from the daisy family. Despite its nickname, calendulas differ from the flowers of the genus Tagetes, commonly known as marigolds. Calendulas have edible petals, whereas Tagetes marigolds are dangerous for both humans and animals to ingest. If you have a plant that resembles a marigold in your garden, you must be entirely sure it is a calendula before eating it or allowing pets near it. To tell if a marigold is a calendula, examine its features and look for telltale signs of its genus.
Examine the shape of the flower's seeds. Calendulas produce brown, U-shaped seeds with small bumps along the exposed surface. Marigolds have straight, black seeds with a white tip. The difference between the two seeds is unmistakable. This is the surest way to determine if your plant is a calendula or marigold without consulting an expert. If the flower is not producing seeds, wait for a bloom to wither and pluck one from the head.
Observe the shape of the flower and its petals. A calendula petal is straight, long and ovular with a yellow, white, orange or pink color. The flower's shape is somewhat flat and similar to a wide and shallow bowl. It has a distinct round dot in the center that ranges in color from yellow to brown. A marigold petal typically looks similar to a rectangle with rounded corners and a wavy, rather than flat, surface. The flower's shape is slightly spherical with a yellow, fuzzy center that seemingly blooms with the petals. Its color can be a solid or mixed combination of cream, yellow, red, orange or maroon.
Measure the flower's height from ground to the tip of the highest petals. Mature calendulas typically grow between 1 and 2 feet tall, depending on the exact species and growing conditions. In contrast, the many varieties of mature marigold grow between 6 inches and 4 feet tall. If the flower is much taller than 2 feet, it is likely not a calendula.
Cut one flower head from the plant, about an inch down the stem and take it to a local garden nursery. If possible, take a picture of the plant as well. If it is potted, take the entire plant to avoid causing it harm. Calendulas and marigolds are similar, so for the most certain determination, always ask an expert. However, if you do not plan to eat the flower or allow animals near it, this final step is unnecessary.