Herbaceous plants are not strictly speaking a botanical category. According to the Académie française, herbaceous plants include "everything of the nature of grass". Yet coleus, date palms and violets have much in common.

When the Immortals refer to the nature of grass, they mean three characteristics:

- color: herbaceous plants tend to be green (any shade is acceptable);
- smell: herbaceous plants exhale a scent similar to that of grass;
- texture: the aerial stems of herbaceous plants are supple, even soft.

The reason herbaceous stems are flexible is that they contain very little lignin, unlike the trunks of trees, shrubs and bristly plants. This is why banana trees, pineapples, palms and bamboos are giant grasses, not trees. Their false trunk is a stem, which contains almost no wood.

Annual herbaceous plants complete their life cycle in one year and then die. Before that, they spread their seeds to give rise to new specimens. The ephemeral flowers of poppies, marigolds, morning glory (Convolvulus tricolor) and sunflowers brighten up your garden for a summer. In the vegetable garden, you'll need to replant tomatoes, radishes, peas and green beans every year. Finally, aromatic plants such as parsley, basil and savory are considered annuals.

Biennial plants have a two-year life cycle. They grow in the first year and flower in the second. If you're not in a hurry to see your garden take on color, you can plant biennial herbaceous plants such as foxglove, snapdragon, pansy, hollyhock or primrose. You'll also need to wait two years before harvesting beet, wild carrots, cabbage, spinach, broccoli and fennel.

Varieties that live longer than two years are called perennials. If you don't feel like sowing every year, opt for sedum, St John's wort, periwinkle, angel's hair, thyme, chives, sempervivum and ferns. Cardoon, spinach sorrel and wild garlic provide a harvest every year.

Our green plants, such as alocasias, monsteras, calatheas and pothos, are tropical perennials. They are not very hardy and, in the open ground, they cannot withstand the rigors of winter. They behave like annuals. Grown indoors, they live for several years. Rejections, layering and cuttings extend the plant's life.