When Is an Annual Not an Annual?


As we move from summer into fall, certain garden activities start appearing on our To Do list. We think about dividing overgrown perennials, planting spring-flowering bulbs, and generally putting the garden to bed for the winter. These activities often bring with them the question of what to do with some of our annuals such as basil and cilantro. Should we pull them up and add them to the compost pile or can we “cheat nature” and keep them alive through the winter?

Lemon verbena is a hardy perennial, tender perennial, or annual - depending on where it is grown.

Lemon verbena is a hardy perennial, tender perennial, or annual – depending on where it is grown.

This got me thinking about what we mean by an annual. The basic definition is “a plant that lives no more than a year” and must be annually renewed. But what I grow as an annual may be a perennial in other parts of the world. Conversely, plants I grow as perennials – lemon verbena, for instance – I know can only be grown outdoors as an annual in more northern latitudes. The same species may be an annual some places and a perennial other places. So I can take annual basil or biennial parsley and “trick” them into living several years, thus transforming them into a perennial. Right?

Not so fast! All plants have a natural life span. Horticulturists define this by what the plant’s life cycle is in its native territory. Lemon verbena comes from Paraguay where it is a hardy perennial but it is a tender perennial in areas like my Zone 8 garden and an annual north of Zone 6. This means the words “annual” and “perennial” used at your local nursery only have meaning locally. Is there a better, more universal way to define the life span of a plant?

I asked this very question of a science-based gardening discussion group I follow online. I gave the example of basil and cilantro as two herbs that gardeners often want to keep alive indefinitely. My personal experience told me it was a waste of time but I needed some scientific explanation of just why this was a lost cause. Here’s what I discovered.

Plant biologists avoid using “annual” and “perennial” when discussing a plant’s true life span because they are not universally applicable. A plant may be an annual in some circumstances and a perennial in others. Instead they define the life span based on whether the plant flowers and sets seeds once or many times. The terms they use are monocarpic and polycarpic. Monocarpic plants flower only once. This can be in the same year, as basil  and cilantro do, in the following year, as parsley and foxgloves do, or even many years later, like the century plant (Agave americana) that lives for decades before flowering once and dying. Polycarpic plants such as oregano, rosemary, and thyme, will flower again and again, year after year.

Once basil begins to flower it will continue to focus on end-of-life seed production.

Once basil begins to flower it will continue to focus on end-of-life seed production.

So who cares? Well, the life span of monocarpic plants is determined generally by when they bloom. Before they bloom they will produce leaves (what horticulturists call vegetative growth) steadily. Then something triggers the onset of flower and seed production. Often this trigger is a change in temperature or day length. When monocarpic cilantro senses the temperatures rising above 90 degrees it knows it is time to set seed and die. When basil recognizes fall is coming, it sends up flower clusters and prepares to set seed and die. Once that trigger occurs, nothing can stop the monocarpic plant from transitioning to end of life. Now let’s translate this science into a practical application using annual basil (Ocimum basilicum) and biennial parsley (Petroselinm crispum) as examples.

Basil will grow and produce leaves until the weather triggers the onset of flowering. Once that happens, the plant continues to attempt to set seed, even when we harvest it. Fortunately for us, the new leaves that show up after an initial harvest continue to have a good flavor although those pesky flower bracts keep showing up. If you bring the basil indoors and attempt to keep it on life support through the winter, you haven’t fooled the plant. It will still try to set seed and die the first chance it gets. Vegetative growth (i.e. production of the leaves we want) will slow to a crawl.

Parsley may take two years to flower, but once it starts, leaf production is over.

Parsley may take two years to flower, but once it starts, leaf production is over.

Parsley is a monocarpic plant that lives two years. The first year it produces lots of leaves but once it experiences the cold of winter, it stops vegetative growth (again, the leaves we want to harvest)  and switches to producing a flower stalk and setting seed. There will still be parsley leaves but they will be old, from last year, and not really worth harvesting as they tend to become bitter.

Bottom line – you can’t fool Mother Nature. Monocarpic plants such as basil and cilantro will change irreversibly when end-of-life seed production is triggered. You may be able to keep it alive through the winter (something of an herbal zombie) but it will be a lot of work for very little reward. Better to go with Mother Nature and start new seed in spring.

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