Some plants that are called perennials don't keep growing back indefinitely - but actually die off after just three or four years. Some people catalog these plants not simply as perennials but as biennials = a plant that takes two years to complete its biological life cycle...or in other words, it comes back for 2 or 3 years but then disappears. These are often referred to as "short-term" perennials. A good example of these types of plants are yellow blooming Echinacea (aka coneflowers). These are flowers that are technically perennials but don't typically stay around for 5 to 10 years or longer.
The table below illustrates the gradient of terms and types of plants as defined by their longevity. Little research has been done on perennial longevity but reports from gardeners and professional landscapers tend to say that some perennial species do die after a number of years but how long these plants can live varies.
Let's go back to S. rosemarinus. Why does it grow back when it's so often called an annual? There's 2 big reasons for that. First, it's called an annual in some parts of the US because it is classified as having a hardiness zone of 8. An 8 hardiness zone means that it can withstand cold temperatures between 10 to 15 degrees F. In places like NJ where the hardiness zone is 6, the herb should (or could) die from freezing temperatures. The 6 zone refers to the potential of low temperatures between -10 and 0. Hardiness zones are not guarantees of low temperatures but merely a range defined by historic winter weather. Some winters are warmer than the prescribed hardiness zone. From experience, I've tracked wintertime seasons with lows much higher than -10 or even 0 in New Jersey. If it never gets below 10 degrees, your rosemary will survive...and it will grow back the next year. That can happen for multiple years and you might begin to think that delicious bush is actually a perennial instead of an annual. However, due to yesteryear firsthand knowledge, don't count on it. Eventually, the cold areas will likely dip down and down below the threshold of survive - and that wonderful herb will go to the by and by of everlasting life.
This is true for many food plants. The main reason you have to replant your vegetable garden every year is that nearly all of the edible plants we grow are not native to the higher latitudes of North America. Solanum lycopersicum (lovingly referred to as tomatoes) is native to Mexico and Central America. It's in the same genus as Eggplant (S. melongena) and Potato (S. tuberosum). Its origin explains why it always important to give it lots of sun and keep it watered. It also explains why your tomato vines can't withstand a winter in Nebraska, Michigan or Connecticut, because parts of Mexico have hardiness zones of 10. This means the tomato plant will die if the temperature gets to 32 degrees.
Complexity is in the Change
Though hardiness zone isn't the sole factor for determining if a plant will grow back or not in a specific location - it is a big reason. Soil is another and sunlight. Lifecycle as mentioned above also contribute. But all of these elements aren't written in stone. Some winters are warmer than others. AND, some winters are warmer than others in specific places while in other states or towns, it's the same or colder. For example, the midwest could have a mild winter while the southeast could have a wildly colder winter. And if that mild winter is dry versus wet...or wet versus dry - this could make all the difference to plant survival.
The more scientific perspective would say that environmental factors and competition have a major impact on survival. The four key indicators listed below assist in predicting the of long-term performance over one to multiple years.
- Inherent longevity. How long are these plants genetically programmed to survive?
- Ability to spread. How well do these plants spread on their own?
- Persistence. Does the plants have the ability to hold their ground and stay in one spot?
- Ability to seed. How well does the plant reproduces itself by seeding?
Different plants combines these indictors in different ways. The mix of the indictors along with climatic influences such as winter temperatures are the governing determination. All plants are programmed to live as long as possible to better allow for reproduction and spread, but some are more suited to the projected climatic conditions. It's critical to know how people catalog plants in these terms so to pick the right plants for a yard....but then again, all the "right" plants might not be what you want to actually see. Aesthetics are just as important. In my opinion, the look of a garden is paramount.