Understanding Annuals, Perennials And Biennials

One sign of an accomplished gardener is someone who can use and understand three of the cornerstone types of traditional garden plants. These are the annuals, perennials and biennials. The theory, at least, is that when you effectively use these three groups you can have a well-designed and thoughtful garden that blooms from spring through the end of fall.

While I’ve been taught by some of the best garden designers (or while they tried to teach me), I’m an outlier, and 99 percent of my garden plants are perennials. That’s just my obsession, however, and by no means a commentary on the grand scheme of good garden design.

The theory has always been that perennials are good early in the season and late in the season. It’s true — that’s when most of them strut their stuff. Yes, it’s much more challenging to have a garden dedicated to perennials and still have three seasons of interest, but it’s certainly possible. To bring in more varied structure and color in the periods when perennials are thought to fade, annuals are used. There are so many annual choices that do well in the dog days of our hot and humid Hamptons summers. Annuals can provide height, flower diversity, foliage texture and good integration with other plant types.

However, as my uber-rich boss once said, “Andrew, why are we using annuals in our gardens when they die every year?” In a sense, her comment was incredibly spot on. At the same time, her comment was naive and clueless. Annuals were also once the inexpensive way to plant a “flower garden,” but when I look at the prices of small pots of some of the more unusual annuals sold in garden centers and online, I recoil as they can often be the same cost as some perennials.

There are also the “new” plants given the moniker of “temperennials” which, for our gardens, turn out to be very expensive annuals as they don’t overwinter here. Throw in a few “tender perennials” and the winter-hardy annuals … well, it can get down right confusing. May I shed some light on what can easily be frustrating and often dishonest plant labeling and information?

Most garden centers have their outdoor plants divided into sections, and you can easily spot the annuals and perennials. Rarely do you find a biennial section, but you may find a shady section, a succulent section and sometimes a spring ephemeral section. While roses are often sold as perennials, you’ll see that by a technical definition they are actually shrubs.

Annuals are plants that we grow from seed, and they complete their entire life cycle in one season. Seeds are sown in greenhouses in winter and are grown as “plugs,” or tiny plants the size of a dime. These are then sold to other nurseries which plant the plugs into pots, grow them for a number of weeks to “finish” them, then ship them to garden centers. At one point, annuals were only sold in cell packs, but now you can buy them in pots from 4 inches up to 6 and 8 inches in diameter. Think large pots of impatiens and begonias.

An annual will grow its foliage, delight us with its flowers, then either go to seed and die, or just die at the first frost. Every spring, though, I get notes about how someone has discovered a perennialized annual that miraculously survived the winter. It didn’t. Some annuals, which we often call winter-hardy annuals, will go through the entire cycle and die but will leave behind seeds that survive the winter. Thus, a “winter-hardy” annual. This also happens with some tomato plants that people tell me are perennial in their gardens. Nope, the seed survived but not the plant.

Annuals come to us from all over the world and mostly from warmer climates. Just to confuse things a little more, there are plants like ageratum and begonias that we use as bedding plants (annuals) that have relatives that are true, hardy perennials, for example, Begonia grandis.

The technical definition of a hardy perennial is a plant that is grown from a seed or root that grows foliage (vegetatively) in its first year then flowers in the second year and in successive years. Most of these die back to the ground every winter then send up new growth in the spring. There are exceptions, there are always exceptions. One notable one is Russian sage (Perovskia). If this plant is pruned to the ground, at the end of the season you can easily kill it since it regrows from “old wood,” or last year’s growth, and not the roots or crown. These should only be pruned in the spring after you can see where the new growth sprouts. Lavenders can grow similarly, and there’s always been some debate if the lavenders are perennials or shrubs. There are perennials that we call “short lived” because they adhere to this regime but may only survive for three years.

Some short-lived perennials can be a few of the newer Echinacea introductions, lupines (except the species perennis), Delphiniums, Columbines, Heucheras, Achilleas and Leucanthemums. A very cold winter with no snow cover can also shorten the life span of some perennials, and this was true of some Geums that did not make it through last winter as well as the Spigelia.

As for roses — since they don’t die back to the ground and generally tend to grow new stems or “wood” every year — I don’t consider them to be true perennials but more like small shrubs. Not sure why so many articles and references call them perennials though.

Now we get to the biennials. This group of plants will generally have a two-year growing cycle. The first year the foliage and roots develop, and during the second growing season they flower. The most common example in this group is the hollyhocks, but there are also many Rudbeckias (black-eyed Susan), sweet William (Dianthus barbatus) and some Campanulas.

This is yet another area of contention since I’m always being told that some gardener has a variety of hollyhock that’s perennial and comes back every year. It doesn’t. It drops seed and gets into a cycle in the garden where every summer there a few plants in their vegetative stage (year one) and a few in their flowering stage (year two). The 2-year-old plants flower while the 1-year-old plants flower the following year. I have an area in my garden where a Dianthus barbutus seems to come back every year and acts like a perennial. It’s not and it isn’t. It simply self-seeds and seems to perennialize but actually none of the plants are more than 2 years old.

And then there are the “temperennials.” I refer to these as a horticultural abomination since most gardeners don’t read the labels. These plants may actually be perennial and return each year in their native regions — but not here. For our purposes, these are just plants that aren’t hardy in our zone but may be in zone 8 or 9, where they are perennials. Some are magnificent looking and are sold at or higher than the prices for perennials, but out here in our zone 7 gardens, they aren’t perennials.

That’s the lowdown. And what really separates the hardy from the marginal perennials can be an excessively hot summer or severe winter. I think this was the issue last winter when so many of us lost a plant that we fell in love with, Spigelia marilandica, or the Indian Pink. It’s native from Southern New Jersey and south and a wonderful plant. When first marketed up here, most of us found it not to be winter hardy. Then a new variety, “Little Red Head,” was introduced, and it made it through two winters. Warm winters. It did not survive last winter for many of us.

Much to ponder here. I still think you should experiment with marginally hardy plants as climate change may present new gardening opportunities. Annual, no problem. Perennials and tender perennials? Well, I wouldn’t design an entire garden with them. Experiment though. Keep growing.