Bloom and bust

Certain plants, and not just annuals, die soon after flowering. It’s one of the mechanisms for ensuring their survival, with some plants living for a very long time, says TIM ENTWISLE, depending on how you measure it

Determining the oldest tree on earth depends on how you measure it

When you propagate a plant from seed, the offspring are different from one another, and from the parent. The seedlings are a new and distinct generation, like our own children.

But when you strike a cutting, the new plant is identical to its parent. You could say it is still the same plant.

What about a strawberry runner? If you split off the new plant, does it become a new individual? What if you don’t sever the connection? And what about suckers from your abutilon or elderberry, which can continue growing after the death of the main stem? Are they new plants, or is it just the old one extending its reach?

These are important questions to consider when it comes to determining the oldest plant on earth, and whether plants can, as some have suggested, in a sense live forever.

live and let die

How long a plant lives depends on the species, and where you grow it. Tiny cress pop up in my garden apparently overnight, flower within a few days, and then… well, I try to weed them out before they set seed and die. Other plants, depending on how you answered my earlier questions, may live, as Sir David Attenborough would put it, for a very long time indeed.

Annuals, of course, flower and fruit within a year – or two, when they become biennials. This boom-and-bust approach, with the plant dying soon after it fruits, is a strategy used by longer-lived plants as well. Most famously, the leafy rosette of the (misnamed) century plant (Agave americana) flowers, then dies, after about 20 years or so. The fishtail palm (Caryota urens) also grows for about 20 years, then produces flowering stalks, from the top down, over 10 years before it dies.

In 2008, a palm was discovered in Madagascar and named Tahina spectabilis. At 18m tall and with leaves up to 5m in diameter, it is no doubt spectacular. Even more so in flower, I gather, when it attracts large swarms of insects and birds. After it flowers, it dies. We don’t yet know how many years it takes the tahina palm to flower, and therefore how many years it lives. The Corypha utan palm on Cape York takes 30 years. These plants that die after flowering are known as monocarpic.

There is a monocarpic legume called the suicide tree (Tachigali versicolor).

I first learnt about it in Richard Powers’ novel, The Overstory, but it’s a real tree, and it really does die after its first flowering. A character in the book says the dying tree opens up the canopy, letting light in, and its rotting trunk provides nutrients and a suitable nursery for new seedlings.

Sometimes a distinction is made for plants where a single stem, not the entire plant, dies after flowering. These are known as hapaxanthic. A good example is the sago palm (Metroxylon sagu). Sago is manufactured from starch extracted from the inner pith of one of the multiple stems just before or during its early flowering.

Most trees, of course, flower and live for a long time. While it’s said that some spruce trees in Sweden may be up to 10,000 years old, a bristlecone pine (Pinus longaeva) chopped down in Nevada in 1964 is often cited as the oldest known tree. The tree was almost 5000 years old when it was accidentally felled, but there are claims that there are other bristlecone pines at least 8000 years old.

A Huon pine covering more than 1ha in western Tasmania is thought to be about 10,000 years old, but there is some debate about whether it is an individual tree or a colony of clones.

shared history

Which brings me back to my opening questions: are suckers and cuttings still part of the original plant? For now, let’s exclude cuttings, which would create a few potentially immortal plants.

Trees like the Huon pine live for so long partly because they spread by suckering or layering. If we allow this in our age calculation, we also have to consider the creosote bush (Larrea tridentata) of California, with at least one suckering plant now more than 11,700 years old.

Back in Australia, there are peach myrtles (Uromyrtus australis) in the Nightcap Range of New South Wales with connected clusters of stems that are each up to about 1500 years old. The entire plant is probably more than 10,000 years old, which could also be how old some of our ‘fairy ring’ mallee eucalypts are, with their original stems long deceased.

Tasmania is home to not only the long-lived Huon pine, but also King’s holly (Lomatia tasmanica). This species exists today only as a small patch of shrubs, each with cells carrying three sets of chromosomes (meaning they can’t produce fertile seed) and no detectable genetic variation. Our best explanation is that a fire fragmented a once interconnected clone, which started life some 43,000 years ago.

I’m sure there are other candidates out there for the oldest plant. What if we accept cuttings? Well, most commercial bananas are only propagated vegetatively, so perhaps we could consider all of them to be part of one very old individual. Perhaps. But that doesn’t feel right.

Let’s say an individual plant has to be physically connected for its entire life, or at least be in the place where it once had that physical connection. That means the oldest plant on earth (keeping in mind fungi are not plants) may well be King’s holly in Tasmania – until, that is, we find another collective of stems with a shared and intimate history.