Omens, plague, and bamboo flowering

In China, Japan, and parts of India, the rare phenomenon of bamboo flowering is considered a bad omen. In China, a proverb warning of ‘pestilence or famine’ presaged by bamboo flowers was reported in a letter to Kew from a British Medical Officer who noted that bamboo flowering in Hong Kong in 1894, 1896, and 1898 coincided with outbreaks of bubonic plague. It was during this epidemic that Alexandre Yersin discovered both the bacillus responsible for the plague and the fact that it was present in rats. Given that we now know that the abundance of food provided by bamboo seeds from mass flowerings creates booms in the rodent population, and that fleas on rats transmit plague, the proverb seems well-founded.

In 1958 the authorities in Mizoram in north eastern India were warned that the bamboo was flowering and famine would follow. They dismissed the reports as local superstition. The bamboo flowered, the rat population exploded and a ‘rat flood’ consumed every scrap of every crop in the area. The resultant famine was so bad it led to an armed uprising.

In India this phenomenon is known as ‘Mautam’, bamboo famine. 90% of all the bamboo in the area is the same species, a tropical clump former with a relatively stable flowering cycle of about 48 years. It all flowers at once (gregarious flowering) and then dies. Not all species of bamboo are so predictable or, fortunately, so devastating in the effects of their reproduction. 

In fact, there is a great deal we don’t understand about bamboo flowering, despite records of the phenomenon going back some 2000 years in China. What triggers it? How do plants of the same clone, growing in different locations, know to flower at the same time? Why are the flowering cycles so varied (between 1 and 120+ years)? Why do many bamboos die after flowering?

Many factors have been proposed as possible triggers over the years, from external environmental factors to an internal clock. It is generally believed now that the flowering does follow an internal time table, modulated slightly by environment and cultivation. The exact mechanism is not yet understood. In the 1970s, the ecologist Daniel Janzen theorised that bamboos flowered in synchrony as a method of overwhelming seed-eating predators. So many seeds would be produced that they could not all be eaten and some would germinate and grow. Plants that fell out of sync would not have this protection, their seeds would be eaten, and their genes would not continue.

A group of biologists at Harvard extended this theory to explain how the long flowering cycles evolved. Supposing in a forest of bamboos which flowered every year, a few mutated to flower every two years. They would have longer to prepare and more stored energy to produce seed. Their seeds would have an advantage and their offspring would increase. More and more bamboos would flower every two years, and eventually those flowering every year would not produce enough seeds to survive predation. They would die out. Then suppose a few bamboos mutated to flower every three years. They also would not have the protection of flowering and producing seed in synchrony and would die out. What about mutations that flowered every four years? Again, they would have the advantage of more time to store energy but still flower in synchrony with plants on the two years cycle. So bamboos which flowered in multiples of the cycle would gradually predominate, and the cycle would get longer and longer over time.

What does all this mean for bamboos in the garden? First of all, bamboos flower at very long intervals, so the plants in your garden may never be affected. However, given the likelihood of them dying if they do flower, it is a good idea to plant a mix of different species if you are planting a bamboo hedge, to protect against expensive losses if you are unlucky.

Secondly, there’s nothing you can do to stop a bamboo flowering. The only thing to do is wait and see. It may only be sporadic flowering, which shouldn’t harm the plant. If it is complete flowering (anecdotally more common in clump-forming bamboos than runners) the plant is both more likely to die and more likely to set seed. You can water and feed it if you wish, and it may recover, but it will take a long time to do so if it does and will look awful in the meantime, so most people dig them out or at least cut them down. I have found seeds more likely to germinate in situ, if they escape the notice of hungry birds.

In 20 years of collecting bamboo, I have had three species flower completely out of a collection of more than a hundred: Fargesia nitida, Chusquea gigantea, and Phyllostachys nigra ‘Megurochiku’. The Fargesia, a clump former, produced viable seed and died. The Chusquea and the Phyllostachys, a giant clumper and a runner, next to one another in my garden, flowered at the same time last year. A forest of seedlings has appeared, but I have no way of knowing their parentage. The Phyllostachys is recovering. The Chusquea has as yet shown no signs of doing so, but I have not cut it down yet as I will not be opening my garden this year and can live with the eyesore if there’s any chance of regrowth. It is a favourite plant and I will be very sad to lose it. Fingers crossed, but as I have learnt over the years, you might be able to predict famine and plagues, but bamboos rarely do what you expect them to do.