Ken Thompson: why are monster-sized leaves typically tropical?

Let’s assume you would like your garden to have a “tropical” look. What do you do? As everyone knows, tropical plants tend to have big leaves, so you grow the plants with the largest leaves you can find: banana (Musa), cannas, palms, ferns and such.

But why do tropical plants have big leaves? To answer that question, you really ought to read a terrific recent paper in the journal Science, but I’ve saved you the trouble. The conventional wisdom, which is indeed entirely correct, is that big leaves are surrounded by a thicker “boundary layer” of air that acts as a kind of blanket, reducing heat exchange with the surrounding air. Thus big leaves have trouble losing the heat they absorb from sunlight and run the risk of overheating. So in hot, sunny climates, leaves should be small.

But hold on a minute, didn’t we just agree that tropical plants have big leaves? The key is plenty of water. Overheating is only really a problem if water is in short supply, since leaves can keep cool by transpiring lots of water. In classic tropical rainforest, water is not a problem and leaves can be as big as they like, keeping cool by transpiring gallons of water.

So by going for big-leaved plants for the tropical look, you’re effectively trying to simulate the hot, wet tropics. So far so good, but that still doesn’t explain why the biggest leaves are in the tropics (which they undoubtedly are). Unlimited water may be crucial if big leaves are to avoid overheating, but there are plenty of wet (indeed, very wet) places outside the tropics. Not only that, lower temperatures mean they don’t have to worry so much about overheating, so why can’t temperate plants have enormous leaves too?

To answer that question, you need to remember that leaves don’t just get too hot, they can get too cold as well, especially at night, and here again leaf size is important. In cold climates leaves rapidly lose radiant heat to the night-time sky. To keep warm, plants need to absorb heat from the soil, the air and from other plants.

But for big leaves, that pesky thick boundary layer reduces their ability to do this, making them susceptible to frost damage. The net result is that across the entire globe, the smallest leaves are found in hot, dry deserts and in cold, high-elevation regions such as Tibet and the Andes, but the reasons for their small leaves differ: shortage of water and danger of overheating in the former, and possible frost damage in the latter.

So, as a visit to the Palm House at Kew quickly demonstrates, big leaves are a reliable guide to the steamy-jungle look. In fact, in the really wet bits of the tropics, temperature imposes no limit to leaf size, and the only thing that prevents the biggest leaves from being even bigger may be the difficulty of physically supporting them.

One final thing to remember is that although climate is certainly the dominant influence on maximum leaf size, that doesn’t tell you how big the leaves of any individual species will be. Many other factors also play a part, so leaves are often smaller than the maximum permitted by the climate in any particular spot.

Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written five gardening books, including The Sceptical Gardener, a collection of his columns for the Telegraph. Visit books.telegraph.co.uk