Almost twenty years ago when a weed scientist proofread the manuscript for my Gardener's Weed Book, he challenged the notion that plants could "drop" seeds. They could shed seeds, he reasoned, but because the word "drop" implies a purposeful action and plants are inanimate beings, they can't drop seeds or leaves or anything else.
Sounds reasonable, and I've accepted it for years, but the more you learn about nature, the more you wonder. Last year one of my winter reading books was What the Robin Knows by Jon Young, and it changed my life in wonderful ways. I understand much more about what I'm hearing from the treetops, bird to bird, bird to dog or cat, or bird to me. If the bird feeder is empty and I go outside, at least two species – chickadees and nuthatches – will immediately chatter at me about the problem, and if I listen closely I'll hear the phoebes clicking in the bushes, too.
This winter's nature reading includes What a Plant Knows by Daniel Chamovitz, an illuminating look at how plants perceive and respond to their environment. As it turns out, the plants in our gardens know quite a lot. Pest activity hidden from our eyes is immediately perceived by plants, and often by insects that eat other insects or use them as nurseries for their young.
Take the tomato, for example. In many parts of the US, a hawk moth lays her eggs on healthy plants, which hatch into leaf-eating caterpillars called tomato hornworms. As the caterpillars eat and their saliva mixes with the chewed tomato leaf, gases are released that are of great interest to two parties – braconid wasps that lay their eggs on tomato hornworms, and other hawk moths in search of egg-laying sites. If a new hawk moth picks up the gaseous chew-plume as she checks out the plant as a possible egg-laying site, she will know three things: (1) there are already hornworms there, (2) the hatchlings have been eating long enough to attract predators, and (3) better find another plant.
Meanwhile, back on the branch, growing hornworms may exhale enough nicotine to repel predatory spiders. In similar fashion, flea beetles can concentrate mustard oils from mustard family plants in their bodies so predators won't want to eat them.
Recent research from Pennsylvania State University indicates that even lowly turnip aphids detect "smells" given off by turnip leaves being fed upon not only by aphids, but by caterpillars, too. Rather than settle in to feed in a crowded house, the cautious aphids will keep looking until they find a healthy leaf. Meantime, it is likely that nearby turnip plants also will have picked up cues that there is trouble afoot, and may have tweaked their physiologies to make their sap less pleasing to aphids.
This does not mean that plants talk to each other, or hear one another's screams, which was the kind of non-science promoted by the widely debunked 1973 book, The Secret Life of Plants. Indeed, the backlash against Secret Life was so strong that important early research into plant signalling, done by David Rhoades at the University of Washington, got swept into its undertow. You can listen to a National Public Radio segment on Rhoades here, but basically he discovered that if you tear tree leaves in half, nearby leaves find out about it and ramp up their self-defence mechanisms. Subsequent research has identified several common plant hormones that become contagious in a cluster of leaves in which some are damaged, which is a simple yet brilliant strategy. If some leaves die, they won't go down without warning their companions to get ready for trouble.
These examples merely scratch the surface of what is being learned about the garden's wisdom. Even slugs play a part. The mucous of some slugs contain hormones that stimulate plants' self-defence capabilities, so that should disease develop, the plant will be able to handle it. Perhaps this is one reason why the slug holes in bean leaves always have clean edges, with no signs of secondary problems. It is even possible that a little slug feeding may help plants protect themselves from disease.
As scientists continue to uncover the mysteries of plant signalling, it will open up a new way of understanding the ten thousand ways that nature's wisdom plays out in our gardens. Stay tuned.
By Barbara Pleasant