In the Garden:
New England
October, 2004
Regional Report

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Lightly petting the tops of your growing seedlings can help them grow sturdier.

Petting Your Plants

My first introduction to the science of growing plants was in 10th-grade biology class. I measured the effects of spraying plants with different amounts of gibberellin -- a plant hormone that causes cell elongation. I mixed up a solution of the chemical and bought ten tomato plants of the same height. Plant #1 was the control, no treatment; plant #2 got one squirt from the bottle, plant #2 got two squirts, etc. Over the next several weeks I tended and sprayed and measured. It wasn't a surprise that the plant that received the most hormone grew the tallest. But it was still fun to watch.

I'm still intrigued by the ways in which plants respond and adapt to their environment, even when the scientific explanation may be a little shaky. You've probably heard about studies that indicate plants grow better when exposed to classical music than to heavy metal music, or that you can improve your plants' growth by talking to them. I can't speak to personal success with either of these techniques, but I'm convinced that petting my plants makes a difference. No, they don't grow better because they are happy to have the attention, like a cat or a dog. Petting your plants seems to have the opposite effect of gibberellin: it encourages them to grow shorter and stockier. And there's actually some science to back this up.

The Touch Response
Scientists have a name for the touch response in plants: "thigmotropism," or the "thigmo response." Plants show this response in a variety of ways, depending on what's doing the touching (people, animals, other plants blown by wind) and what kind of plant is being touched. When the growing tips of morning glory touch an object, for example, they begin twining around it. But a typical effect in many plants is decreased height and thicker stems.

Tomato plants, for one, have been found to respond quite dependably to being touched. We can use this to our advantage when we're trying to grow strong seedlings for transplanting outdoors. The biggest drawback to starting seedlings inside is the legginess that results from the plants stretching because of inadequate light. You can help overcome this by touching your plants.

Even though I use grow lights, I gently brush my hand over the tops of all my seedlings every day or so, just barely touching them. The growing tips and unfolding leaves apparently are the most sensitive, but structural changes also can occur in the roots. There's evidence that this kind of disturbance early in a plant's life may even affect its growth much later. Even larger plants respond to touch. Some people recommend petting houseplants to keep them in bounds.

Wind can have an effect similar to touching. You may notice that plants in windy areas of your yard grow stockier than those in protected areas. You can mimic the wind by setting up a small fan to move the air around your seedlings to help them grow stronger. I use an oscillating fan and move it every couple of days to different flats, turning it off at night. (This also can help keep disease from getting a foothold.)

Perhaps this thigmo response helps plants better survive in an environment where there's a lot of wind, animals, or other mechanical stimulation. Scientists aren't sure. Hmmm, now that I think about it, my daughter has a science fair coming up in the spring ...


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