Touch Your Tomatoes: thigmomorphogenesis

Yes, that’s what I said: thigmomorphogenesis. It may sound like a horrible spontaneous infection or a non-word entirely, the pseudo-scientific version of a thingamabob, but it’s not; it’s the horticultural term for the fact that plants’ growth patterns are altered by physical touch and agitation, whether by wind, rain, snow, contact with other plants, or—and now we’re getting to the heart of the matter—touch. All of these things cause upward growth to slow and stems to become sturdier.

The picture below shows a tree that didn’t get to experience it.

Source: Linda Chalker-Scott, Aug. 2005 horticultural myth on thigmomorphogenesis.

This phenomenon (but not, I’ll admit, the term, which I had to look up) came to mind as I read a recent post at A Way to Garden. Margaret’s comprehensive list of “tips for growing better tomatoes from seed” is all-around useful and informative, not to mention engaging. No surprises there, to anyone who reads her regularly.

But what caught my eye was her advice to set up a fan to give tomato seedlings a bit of a breeze, which will make them stronger and sturdier. In fact, Margaret also advises brushing the seedlings daily (with your hand, not a hairbrush.)

I’d have regarded this advice with some skepticism if I hadn’t recently read a short article on touching plants by a horticultural professor out Washington way, Linda Chalker-Scott. She used to write a monthly column on one gardening myth or another for the endangered Master Gardener Magazine, but since becoming editor has cut back to a few times a year. (Read Susan Harris’s post at Garden Rant, “Help Keep Master Gardener Magazine Alive” on why the magazine is endangered and what we can do to save it.) All of Chalker-Scott’s pithy articles are available on-line in PDF format at her website under the heading "Horticultural Myths."

In August of 2005, she tackled the following myth: “Unless It Causes Visible Damage, Touching or Brushing Has Little Effect on Plants.” She acknowledges that “plants responding to gentle stroking is a little too ‘touchy-feely’ for many of us,” but she summarizes the scientific literature, explains the term (and it does need explaining!) and concludes that “The result of thigmomorphogenesis is a stocky, sturdy plant that is more resistant to breakage or windthrow than one that has been untouched.”

Chalker-Scott never mentions tomatoes; she concentrates on staking trees, a practice that she refers to as “tree bondage” in another column (Mar. 2001; "Newly planted trees should be staked firmly and securely"), where she confesses to “playing tree liberator on more than one occasion.” As she explains, unnecessary or overly tight staking deprives trees of the chance to move with and resist wind. As a result, their roots tend to grow shallow and their trunks thin. If and when the stakes are removed, such trees are easily uprooted or snapped.

By now it should be clear why Margaret recommends providing a bit of fake wind for tomato seedlings started indoors. There’s no wind inside, and generally no insects landing on leaves, no plants brushing up against each other, no rain pattering down for hours, indeed none of the usual sources of stimulus or stress. Of course, that’s one reason we start plants indoors in a protected environment (that, and a chance to get a jump on the season.)

But just as children can be over-protected, so can plants. Which is why you should touch your tomato seedlings, and probably any plants started indoors. Give them a slightly less protected childhood, and they’re more likely to make it to adulthood.