The language of leaves

I’ll grant that the notion that there’s a “language of flowers” is amusing. For Victorians, rosemary stood for “remembrance” and red tulips announce a “declaration of love.” But choose carefully: yellow tulips indicate a “hopeless love.” These are just three examples from hundreds out there. Amusing yes, but not useful in guiding us on placing plants where they’ll thrive.

The little plastic tags that come with plants are only slightly more helpful: just how much shade is part shade anyway? Thankfully, many plants – though not all – come with idiosyncratic tags that provide a few clues about their preferred growing conditions. These tags are custom-designed by a few million years of evolution: their leaves.

So forget the language of flowers. What every gardener needs to learn is the language of leaves.

I first began to appreciate the language of leaves as my understanding of hostas developed. In the ’90s, I used to plant sweeping masses of green, blue and variegated hostas in the shade for my clients. But after five or 10 years of our Southeastern droughts (and haphazard watering), only the blue hostas survived.

The variegated leaf hostas died first. A leaf is variegated because the white part of the leaf does not contain green chlorophyll. All cells need water and nutrients, but the variegated ones aren’t pulling their weight: without chlorophyll, they can’t turn sunlight into sugars to feed the plant. Despite their visual charm, be aware that variegated leaves are telling you that they just aren’t as tough as a comparable plant without variegation.

Next, look at bluish leaves like those on blue hostas, dianthus, bearded iris (Iris germanica) and giant coneflower (Rudbeckia maxima). Rub a spot with your thumb and the blue comes right off, revealing that the leaf is just as green as any other plant. The blue we see is just a thin coating of wax. It keeps the leaf from drying out during droughts.

The wax reflects light and therefore sheds heat. That keeps the temperature of the leaf down. That’s valuable because plants are similar to cold-blooded animals like frogs and turtles: their metabolism rises with the temperature. So keeping the temps lower means the plant needs less water.

A plant would only go to the trouble of making a waxy skin if evolution favored the waxier plants during dry spells. Their progeny lives on while less waxy siblings die out.

So when you see a plant you don’t recognize at the nursery and it has a bluish tinge of wax, you’ll know that it can handle dry spells better than most.

Hairy leaves tell a similar story. Like wax, a coat of fuzziness reflects light. Dense hairs on a leaf also create a very slender windbreak that reduces transpiration, or evaporation of water from a plant’s leaves. Lamb’s ear, wooly thyme, rose campion and catmint are all good examples of this drought fighting technique.

The size of leaves can reveal a lot too. Generally, the bigger the leaf, the more water the plant expects. The smaller the leaf, the more drought it can handle.

Think of the spaghetti-thin leaves of pine trees and other conifers like cedar, spruce, fir and juniper. By reducing the surface area of their leaves to a bare minimum, these plants can survive droughts that would fatally drain large-leaved plants. Most of our Mediterranean herbs also have small, skinny leaves: rosemary, lavender, chives and thyme. These plants are telling you they can handle life on a high, dry, sunny site.

At the other end of the size spectrum are very large leaves like banana trees, cannas and brugmansias. Big leaves absorb a lot of heat and transpire a lot of water in summer. They’re telling you they want to be in the parts of the garden that stay moist, like the bottom of a hill, near the drain line for the air conditioner or someplace easily watered.

Size also explains the challenge with green hostas. Their big leaf tells you they expect more moisture than woodsy soils in the Piedmont normally provide.

There are, of course, exceptions to every generalization about leaf size or color. But by learning some of the language of leaves, we gardeners can bump up our batting average on choosing and siting plants correctly. And this “language” also gives us one more thing to talk about over herb-infused cocktails in the garden.

This story was originally published August 5, 2016 at 11:46 AM.