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| Gray Leaved Herbs for a Shade Garden |
Answered by: Conrad Richter
Question from: Kay O’Connor
Posted on: April 20, 2000
The head horticulturist from Montgomery Place, Susan Levy, in Columbia County in upper state New York came by my country home in the same area (Zone 4-5) and suggested that I improve my herb garden by using gray leaved herbs in two of the four triangles I have around a small fountain and a deeper shade of green in the other two triangles to give a more unified look. The only problem is that one of each of the gray leaved beds and deeper green beds are in shade under large maple trees with only late afternoon sun for a couple of hours. The other two beds are getting enough sun to grow lavender, oregano, lovage, and rosemary.
I have tried desperately to locate plants, especially gray leaved plants for the shade but to no avail. Could you please give me some suggestions? Susan is too busy to be able to help it was a miracle that I got her attention when I did. Sweet woodruff grows very well in the shadiest bed that requires the deeper green leaved plants but it is only a ground cover.
I can’t think of many gray-leaved plants that thrive in shade. All of the grey-leaved herbs that I can think of do best in full sun and only a few will tolerate anything less than full sun. A possible reason why this might be is quite interesting.
The gray colour comes from tiny hairs on the leaves. It is thought that hairy plants have hairs to slow down transpiration or water loss. The tiny hairs create what is called a "boundary layer" of air around the leaves that impedes air movement. Slower moving air around the leaves would have the effect of slowing the flow of water vapour out of the leaves.
Why would some plants want to slow water loss? Obviously, plants that grow in dry areas will want to conserve water. Thus any strategy that helps to slow water loss would help ensure the survival of those plants. But if you think about it, loss of water is more of an issue with plants that grow in the full sun. For shade plants, water is usually less of an issue because the soil in shades areas is usually moist. So, following this through to its logical conclusion, it is perhaps not surprising that there are so few gray-leaved shade loving plants.
Water conservation is not the only benefit of hairs on plants; hairs also interfere with the movement of insect pests, so the water connection is not necessarily the whole story. Nevertheless, the vast majority of gray-leaved plants probably are aiming to reduce water loss more than anything else with the hairy leaves.
Gray plants can become less gray when grown in the shade. Instead of a bright grey sheen on, say, woolly lambs ears when grown in full sun, the plants will have far fewer hairs and will look greener in the shade. So, even if a herb is known to tolerate shade conditions it is likely to be less gray (though not necessarily green) in the shade.
The following is a partial list of gray or silver herbs from the Richters catalogue known to tolerate partial shade (but not full shade): woolly thyme, silver mint, woolly lambs ears and silver speedwell.