Dry Shade Revisited

Since I started this blog, nearly two years ago, by far the most popular posts have been the two on tough plants for dry shade, especially in spring when people start planning their garden activities for the coming season. It seems that there are many gardeners struggling with dry shade, trying to find plants that tolerate dry shade conditions and look good.

I’ll bet this is a common scenario: someone buys a house and lot and decides to make a garden. But the lot harbours at least one, and possibly several, large trees. Even if the effort of digging up the rooty soil under the trees to make planting beds doesn’t discourage the prospective gardener, the eventual failure of most of what is planted will do the trick. Unless the gardener is willing and able to do tree removal, that may be the end of their garden.

Or it may be a more insidious situation, like my own. When I began gardening here in 1992, the two large Norway maples on the western edge of my lot had been lopped by the previous owner of the place. Ugly though this treatment was, it meant that the back yard got a fair amount of sunlight. The first year or two, I was able to grow cucumbers and even corn in my minute vegetable patch, not to mention tomatoes and pole beans.

Ten years later, the maples had regrown and moreover had sent out a mat of roots into the vegetable patch (not to mention the perennial beds that are closer to their trunks). Shade was way more prevalent. Cucumbers? Forget it. (I gave up on corn once I realized how many raccoons lived in the area). The tomatoes did less well every year, until by 2009 I had resorted to growing them in big pots parked in whatever sunny spots remained.

In the perennial beds, it soon became clear which plants could tolerate shade and root competition and which could not. Delphiniums and lilies, even the tough Asiatics, dwindled away over several years. Blue poppies gave up immediately when roots re-invaded their carefully prepared bed. This spring, things don’t look good for a tree peony that did relatively well for years. About the time I gave up on tomatoes in the vegetable patch it started to weaken and may now be in its final gasps.

Shade is a problem for sure, but it’s more than that. There’s also the matter of what’s happening underground. Roots, in other words. Most of a tree’s feeding roots are in the topmost one or two feet of soil, right there with the roots of herbaceous perennials and small shrubs. They compete with the smaller plants for water and nutrients, but they may also do something else — a phenomenon known as allelopathy, which is a process by which a plant produces biochemicals which affect the growth of nearby plants. Norway maples are suspected of negative allelopathy, especially in places where they are an introduced species. So is the Tree of Heaven, under which I planted my doomed blue poppies in the fall of 2000. They bloomed beautifully the following spring, but died soon after.

There is almost no mention of allelopathy in most gardening books, even those that have sections on shade gardening. From my rather superficial internet research on the topic, I see that much of what has been written is academic papers in forestry and agriculture, or to do with the interactions between introduced trees and native understory plants. A few bloggers have written about it as well, for example, this post by Curtis Adams. There is definitely scope for investigation of this subject as it pertains to ornamental gardening.

When I first discovered allelopathy, I had an Aha! moment. Yes, those tree roots are indeed evil, poisoning my delphiniums as well as bullying them. But in the end it comes down to this — either I get the maples cut down, or stop gardening under them, or do as well as I can with conditions as they are. Which I’ve known for years.

Another option, of course, is to move. Drastic, perhaps, but if I ever purchase another patch of land with the intention of gardening upon it, that patch will be tree-free. In the meantime, this year’s massacre of maple seedlings has begun.

Happy Plants in Dry Shade