dry shade gardening

My garden, as I’ve said many times, is a 50-by-120-foot suburban lot with the usual house, driveway, garage and paved walks. Among those elements are half a dozen beds or borders containing combinations of shrubs, perennials, and self-sown annuals. There are also small areas of grass (sometimes called “the lawns,” although that sounds rather grand).

The soil is a light sandy loam, with good drainage. The climate is semi-Mediterranean, which means mild wet winters and dry summers. Recent summers have been drier and warmer than normal. Most of the plantings are affected to some extent by four large Norway maples, a big birch and a medium-sized Tree of Heaven (Ailanthus altissima).

In this difficult situation — dry, rooty shade — a few reliable tough plants perform quite well, because they are tough and almost weedy. Properly managed, they can be quite ornamental. I plan to write several posts this summer about how I use these plants in my garden.

First up is Purple Toadflax (Linaria purpurea). Apparently, the leaves resemble those of the flax plant, which explains the “flax” part of the common name and the “lin” (linum is Latin for “flax”) in the genus name. I’m not sure where the toad comes in.

The plant grows between 1 and 3 feet (30 cm to 1 metre) tall. In shade, it can reach 4 feet. It’s somewhat slender, so inserts itself easily among other plants without crowding. Most plants have between 4 and 10 stems. The stems are stiff and never need staking.

The individual flowers are like tiny snapdragons. They cluster in spiky racemes at the tops of the stems. The standard colour is medium to dark purple. I’ve had a few plants whose flowers are purple and white, but they’re rare. Pink flowers are more common than the bicolours. They’re a fairly pale pink, though; their main interest comes from being different from the norm. The standard purple is a fine colour, contrasting well with the grey-blue-green of the slender leaves.

Purple toadflax is of no interest to deer. It grows well and blooms reliably in dry, rooty soil and shade. It thrives in sunny spots. Bumblebees love it.

What’s not to like? Well, I did say it’s a quasi-weed. Left to its own devices, it blooms in May and June. The flowers are succeeded by seed pods that look like tiny green beads. At this point flowering stops. Seeds ripen by summer’s end and scatter when the plants are disturbed. The following spring hundreds of seedlings emerge, producing more toadflaxes than anyone wants. An untended patch of toadflax is reminiscent of gravel parking lots, not garden borders.

Rigorous deadheading is the way to curb the plant’s weedy tendencies. Clip off the spikes of bead-like seed pods as soon as they form. You can either cut each thin side-stem or wait until the whole raceme has pretty much finished flowering and cut the main stem just beneath the flower cluster. The plant will put out new flowering side-stems and keep blooming all summer. A bonus of this process is that the foliage of plants growing in sunny spots turns interesting shades of pink in autumn.

The trick is to deadhead before the seeds ripen. Inevitably, a few plants escape my notice. I discover them when I’m doing fall cleanup, by which time the tiny seeds scatter far and wide. A desperation measure for this situation is to carefully clip the seed-bearing top of each stem and transfer it into a pail or similar container, preventing seed scatter. I suspect semi-ripened seeds are able to finish maturing in the compost pile, so it’s best not to compost them unless you’re sure your heap achieves temperatures high enough to kill seeds. Mine doesn’t.

This is why I know how well toadflax blends into plantings. I don’t remember ever actually planting it. I go through the beds in spring and remove excessive seedlings and badly-positioned plants.

Toadflax is a short-lived perennial. Individual plants persist for two or three seasons. Eventually, they get woody at the base and perform poorly. By that time, though, there are more than enough young plants to replace them.

I wouldn’t recommend purple toadflax for mass plantings. The skinny, diagonally placed leaves produce a visually tedious herringbone tweed effect. Besides, this mass planting effect is what you get when you let toadflax self-sow with abandon. I like them best among other plants with contrasting leaf shapes.

More info about Purple Toadflax is available at the Missouri Botanical Garden site and Wikipedia.

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.