We’ve all got places in our gardens where it seems nothing will grow. Only occasionally is this impression correct. If a previous owner dumped crankcase oil in the spot or applied a particularly persistent and virulent herbicide, the soil could be toxic. (To check for petroleum products, smell the soil. For herbicides plant radish seeds and keep them moist. If they don’t germinate within a couple of weeks, something is amiss.)
Commonly, dry shade is the culprit. Spruces are notorious for creating this condition: Their shallow roots suck up moisture from the soil. But just about any mature shade tree can have the same effect, turning the real estate around it into the garden equivalent of the Sahara Desert.
A large number of plants can grow where conditions are dry – the deserts of the world are not plant-deprived. An equally large number of plants are adapted to shady places. It’s the combination of these two stresses that is the killer. Natural habitats reflect the reality. Think back to the last time you hiked through a climax forest. Unless it was in the rain forest of the Pacific Northwest, the forest floor was likely wide open and relatively free of vegetation.
We can mitigate the extreme dryness, at least somewhat, by installing an irrigation system. There is a slight risk of harming the tree with overwatering, but since mature trees consume huge amounts of water, this isn’t a great risk. The trick is to water lightly and frequently enough to keep the surface of the soil slightly damp for the garden, as well as soaking the soil more deeply from time to time to meet the needs of the tree without keeping the subsurface continually saturated.
The dry shade beneath a low deck or other structure is no less problematic. While you may not have tree roots to deal with, the issue here is preventing damage to wooden posts or the foundation of the house. Sodden soil and moisture fluctuations can harm structures, which explains the tradition of latticework “skirts” used to hide the bare dirt underneath a deck. Lumber made from recycled plastic won’t rot, but there are undoubtedly easier places on your property to garden.
Though I’m not personally a fan of the grottos popularized in English historic gardens, if you must have one, this might be the place – hermit optional.
Adding organic matter can help improve the water-holding capability of soil. For this, peat moss is best but any good quality compost also is beneficial. Use care, though, when amending soil beneath a valuable tree. Digging can harm a tree’s roots and eventually lead to its demise.
There are a few plants that come close to meeting the challenge of dry shade. The following are worth a try, as long as your site is not too dry or too dark for any living thing:
Periwinkle Vinca major, partridge feather Tanacetum densum amanum, ‘Claude Barr’ penstemon, sweet woodruff Galium odoratum, hardy cyclamen C. coum and C. hederifolium, bear’s breech Acanthus hungaricus, pearly everlasting Anaphalis margaritacea, heartleaf brunnera, English ivy, candytuft Iberis sempervirens, archangel Lamiastrum galeobdolon, Sedum spurium, compact Oregon grape, Juniperus communis, stinking hellebore, and wild ginger Asarum europaeum.
Marcia Tatroe is a garden writer and lecturer. E-mail her at rltaurora@aol.com.
