Documents: Special Interest: In The Yard:

Care and feeding of shade plants
by Lorraine Flanigan
by Lorraine Flanigan

email: gardenmart@sympatico.ca

Lorraine Flanigan is a freelance garden writer living in Toronto.

She is contributing editor for suite101.com's Gardening in Southern Ontario web site and her City Gardening column appears in Toronto's Town Crier newspaper.


November 10, 1999

With roots firmly planted in the ground, it's hard for plants to run to the local grocer for their daily diet of carbohydrates and sugars. Instead, they've become little green food factories, producing their own food using what's around them soil, air, moisture and light. Plants that grow in the shade are operating food factories with less light than most. To encourage healthy growth despite these low-light conditions, shade gardeners have learned to compensate by improving the other elements needed in the manufacturing process soil, air and moisture.

Here's how:

The delivery system

Soil is the delivery system in the food manufacturing process. Rich in minerals and nitrates, soil delivers nutrients to the roots of plants. How efficiently the delivery system performs depends upon a few things. The first is how well the soil retains water. That's because the roots of plants can only absorb nutrients that are present in water. The best soil for retaining water is a rich, light, humusy soil the kind most often found in woodlands. To make rich woodland soil in your own backyard, work compost, pine needles, well-rotted manure, or leafmold into the soil to a depth of at least six inches. This is not a one-time fix. Humus must be added every year to maintain a light, fluffy texture to the soil. The intake of nutrients from the soil is also dependent upon the concentration of positive ions in the soil. Most of us know this phenomenon as pH, a measurement of soil acidity and alkalinity. Most nutrients are more easily absorbed by roots growing in slightly acidic soil around 6.2 to 6.8 on the pH scale. Add peat moss, oak leaves, and pine needles to increase levels of soil acidity.

Keep cool

Soil temperature is the third factor affecting the efficient delivery of nutrients from the soil to plant roots. Cool soils retain water longer than faster-draining warm soils. The longer water is present in the soil, the more time roots have to absorb the nutrient-rich water. The trick is to keep soil cool and moist, but not soggy. One of the best ways to achieve this balance is through mulching. Mulch provides a barrier against warming summer winds and sunlight and at the same time, allows moisture through to the soil. Use a mulch of smaller leaves, or shred larger leaves. The larger leaves of trees like maple tend to matt, acting as a moisture barrier, preventing rainwater from entering the soil. In early spring, spread organic mulches such as oak leaves and pine needles over woodland beds to a depth of about three inches. To prevent the crowns of plants from rotting due to excess moisture, though, carefully work the mulch around the crowns.

Just add water

During the long, dry days of summer, mulch may not be enough to keep moisture-starved shade plants from wilting. Shade plants face extra competition for moisture from the greedy roots of overhanging trees. Shade plants facing root competition during dry summers may need extra watering. Rather than using an above-ground sprinkler system, though, use a soaker hose or an underground drip irrigation system which leave plants dry and healthy. This is important because in the shade of trees and buildings, the sun has less chance of drying off the leaves, and water left too long on leaves and flowers may lead to disease. To some extent, improving the air circulation in the shade garden also helps keep leaves dry. This can be achieved by high-pruning the lower limbs of trees and thinning shrubs each year.

Working worms

Not only do natural mulches shade and cool the soil, they have an added advantage of providing nutrients to the soil as they decompose. They do this indirectly by attracting worms. Worms love nothing more than a mulch of decomposing leaves. Leaves are rich in nutrients that, until digested and deposited as worm castings, remain inaccessible to plants. As worms pull leaf mulch down into the soil, they also push nutrients and humus to the top where they are more accessible to the shallow feeder roots of plants. As if this wasn't enough to convince you of the benefits of worms, their burrowing also helps keep the soil light and crumbly by making air spaces to trap nutrient-rich moisture.

Diet supplements

The more nutrient-rich the soil, the healthier your light-deprived shade plants become. Often, ensuring that your garden soil is cool, humusy, moist and slightly acidic takes care of most mineral needs of shade plants. However, you may want to supplement their diets with slow-release organic fertilizers. Add compost, liquid seaweed, or manure tea early in the spring as plants start into growth. Alternatively, use one of the many granular organic fertilizers now available in garden centres. Spread a slow-release complete fertilizer such as 20-20-20 over the bed, working it into the soil surface using a garden cultivator.

Lorraine Flanigan Editor: GARDENMart

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