Simply saying a plant will grow in the shade is too simplistic a statement because not all shade is the same. There is filtered shade, partial shade, open shade, and dense shade. Shade changes with the time of day and from year to year as trees grow. Sites that might be in full sun part of the year may become heavily shaded as the season changes or as trees leaf out. Light is also influenced by topography. For example, a south-facing slope receives more light than a north-facing slope.

Filtered shade (dappled sunlight) is suitable for growing many plants --- even plants that prefer full sunlight such as iris or daylilies. Light intensity is relatively high with sunlight and shade constantly changing from minute to minute during the day. Filtered shade occurs under birch, mimosa, and honey locust trees. Partial shade changes as the day progress. The area may be in the shade until some point in the day then the area received full sunlight. Open shade occurs where there are no trees overhead to block sunlight but the plants are in the shade due to the shadows of a building. Open shade is well lighted but does not receive direct sunlight. Deep shade is the most restrictive type of shade. Deep shade is found in heavily wooded areas and in landscapes where large evergreens or broadleaf deciduous trees (maples, oaks, hickories, beeches) occur. Deep shade can also occur in a narrow side yard on the north side of the house when another building is located close by or in a recessed entry way on the north side of your house.

Many shade plants are native to wooded areas and grow best in soils exposed to decomposing leaf litter and compost. Moisture in shaded areas is different than sunny areas. The cooler temperatures and less exposure to wind decrease water loss. However, competition from tree roots and the large, tender leaves of many shade-loving plants can cause moisture shortages. Trees vary in their competitiveness for soil moisture. Some shady sites can be quite dry. Many plants will grow in the direction of the strongest light; one side of the plant will be thick and full while the other side will be sparse.

Tree shaded gardens become more shaded with time. As trees grow taller and wider they cast larger shadows and less light will penetrate the increasingly dense shade. The quickest way to admit more light is by removing lower tree limbs thus raising the height of the shade. This will decrease humidity and allow some filtered light to reach understory plantings especially in the morning and afternoon.

Canopy thinning involves selectively removing trees limbs. This could involve removing a few, large limbs or many, small limbs throughout the tree. The results is a less dense shade or even perhaps a dappled sun-shade situation. Thinning is not a one time procedure; it will need to be repeated as trees grow. The same process can be used on large shade-casting shrubs. Some shrubs can be pruned into a tree form thus allowing more light to plants growing near their base.

Trees differ in the number and depth of their roots. Maples, for example, have numerous shallow roots which makes digging, planting, or growing plants under them difficult. While tilling, creating raised beds, and root pruning are methods to cope with tree roots, these methods can lead to the decline or even death of the tree. The worst location for starting a raised bed is at the base of a tree; the additional soil can lead to decay organisms attacking the trunk or the main roots that support the tree. These methods are temporary at best since tree roots will grow into the newly amended soil or raised beds. In order to protect the tree, limit tilling and the addition of topsoil to a small percentage of the total root system. Dig individual holes for shrubs and flowers instead of preparing beds. Filling around trees with compost, bark, and sandy loam for a raised bed is another option --- provided it is done in moderation.

Consumer Horticulture | Quick Reference

© Erv Evans, Consumer Horticulturalist
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