What, to plants, is the meaning of shade? Have we ever asked ourselves why the sunflower loves the sun, the wood anemone the woods? Why is it that certain plants can be grown successfully in partial or even dense shade while others scarcely seem able to exist without abundant sunlight?
The two plants cited have evolved under different light and growing conditions. For an explanation of their continued behavior let us see whether these conditions may not have affected their present structures.
Light is Essential To Plants
It provides the energy which is required in the manufacture of sugars and other foods from the carbon dioxide of the air and the water of the soil. Carbon dioxide is taken into the plant through the thousands of small pores, or “stomates”, which occur on one or both sides of the leaf.
It is the green pigment, chlorophyll, which entraps the light energy required for the chemical processes involved. Now, the rate at which gasses can be taken into the leaf varies greatly with different plants, for some plants have many more pores upon their leaves than others.
A sunflower, growing in full sunlight, may be able to utilize all the light it receives because it is well-supplied with stomates; it can take in carbon dioxide freely and can manufacture food at a maximum rate.
A shade-loving plant, however, like oxalis or Thalictrum, may have many fewer breathing pores; its capacity to absorb carbon dioxide may be very much lower. In partial shade, this will not matter, for available light energy and carbon dioxide are both comparatively scarce, but if the plant is moved into open sunlight its process of food manufacture will still be limited by its slowness in absorbing carbon dioxide.
The extra light received by the leaves will be absorbed as heat; an increased water loss will result and the plant will suffer as a consequence. In extreme cases, the beating effect may be so marked that the essential green pigments may be partially destroyed.
In other words, with additional sunlight, the sun-loving plants can manufacture more food and grow more rapidly while the shade-loving plant may even produce less food than formerly and will certainly suffer from excessive loss of water. The rate of growth, instead of being increased, may be diminished.
Reverse Process
The shading of plants that normally grow under conditions of full sunlight. In partial shade, light energy is reduced and the rate of food manufacture by such plants will almost certainly be slowed up. Water and mineral nutrients, however, may be absorbed at an excessive rate. The result is a soft, elongated growth that uses up most of the food and leaves little or none for the production of flowers or fruits.
Or again, in fairly dense shade, the effective period of light duration or “day length” may be sufficiently reduced that flower production by certain plants requiring long days for Power bud initiation becomes impossible. A scarcity of flowers is frequently associated with insufficient light.
Some plants, when shaded, may manufacture so little food that they are almost incapable of surviving, particularly if the shade is dense, and as is frequently the case under garden conditions, soil and other factors are not as good as they might be.
Shade tolerance is most certainly increased by any improvements which can be made in these other growing conditions. It is an interesting fact that many woodland plants, especially bulbous types such as Trillium, can survive and increase because most of their food manufacturing is done early in the year.
They grow, blossom, and store their food before they are shaded at all, for the trees above have not yet conic into leaf. For this reason, very many garden bulbs can be grown successfully beneath trees, for shade after blooming is no longer important.
Growing Plants Under Shaded Conditions
In garden practice, if plants are to be grown under shaded conditions, the factors which determine whether any one plant will succeed or not are twofold: Firstly, how intense is the shade, and how long will the plants be subjected to it? Secondly, how suitable are other environmental factors such as soil and moisture?
The “natural” shade most frequently found in gardens is that of buildings or trees. Shade cast by buildings is a very constant type. Its duration may be for a part of each day or, in a northern exposure, the whole of the day.
Shade intensity will vary with the proximity of other objects. Soil and other growing conditions may likely be reasonably good.
The shade of trees is rather different. A heavy canopy may be too dense to permit anything but a few weeds to grow beneath it however good the soil. On the other hand, the leaf covering may be such that sufficient light may penetrate, but the soil beneath may be poor, dry, or filled with a tangled mass of tree roots.
Should the features of both be unhappily combined, as we do not infrequently find beneath such trees as Norway or sugar maple, the situation then is apt to resemble a dragon which no St. George, be he sturdy myrtle or persistent fescue, can conquer. This is an extreme case.
The dappled shade of open woodlands, where the sun shines through for perhaps an hour or so each day, presents little if any difficulty. Soil alone will be the guide in selecting from the host of plants that revel in a condition such as this.
What are some of the better plants for use in these various conditions?
Beneath a heavy canopied Norway maple, it may be possible to find one or two plants that will exist – certainly, none will flourish. If the soil beneath such a tree is dry and filled with hungry roots, it will probably remain bare no matter what is planted. Such a place would conceivably be unusable and less unsightly if covered with paving stones and furnished with a seat.
Most trees, notably oak, elm, sycamore, hickory, and so on, do not cast so dense a shade but that some plants can be grown beneath them with fair success. If the grass is desired, and the Kentucky bluegrass of the regular lawn mixture has already died out, it would be well to break up the soil and reseed with red fescue, or rough-stalked meadow grass, using the red fescue for dry places, the other where the soil is fairly moist.
The little evergreen Vinca, or myrtle, will often succeed in dry soils under the comparatively heavy shade where the grass of any kind would not. It is one of our most valuable ground covers, particularly for use under surface rooting trees such as elm.
Japanese spurge is another first-rate ground cover for any shady location in rather better soil. It likes peat moss and a moist root run. Where spurge will grow, Spring bulbs flourish, too, and an interplanted mass beneath an old beech can make a delightful picture in the early part of the year.
In the continuous shade, such as on the north side of a building – a situation frequently met with on city property – several plants will grow well but not very many can be relied upon for flowers. Heading the list of those that will flower under such conditions is the plantain lily, Hosta or Funkia, in several forms.
The luxuriant heart- or lance-shaped foliage and the profusion of white or blue-purple flowers are produced in abundance whether this tolerant plant is grown in full or partial shade, or in soils that are dry, moist, or even wet.
Lily-of-the-valley; Thai Id rum; various forms of Dicentra or bleeding heart; Spider-wort, Tradescantia; and Jacob’s ladder, Polemonium, will all succeed and flower in heavily shaded situations. In proper soils, Leucothoe and a host of ferns can usually be relied upon. For ground cover, to Japanese spurge and myrtle, we might add English ivy.
Against a brick or stone building, it will act as a vine. Although only partially hardy in our northern regions, it will often succeed when entirely removed from direct sunlight.
It is an example of the use of shade to increase hardiness, for many plants are injured not directly by low temperatures but by hot sunlight which dries out their leaves and branches at a time when the soil is frozen and their roots are incapable of supplying more water at a sufficiently rapid rate.
Another good vine is bittersweet but, as explained earlier, it cannot be expected to fruit so freely in the heavy shade as it does in partial or full sunlight. For taller evergreens, there are none better than ever-reliable Japanese yew and the hardy rhododendrons. For tall plantings, hemlock is next best to Japanese yew.
It is good for partial shade but becomes thin if the shade is too intense. Most other conifers, particularly the junipers, need a generous supply of sunlight.
Under Light Shade
The shade of open woodlands or single high-branched trees – a great many garden plants will grow and thrive. A host of native wildlings such as Hepatica, Trillium, Mertensia, Cypripedium, blue phlox, violets, the bergamots, and bee-balms come to mind. One picture drifts of fluffy, cream Tiarella, multicolored Polyanthus, tall lilies, and scattered spikes of purple foxglove.
For the shrubs, it will suffice to mention two groups by name for the range and number of good varieties are very great. These two are the azaleas and rhododendrons, the best and most colorful of all plants for brightening the open woodland. They flower best when the shade is not too dense; they require fairly moist, acid soil.
As already suggested, it is the combination of shade with dry soil which presents the most difficult problem. Even here, however, we need not be too discouraged for it is possible to find certain ferns, flowering plants, and shrubs of truly astonishing endurance. The five-leaf aralia, with its pleasing glossy foliage, is but one example of these toughest of plants.
If plant needs are understood, if dry soil can be improved by the addition of moisture-holding organic material, and if plant varieties are wisely selected, there is no reason why the garden in shade need be any less beautiful and satisfying the year around than the garden in open sunlight.
44659 by Henry T. Skinner