The plants which most successfully fill the requirements for undergrowth planting are our native woodland species. These requirements are mainly the ability to succeed in partial or dense shade and also to survive the struggle for moisture, food, and room which always exists in a naturalistic planting where the ground below and the air above are already well occupied by large trees. Our northern forests contain a profusion of plants which will succeed as undergrowth. But too often in the past not enough care has been taken to choose only those plants which are desirable from an ornamental point of view. It is possible, however, to select from the large amount of available material all the plants which are necessary to carry out an undergrowth planting on any scale, and at the same time use only plants which are desirable on account of their flowering habits, their fruits, the autumn colouration of their leaves, their evergreen character, or some equally valuable characteristic quality.
One of the best examples of a successful shrub for undergrowth is the well-known maple-leaved viburnum, which produces white flowers in June and blue fruits in the autumn, and whose leaves in autumn have a striking pink colour. Other shrubs which lend themselves very readily to undergrowth planting are the sweet pepper bush, with its profuse white flowers; the Carolina allspice, on account of its fragrance and autumn colour; and the chokeberries and winterberry, useful for their striking autumn fruits. Among the coniferous evergreens the native hemlock and the balsam fir are very satisfactory plants. The Douglas spruce also promises well and is adaptable to almost any soil.
In choosing smaller plants of perennial herbaceous character much care should be taken to avoid weedy species or those without some especially worth-while characteristic. There are many native forms which are valuable for their green foliage alone, such as the Christmas fern, which is evergreen also, the ostrich fern, Clayton's fern, and the maidenhair fern. Among the valuable asters are several which bloom profusely during the late summer and autumn months when other woodland flowers are scarce. The old-fashioned dead nettle, which does not sting, and its variegated leaved variety, provide flowers from May to September, when colonized in moist shade, and the goldenrods also, such as the blue-stemmed and the variety called speciosa, enliven the woods from August to October. Among the plants which will be found valuable for forming mats of ground cover and some of which are evergreen in character are the moneywort, English ivy, running strawberry-bush, spotted wintergreen, and the dwarf cornus or bunchberry.
As a rule these plants succeed best when planted in small colonies and when used to face down clumps of shrubs which may in turn be used against an evergreen background. Thus, plantings may be grouped so as to provide interesting combinations along the sides of paths and at ends of vistas. If the natural mulch layer has disappeared an effort should be made to reproduce it as soon as possible after planting and care should be taken to see that forest fires do not burn off the autumn leaves which nature provides for a winter cover. When leaves drift in so thickly as to threaten to smother the smaller plants a portion of these leaves may be removed; but as a general rule it is not wise to do too much cleaning up unless the desirable plants are in danger of being overwhelmed by the mulch or by larger native plants. One of the most common faults in woodland landscape developments is the attempt to "clean up" existing undergrowth rather than to study its interesting possibilities in combination with many types of plants valuable for foliage, flowers, and fruit.
All these types of plants are adapted for use in woodland wild garden areas, and without exception they will in time naturalize themselves. In developing plantings of this type it is much better practice to lay the foundation during the first season by planting sparsely over the entire planting area. During the succeeding season many plants can be added to supplement the planting which is in place and to replace those which have died in the process of establishing themselves. The best results are obtained by so laying out the planting development that a period of at least three years is required in which to put all of the material into its permanent location. The development of mass plantations under heavily shaded conditions is quite a different problem from the development of mass plantations on open and refined lawn areas where planting should be practically completed during the first two seasons. Experience has taught those who have watched this type of plantations develop that a great percentage of loss must be anticipated, for two reasons: In the first place, plants are placed under abnormal conditions of lack of sunlight, and second, the available artificial water supply is apt to be very limited. The process of naturalizing plants and acclimating them to conditions of this kind must naturally be a slow process if the results when the work is completed are to be a success from a landscape standpoint.
It is quite essential in naturalizing perennials in a wild garden that conditions similar to those under which the plant was previously growing should be reproduced. A number of wild garden developments have at different times become failures because as trees have died, thus changing the conditions of shade, these trees have not been replaced, and the result is that this changing of shade conditions has caused the killing out of many types of perennials which are especially susceptible to changed conditions of this character.