In the use of perennials another problem is often met: that of selecting proper types for planting at the water's edge. These plants must be of the kind that will thrive with their "feet in water." The group from which selection can be made is comparatively limited, especially with reference to genera. Many of these plants, such as the lemon lily, the loosestrife, and the iris will soon spread beyond control if not carefully thinned out and kept within bounds during the succeeding years. Most of this material is adapted to growth in the open sun and will not withstand extreme shade conditions.
In the selection of perennials to be used in the development of the large flower garden areas the entire field of perennials is open from which to choose. The development of a small, refined flower garden, the intimate details of which add to its charm, requires a more careful knowledge of perennials, especially of those which usage has proven cannot be safely introduced into such limited areas. These types of perermials should be avoided in the development of a small flower garden, They can be used by one who will give them consistent attention to keep them within bounds by staking and cutting back. Otherwise they will produce a loose, ragged effect not in keeping with the neat lines desired in a small garden, and will often crowd out interesting types of smaller perennials which mean more to the success of the garden.
Most varieties of the hardy aster or Michaelmas daisy, the sneeze-weed, the loosestrife, and the plume poppy are too rampant and vigorous in their habit of growth to be successful in a small garden.
The most important requirement in the development of any flower garden is to provide perpetual bloom throughout the growing season. There are a variety of combinations of perennials which can be used to accomplish this purpose. As illustrative of a possible range of plants it is well to list some of the thoroughly tested varieties that will grow in any good garden soil and which will provide flowers from early spring until late fall. For a person who understands plants to some extent the lists of perennials, grouped according to colour and season, will make an excellent source of reference from which to select types for continuous blooming effects.
There are some perennials, among the most important of which are the peony, of which the blooming period is very short and the foliage effect during a great part of the summer may be consequently monotonous and uninteresting. In such plants it is highly desirable to have touches of colour throughout the later parts of the season. To accomplish this there can be introduced, among the peony plants, such types as the monkshood, the blazing star, lilies and gladioli, to provide flowers and add interest to the otherwise monotonous mass of green leaves.
A considerable part of the success of any perennial flower garden is the presence of groups of perennials which present good blooming combinations. Often a garden is seen where some particular colour note attracts special attention. On close examination it is found that this effect is produced by a combination of colour brought about by the successful grouping of two or more perennials. The average garden lover cannot become familiar, from his limited study of plants, with all of the interesting types of perennials which produce colour effects that harmonize with each other. A list of these groupings has been included in this chapter, and through further study many others may be found which will be equally effective.
Annuals are most often planted because of their ability to produce flowers for cutting. Many perennials are planted for this same purpose. There are a few perennials, such as the blanket flower, ball of snow, larkspur, and marguerite, which are benefited by constant cutting, and the flowering season of which is lengthened through this process. There are other perennials, such as the foxglove, peony, and iris, with which the process of cutting flowers does not encourage. growth of others during the same season. If such perennials are to be used for cut-flower purposes it is best to plant them in a distinct cut-flower garden. The same discussion concerning a cut-flower garden, and a flower garden as an interesting design, applies to the planting of perennials in the same manner that it applies to the planting of annuals (See Chapter XXXII (Annuals), Page 238).
The majority of perennials will continue to increase from year to year and will require "dividing" and transplanting every two or three years. There are other perennials which should be treated as biennials and accordingly replaced completely by new plants at the end of every second year. These plants, such as the foxglove, white pink, English daisy, and bellflower will "run out" after a period of two or three years. They will still continue to grow, but their vigour will be so much less that their presence will be but an apology for strong, healthy specimens. All of these plants are known as perennials; but in reality they develop only as biennials. Other perennials, such as the aster, phlox, and iris, which grow into large clumps, should be divided at least every three years. If they are not so treated they will become crowded and the plants will not have space to develop properly and the result will be spindly, unhealthy plants which will not produce normal flower effects. But the peony, in good soil, with space of a diameter of approximately three feet in which to grow, is best left to grow undisturbed for a score of years or more (Page 88).
Taken on the whole, no more picturesque or graceful effects can be produced anywhere than by appropriate planting along the banks of ponds and streams. The more bold and picturesque a planting mass is, the better it looks when reflected in a still pool; while the flowing lines of a stream are supplemented by the graceful, arching branches of shrubs and vines. Among the most successful and beautiful plants for watersides are the herbaceous perennials.
Perennials for planting in deep water are largely confined to the lotus and water lilies. These plants should not be permanently planted in ponds which freeze solid during the winter, nor where there is not plenty of rich soil on the bottom, and an abundance of clear water and uninterrupted sunlight. The best locations are on the margins of sluggish streams and of bays and in sheltered nooks. Water which flows too swiftly or is too cold or contains mud is not good for aquatic plants, nor should they be planted in newly constructed cement tanks which have not been thoroughly washed and rinsed so as to remove all the caustic property of the new cement. The best fertilizer for aquatic plants is cow manure, which may be mixed with twice its bulk of strong loam and used for planting beds.
In the water near the margin of a pond many more sorts of aquatic and bog plants may be used, such as the native irises or flags, water plantains, bulrushes, arrowheads, and marsh marigolds. These plants are more hardy and less exacting in their requirements. Indeed, they are likely, when congenial conditions occur, to grow so luxuriantly as to prove annoying if planted in very large quantities. A rich alluvial mud provides the proper soil for most sorts, and once established where there is not too much lime in the water, or too swift a current, they will take care of themselves.
For planting on the land at the water's side, a still larger list of plants is available. These include many of our common herbaceous garden perennials, such as sneeze-weed, Japanese iris, and lemon lily, as well as native herbs, such as gentians, cow parsnips, and some of our native orchids. With these perennials should be combined, if possible, some of the moisture-loving native shrubs. For this purpose nothing is better than the swamp honeysuckle, button bush, red chokeberry, rhodora, leather leaf, and wild rosemary, not to mention the more commonly known dogwoods or cornels.
If no special place is assigned to perennials, room may always be found for some in the shrub border. Here there should be reluctance to place any sorts that require considerable culture or the full development of which might be desired, particularly if they be sorts that are prized. One would be loath to subject a valuable variety of the peony, for example, to a life-long competition with vigorous shrubs which, in addition to sending out more rapid-growing roots, would have the advantage of overtopping it. But there are certain types of perennials that can, in every way, be appropriately used to fill bare spaces among shrubs that do not yet cover all the space, or at the front edge of the border. Here at the edge, if the shrubs do not droop too low or are not too vigorous in their habit of growth, may be found a place for a fine thing like the evergreen candytuft. In the edge of the shrubbery bed can always be found room for some bulbs; they really seem to prefer the slight protection of the overhanging branches and the soil around the roots of the other plants. An additional consideration is that the flowering season of bulbs is not encroached upon by the foliage of the shrubs, as would occur to the detriment of perennials that flower later in the year. Formal regularity in planting should be avoided because most perennials and bulbs appear best in small masses or clumps.
In designing a border planting of perennials or annuals located at the edge of masses of shrubs an ample width of four or five feet should be allowed, especially if this is the only place for the development of a flower border. Unless this provision is made and frequent pruning of the shrubs resorted to, the branches of the shrubs even then are apt to encroach upon the smaller plants at the front. This does not, however, apply to bulbs. Where it is necessary to develop a flower border in combination with a border of shrubs which shall serve as its background, little success will follow the attempt to develop such a border, especially in relation to tall-growing shrubs, if the flower border is placed upon the north side. If the shrub border or hedge is to consist of tall and vigorous-growing shrubs or columnar trees to provide a screen against objectionable views, the designer must always remember that competition of perennials with the greedy root systems of such plants will starve the perennials.