This section is from the book "Flower Gardening", by H. S. Adams. Also available from Amazon: Flower gardening.
Next, sort the cards according to season of bloom—going by the month or, better still, by fortnights; they cover better the average period of perfection. Lay the resultant packs of cards, chronologically, in a line on a table and see if there are any distinct breaks in the succession or any fortnights that do not admit of enough choice. Should these deficiencies exist, return to the catalogues and garden books for additional material before proceeding.
The last step is to take up each little pile of cards by itself and either subdivide according to this or that feature of the memoranda or at once choose for the planting. The selected cards will then answer as notes from which to make the garden, or border, plan.
Even with this preliminary study, it would be far better for every one who is growing perennials for the first time to plant most species in rows like so many vegetables; this for a year or two. No matter how much one absorbs from books, it is only by watching a perennial grow a season or more that it is possible to sense its character in every particular, and if this is done in a little home nursery the acquired practical knowledge makes every definite step in the use of such plants as permanent garden material infinitely easier and more effective. No time is really lost and much working experience is gained.
A good reason for this preliminary planting is the difficulty of getting a clear idea of the foliage spread of a perennial without actual observation. The kinds are too numerous to permit of the spacing tables by which tulips, hyacinths, pansies and geraniums are set out; very few go into the ground excepting by what seems guesswork, but is really an acquired knack.
The foliage spread is important to know before planning a hardy border or garden, in order that enough and not too many plants may be acquired and set out—thus saving money at the outset and time spent in replanting later.
Suppose, to get away from the abstract, half a dozen oriental poppies and as many plants of "baby's breath" (Gypsophtla paniculata) are set out in a home nursery bed—in parallel rows, about fifteen inches apart and the plants nine inches apart in the rows. If the plants are of commercial size they may not seem too close together in the row the first year; but the second year they will look crowded and there will be every sign that thinning or complete replanting must be done earlier than ignorance had suspected would be the case at the time they were so very carefully set out at apparently wide spaces.
Possibly ignorance, had the planting been done in a garden, would have taken it for granted that no change would be necessary for years. The second season it is noticed that an oriental poppy is likely to have a spread two feet in diameter while the masses of "baby's breath" in the blooming season will perhaps be twice that distance across. Meanwhile this will have been discovered the first year and will be still plainer the second; the poppy blooms early in summer and soon the plant turns brown and dies down to the ground, the while the later-blooming "baby's breath" is spreading out toward it and gradually concealing its unsightliness. It is also seen that by the time the "baby's breath" is turning brown a couple of vines of Thunbergia alata, from seed that happened to fall there, are making their way over the drying masses—because partly to hide the ugly . is one of the special errands on which nature sends the five-foot climber.
By autumn another thing is noticed; the poppy has begun to make a considerable second growth of foliage and, lest this be too shaded, there is need of cutting away some of the branches of "baby's breath"—or else diverting them to one side. Obviously, the oriental poppy and "baby's breath" are one of those dovetailing perennial combinations to know which is among the secrets of successful hardy gardens and borders.
Here then is a whole lot, and not all at that, learned by the exercise of a litle patience in the study of plant character before attempting to bend that character to one's own use. And the observation of the plants was the easier because of their being in a row.
The only safe general rule for the planting of perennials is to allow a space of ground six inches square for each plant known to be of dwarf or fairly low habit and a space a foot square for the taller ones. This is a good rule. Unless the plants are seedlings or small cuttings—sometimes then—the ground will be nearly or quite concealed when the first summer is well along on its course. And there will be ample room for two, three or more season's growth—according to the plant's normal rate of increase and the way that this is helped or hindered by weather conditions.
Whether the plants are set out in rows or a more or less naturalistic fashion, the rule in question need occasion no complete replanting for a long time. This is avoided by removing alternate plants, or one here and there, as the colony becomes crowded. In some instances the plants may be left in the same number, but the individual size reduced by cutting off portions with a trowel— which may be accomplished without lifting 'the plant from the ground. Peonies are an exception to the rule; they should be planted two feet or more apart, as they dislike frequent disturbance.
Perennials usually are planted for permanent effects, but there is a growing tendency to use some of those that bloom in the spring and very early in the summer as bedding plants. Seedlings or small plants raised from cuttings are bedded out in the autumn, after the summer flowers have come to the end of their tether, and the year following, directly the height of bloom is past, they are rooted out and either thrown into the compost heap or divided and placed in nursery rows. This is the plan of Belvoir Castle, where every spring there is a superb display of bedded-out perennials on a scale that may be imagined from the fact that the annual consumption of aubrietias alone is some seven thousand.