There are some perennials which are benefited by being allowed to remain permanently in one place, such as peonies, gas plants, and bleeding-heart, but the vast majority of plants are not likely to improve unless taken up from time to time, divided, and reset. This is especially true of plants like the iris and the sneeze-weed which spread underground and form mats, soon exhaust soil fertility, and engage in a severe struggle which hampers their ornamental development. Likewise, crowns of perennials which give the most desirable flowers bloom only two or three seasons and then die. There is also the necessity for renewing the biennials such as foxgloves, canterbury bells, hollyhocks, and sweet williams. Some perennials such as oriental poppies should not be moved but may be divided with a sharp trowel. The general practice is to lift and divide the roots of perennials every two or three years. Spreading and shallow-rooted perennials will thrive and flower much better if divided and transplanted each year. If this "dividing" is not undertaken each year the "clumps" become crowded and the individual flowering stocks do not have an opportunity to develop normally. The result is that many dead stocks remain in the middle of the clump and a much inferior type of flower is produced.
Illustrations of this group of perennials which should be divided each season and preferably during the early spring before growth begins are the hardy asters, the sneeze-weed, yellow marguerite, ball of snow, English daisy, and the pompom chrysanthemum. If these plants are not taken up, divided, and replanted in good rich soil, the plants during the succeeding year will not produce large flowering heads, neither will they produce the strong, vigorous growth which they are accustomed to produce. Most of these plants are vigorous growers and heavy feeders and it does not take them long to sap from the soil much of the good food matter which is so necessary to their normal growth.
The plume poppy, Shasta daisy, and yellow marguerite will be surrounded by a large number of young plants, which spread out and surround the parent plant, with the result that a number of inferior plants occupy the space which should be occupied by only a few fine, thrifty specimens, providing the parent plants are not divided and transplanted each year. With the yellow marguerite especially, it is much better to discard the old roots and to preserve in the dividing only those roots which are the result of the previous year's development as offshoots from the parent plant. Many times the seedlings that spring up around these plants are equally as preferable if transplanted and given ample space to develop normally.
In the group of plants which should be divided at least every two years are included a few of the more vigorous types of the garden phlox together with the boltonia and the bee-balm.
There is a group of perennials which should be divided on the average of once every three years. They will not be benefited through the process of being divided at more frequent intervals. This list includes the common garden phlox, painted daisy, most types of the larkspur, the lily-of-the-valley, a few asters, and the hardy sun-flowers. The common impression with reference to the lily-of-the-valley is that it should not be divided or transplanted after the time of transplanting the original plants. Those persons who have had an intimate acquaintance with the flowering habits of this plant state that it should be divided at least once every three years if an abundance of large blooms is to be obtained.
Such plants as the larkspur should be divided with great care every three or four years and each division of this plant should be left with a good crown attached to which is a quantity of good fibrous root growth.
The greatest success in dividing perennials will be attained with those plants whose roots can be readily pulled apart with no severe ruptures. Those plants which have heavy roots like the larkspur and some of the irises should be subdivided with much greater care. Biennials will renew themselves by seeding if the soil is not disturbed around them to any great extent. A few perennials such as the yellow marguerite and some of the hardy asters will also renew themselves each year by seeds dropped from the parent plant. Every perennial garden is benefited by a thorough spading over at least once in three years. When a garden is filled with perennials the spading does not in general reach deep into the soil, nor does it cover the entire area. The best soil for these plants is one which is friable and not too compact. This is the reason for spading every two or three years.
Plants may be dug up and separated by hand or thinned out in the beds by cutting with a clean, sharp spade or trowel and removing the excess plants. Replanting should be done in fresh soil if possible, or some new soil and bone meal would better be worked in. Care should be taken in the replanting that the crown of the plant is not smothered. It is equally fatal to the plant to be set too low or too high. Divide and replant in the spring those fall-blooming sorts which continue in full bloom until late in the season, such as chrysanthemums and anemones, and all fleshy-rooted plants except the peony. (For the peony and the iris in particular, and for planting perennials in general see Pages 39 and 47.) The best time of the year for lifting and separating perennials in general is probably the fall. The early-flowering perennials like some of the irises and the leopard's bane should give the best result by dividing and transplanting shortly after they have completed their flowering period. One objection to fall planting, however, is that the smaller plants heave out if planted too late or are apt to get lost during mulching or inthe "cleaning up" work of the spring. Plants with heavy tops or fleshy roots in general, except the iris and the peony, are more liable to rot if planted in the fall. If the planting of them is done at this season it should be begun in the latter part of August, if possible. The regular mulching, so necessary for all plants set in the autumn, should not be forgotten when the winter comes on. During the summer a fine mulch of some sort may be kept on a perennial border to good advantage as it prevents loss of moisture, saves labour otherwise necessary in cultivating, stops spattering of mud over the leaves of smaller plants, and prevents baking of the soil after rains.