As I shall treat of the management of the greenhouse plants, which are to succeed the annuals, in my next, I may as well fill up my present letter by saying a few words on the management of hardy perennials, in case you should prefer planting biennials and perennials in your beds at once, to going through the routine that I have marked out for you in the former part of this letter.
_Perennials_ are those permanent plants which are not woody, and yet remain in the ground as long as most kinds of shrubs, producing flowers and seeds every year. Perennials are of two kinds: those that die down to the ground every autumn, and send up fresh stems from the root the following spring; and those which remain green all the year, as, for example, the pinks and carnations. Besides these kinds, there are other sorts of perennials, as, for example, those that have tuberous roots, such as the dahlia. Bulbs are also perennials; but of these I have already spoken.
Most kinds of perennials are propagated by dividing the roots; but, in the case of the dahlia, ranunculus, and anemone, care must be taken to choose only those portions of the tubers that have buds or eyes, as they are called, as otherwise the tuber, though it will send out fibrous roots, will never produce a stem; and, in dividing fibrous-rooted plants, care must be taken that the divided part is furnished with buds. Almost all kinds of perennials may also be propagated by cuttings; and those of pinks and carnations are called pipings, because, instead of being cut, they are pulled asunder at a joint, and this gives the separated parts a hollow appearance like small pipes. Tubers are frequently taken up every autumn; and those of the ranunculus and anemone are replanted in November or January, the former season being much to be preferred. The tubers of the dahlia are generally taken up in November, and replanted in May or June.
Most perennials are improved by taking them up occasionally and replanting them in another place. This used to be accounted for by supposing that plants threw out excrementitious matter, which, after a few years, poisoned the soil in which they grew; but it is now supposed that, as every plant requires peculiar earths for its nourishment, they must be removed when they have exhausted all the particular kind of earth they want which lies within their reach. It is rather difficult to explain this without entering into long details; but it will be sufficient for our present purpose merely to state the fact, that plants do require their roots to have a constant supply of fresh earth; and, to meet this want, nature has provided that the roots of trees, and of all plants that are intended to remain for several years in the soil, elongate themselves every year, so as to be continually able to obtain a fresh supply of nourishment. In gardens, however, the constant digging that is going on for the culture of annual plants is unfavourable to the elongation of the roots of the perennials, and consequently it is absolutely necessary that those plants should occasionally be taken up and replanted. The season for taking up and replanting perennial plants is either in autumn, after they have done growing, or in spring, before they begin to shoot; and, if the soil about the roots looks black and wet, or, as the gardeners express it, sour, the roots should be washed quite clean before replanting. When the roots of plants are divided, it is either done with a sharp spade or a knife, care being taken in both cases to make what is called a clean cut, and not to leave any part bruised or jagged.