Tender annuals, such as balsams, cock's-combs, globe amaranths, &c., must be sown in February or March in pots of light rich earth, and plunged in a hotbed. As soon as the plants come up they should be transplanted into pots of the very smallest size, one in each pot; and these small pots should be set in the hotbed again, as near the glass as possible, and slightly shaded during sunshine. In a week or two, as soon as the roots have made their appearance on the outside of the ball of earth within the pot, which is known by turning the ball of earth with the plant in it carefully out of the pot without breaking it, the plants are shifted into pots a size larger than what they were in before, and the space filled up with light rich soil. In another week or two the plants must be shifted again into pots a little larger, always using light rich mould to fill up the pots, and taking care that the pots are well drained, by putting potsherds, that is, pieces of broken pot at the bottom. As soon as the plants are shifted, the pots must be replunged in the hotbed, and shaded for the remainder of the day. The shifting and replunging must be continued till the plants begin to show flower-buds; after which they must neither be shifted nor plunged in the hotbed any more, but gradually hardened, by the frame in which they are placed being left open all day, and, at last, only partially closed, even at night, till the plants will bear setting out entirely in the open air; unless they should be intended to flower in a greenhouse, in which case they may be removed to the greenhouse as soon as they have formed flower-buds.
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.
Biennials are plants raised from seeds, which do not flower till the second year, but which generally die as soon as they have ripened their seeds. Biennials are usually sown in a bed of light rich earth in the open air in the reserve-ground, and then transplanted in September to the place where they are to flower the ensuing year. The finer kinds, such as the Brompton stocks and hollyhocks, should have a bed or pit prepared for them, of rich loamy soil, in which they are planted, with a small quantity of manure. Wallflowers, snapdragons, and Canterbury bells do not require any further care than transplanting to the border; and, though they are called biennials, they will frequently live and flower three or four successive years.
A hotbed may be made of any material that will ferment, so as to produce heat. Stable manure and dead leaves are, however, generally preferred to all other materials; and stable manure is unquestionably the best. A cart-load of this manure will make a hotbed sufficiently large for rearing tender annuals; but as, when it is taken out of the stable, it consists partly of the dung of the horse, and partly of what is called long litter, that is, straw moistened and discoloured, but not decayed, it must first be thrown together, so as to form a heap till the straw is decomposed. A most violent heat is produced by the fermentation of the straw while decomposing; and, as this heat would be too much for any plant exposed to it, it is absolutely necessary to let the heap remain for about a fortnight, turning it over two or three times during that period with a fork, till the straw is sufficiently decomposed to be easily torn to pieces with the dung fork. When the manure is in this state it is fit for use. The hotbed should be formed in an open situation, on a surface raised about six inches from the surrounding ground, with a gutter or shallow ditch cut round it, to allow the water to drain off. The bed is then made; and, if only intended for raising annuals and striking cuttings, it may be five feet long by four feet wide. The manure should be first regularly spread over the lower part of the bed, and then continued, in successive layers, made as smooth and level as possible, till the whole of the cart-load of manure has been used.
As soon as the bed is finished the frame should be set on it. The frame consists of a box without a bottom, and with a movable top, formed of a glazed sash or sashes. A frame for a bed of the size I have mentioned will only require one sash, or light as the gardeners call it; and it should be three feet wide and four feet long, so that the bed may be half a foot larger than the frame on every side. The back of the box may be two feet high, and the front one foot, so that the glass may slope from the back to the front. About two days after the bed is made, the fermentation will recommence, and a steam will be observable on the glass. The surface of the bed should now be covered two or three inches thick with light garden mould, and any common seeds may be sown in this. It is more general, however, to sow the seeds in pots, and then either to set them on the surface of the bed, or to plunge them into it up to the rim. No bed for raising annuals should ever be hotter than 60°, and when it exceeds this heat the glasses should be left open so as to cool it. The thermometer for ascertaining the heat should be put on the surface of the bed, with the glass shut above it; and it should be examined in this situation, as it will fall a degree or two immediately on being taken into the open air, if the weather should be very cold.
You will, of course, have your hotbed made in the reserve-ground; and, as the one I have given directions for will be a very small one, you will probably find it necessary to have one much larger for your cuttings, or to have three or four small ones. I should advise the latter course, as small hotbeds are much more easily managed by inexperienced gardeners than large ones. A hotbed of two or three lights will require two or three cart-loads of manure, and will, of course, produce a great deal of heat from the immense mass of fermenting materials it contains; and, as you would find this additional quantity of heat very difficult to regulate, you might chance, some fine morning, when you visited your plants, to see them turned black, with their leaves shrivelled up, or, as the gardeners term it, burnt, from the too great heat of the bed. There is also danger of a hotbed getting too cold instead of being too hot, and, when this is the case, the heat should be renewed by the application of dung linings, that is, a quantity of fresh stable manure, round the outside of the bed; or by having linings of dead leaves piled up round the outside of the bed. If, however, you use your hotbeds only for raising seeds, they will not want any linings; as it will be advantageous for the young seedlings to let the beds gradually become cool as the plants increase in size, so that they may acquire strength and hardiness before they are turned into the open ground.