This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
To grow Celery to perfection, rich soil is essential. Indeed Celery grows best in decayed manure, a year old or so. It is generally grown in trenches or beds sunk below the surface, by taking out the soil and building it neatly on either side of the trench, - the earth thus stored up to be used for blanching the plants after they have grown sufficiently. It is best to prepare the trenches only a short time before planting; because in spring the frosts cause much of the earth to fall in, and part of the work has to be done over again. Particular care must be taken to beat the earth firmly on either side to prevent its falling back into the trenches again by the action of the weather. Some prefer single trenches; others prefer "beds." We will describe both methods. Single trenches are made 1 foot 8 inches wide and 1 foot deep. When more trenches than one are required, allow 3 feet between them. In forming them, run the line along each side of the trenches, and cut close down the inside of the line the full depth of the spade. In throwing the earth out, keep it close by the side of the trench. Cut the trench perpendicularly, and slope off the loose earth at an angle of 45° or so, beating it neatly, so that it may keep its place.
Do not lay it down to a greater depth than 6 or 8 inches, and make it flat on the top, as it will do for growing small Salads on. The spaces between the trenches are to be piled up with earth in the same way. Break the earth fine, and smooth it over nicely. Lettuce, Radishes, Mustard and Cress, etc, may be sown on the ridges, and they will all be removed before the earth is required for blanching the Celery. After the trenches are dug they will require manuring, and there is nothing better than well-decayed stable-yard manure. Put 6 inches of this in the bottom of the trench, and if the soil be heavy and wet, an inch of sand. But in heavy wet soils it is best to make very shallow trenches and fill them to the brim with a compost of manure and some light soil, or a little sand, if light earth cannot be got. Of course the earth for blanching has to be dug from the sides, but this leaves deep drains, and the Celery is then raised on ridges, which is in its favour in heavy or wet ground. In ordinary soil, after the manure is in the trench, mix it with some of the soil in the bottom of the trench, and leave until planting time.
Such trenches, in light dry soils, favour the retention of moisture about the roots of the plants, and moisture they must have or they will not grow satisfactorily, Celery being a native of damp situations. At the same time too much wet, in winter especially, rots it, and hence the reason for planting on the surface on heavy wet soils. The "bed" system of growing is merely an extension of the trench, and is a good enough plan in light dry soils, especially when the crop is to be cleared off early in autumn; but as it presents a wider, flatter surface, the rain is not thrown off as in the trench system, and hence Celery is more liable to rot than in beds. Beds generally contain four rows of Celery, while trenches as a rule contain one, and seldom more than two; and in earthing up those in trenches, if the heads of the Celery are inclined slightly inwards, the earth which is used to blanch them may be built so as to present a thin ridge top, which helps to keep the plants and soil drier than in the case of beds. Frequently only one row is planted in the trench, but we prefer two, as double the quantity is produced with only a little more room, labour, and manure.
The chief recommendation which can be given to the bed system is that greater quantities can be grown in small space; but it often happens that there is room for a trench in small gardens when there is not for a bed; and when the sides are utilised for growing small Salads, not an inch of ground is wasted.
The best way of raising Celery plants is by means of a slight hot-bed; and should the amateur possess one for striking cuttings and raising half-hardy annuals in March and April, space may be spared for holding a small seed-pan, for a very small space will raise a large number of plants while they are in the seedling state. For soil use one half well-decayed manure, and the other half light loam. Drain well the seed pan or pot in which they are to be sown; use the surface soil in a fine state; sow thinly, and cover very lightly. A piece of brown paper over the pot, etc, will prevent evaporation, but this must be removed the moment the seedlings appear. Keep the pots near the glass, and where they will get fresh air, in order to make them as sturdy as possible. Never allow them to suffer for want of water; and indeed take care that they receive no check, or they may become stunted and useless. When they are in the rough leaf, they must be "pricked out" or transplanted into boxes at from 3 to 4 inches apart, using rich soil, such as we have recommended for sowing. Use the soil in a moist but not wet state; and after they are pricked out, give a gentle watering through a fine rose. Put them back in the frame, and shade slightly for a few days, should the weather be bright and hot.
When quantities are grown, the usual and best practice is to put down 3 or 4 inches of well-decayed manure on a hard bottom, covering it with 2 inches of light soil, and to prick out the plants on this, and, if possible, covering them with glass lights until they are strong enough for the trenches or beds. When this takes place, the plants move with nice balls of the rotten manure, and receive but a very slight check, scarcely a root being sacrificed. But in most cases those for whom we write do not require such quantities as would necessitate the use of a whole light. Moreover, the plants come out of ordinary cutting-boxes almost as safely as from a bed or frame. Those who have nothing beyond a greenhouse or a cold frame can raise the plants in these, or even under a common hand-glass in a sunny sheltered place out of doors; but plants raised by these means will be later, and the produce scarcely so good, although with good management in favoured localities very fine Celery is raised without the aid of glass at all.