This section is from the book "The Florists' Manual", by William Scott. Also available from Amazon: The Florist's Manual.
What florists understand when we speak of cinerarias are those which have originated from C. cruenta. The hardy species, although acceptable plants for the herbaceous border, where hardy, are not of much value to the florist. Since the introduction of the cineraria, or rather since its common use as an ornamental plant in our greenhouses, a wonderful improvement has been made in size, color and form of flower as well as in the habit of the plant. They are of easy culture and it may be said that any glass structure, where it does not actually freeze, will grow cinerarias. But like many other of these soft-wooded plants which can be called "a. cheap plant and easy to raise" a slight mistake or neglect will ruin the whole lot. A palm or an orchid will be much less liable to permanent injury by neglect or mismanagement, for what is a cineraria but an abomination unless it has broad, stiff, healthy leaves, and if it has those it will be sure to have a handsome head of flowers.
It is quite possible that some choice varieties are still perpetuated by cuttings, as they commonly were years ago, but that with the American grower is never though of, neither is it at all necessary, for a fine strain is readily produced from seed supplied by our leading houses. Double varieties were also a novelty a few years ago and supposed to be a great acquisition, but the cineraria, like some other florist's flowers, is not in the slightest degree enhanced in beauty, either as an individual flower or as a decorative plant, by its being double. It is simply a monstrosity and the craze for the double varieties has vanished.
If flowering plants are wanted by November and the holidays, you must sow at the end of May or early in June. Except on private places this is not to be recommended. For the commercial florist they would not be very profitable, for they are a troublesome plant to carry through the hot months and the bulk of your customers are not ready for them till February, March and April. It is well to make two sowings, the first early in August, the latter the middle of September; the last sown will usually come in right for Easter. The seed is not so small but what it can have a slight covering; finely sifted leaf-mold or sand will do, and keep uniformly moist till the seeds are up.
When they have made a small character leaf, transplant into a flat or 2-inch pots. From this time on they must be shifted on as they need it, never by any means allowing them to become stunted for want of larger pots. After they leave a 3-inch pot the soil should not be sifted. If it is a little rough or lumpy so much the better. I have seen hundreds of cinerarias in 4 and 5-inch pots die, not with a slow death, but suddenly droop and die, and the cause was a close, adhesive soil through which the water did not pass freely. They may not be a profitable plant, but if worth growing at all will surely pay to grow well. They must have room to spread their leaves, and until flowering time 40 degrees at night will suit them better than a higher temperature. You will often hear instructions given to "keep plants near the glass," in other words this means light. They must have light, room to spread out, a cool temperature, and although a stagnant state of the soil is fatal to them, should never be allowed to wilt from dryness or they will lose some of their best leaves. After light, air and a low temperature, the remaining great object to watch in their successful culture is never to let a greenfly be seen on them. Fumigate regularly and faithfully.
Those that are summered over do much the best in a pit or coldframe, but it should be deep enough so that when ventilation from the raised sashes is given it should pass over their tops and not be playing too freely on their soft leaves. Specimens can be given an 8 or 9-inch pot, but the commercial florist will find that a 6-inch will flower them sufficiently well. I have seen some growers pinch out the leading flower shoot to induce a broader head of bloom. If grown cool and light this is entirely unnecessary. It is seldom that cinerarias are troubled with thrips or red spider, but a dusting of water in summer and fall is beneficial to them.