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.
Perhaps amongst autumn, winter, and spring flowering plants there is none that can surpass the Primula for general usefulness, and nothing better exists for the amateur's greenhouse or as a window-plant. For room-decoration it continues in good condition for a long time, and is invaluable. The Primula is easy of cultivation and most floriferous, continuing to produce its flowers during winter in continuous succession. Many beautiful new forms are from time to time offered, but when really good strains of P. sinensis, fimbriata, rubra, and alba are obtained, none are much if any better.
If wanted to bloom in autumn, seed should be sown any time during the present month : if not until spring, the operation of sowing can be deferred some few weeks longer. Hitherto I have failed to see the utility of making successional sowings, as out of one pan of seed there is generally a good succession of plants, which should be sorted, when pricked out of the seed-pan, into two or three sizes, and placed in separate pans. Many experience difficulty in getting Primula-seed to germinate, which need not be the case if the seed is good to start with. Square pans or 6-inch pots, according to the quantity of seed to be sown, should be liberally drained, and the drainage covered with a layer of moss. They should be filled with a light compost of equal parts of loam and leaf-mould, with a liberal dash of silver sand. The soil placed on the top of the pan or pot should have passed through a rather fine sieve. The surface of the pan should be made level, but not pressed too firmly, and the seed should be evenly sown over the surface. No attempt should be made to thoroughly cover the seeds; a little fine leaf-mould I have found the best to scatter amongst the seeds. Many failures result from covering the seed too deeply.
After sowing, the pans should be thoroughly watered through a fine rose, and a square of glass placed over it covered with a little moss. The more heat the pans are placed in, the sooner germination takes place, but a temperature of 60° is ample; or the seed will do in a much lower temperature in case the former cannot be maintained. Care must be taken that the pans do not suffer for the want of water until the seed has germinated. When growth commences, the glass over the pans must be tilted so as to admit light and air to strengthen the seedlings. Exposure to light must be gradual at first, until the glass and moss can be entirely removed : at no time should strong sun strike upon the seedlings while so tender and young. It is surprising how soon their tiny roots lay hold of the leaf-mould scattered amongst the seed. When large enough, the seedlings should be pricked into pots or pans and sorted as referred to above, using much the same soil with a little larger proportion of loam. This time none of the soil will need to go through a sieve - an operation I never practise except in the case of a few fine seeds.
After the seedlings are pricked off, they should be placed in the shade for a time until they commence rooting afresh, when they may be put as close to the glass as possible, shading them from direct sunshine, but allowing them all the light they can bear. Air must be admitted on favourable occasions, so as to keep the young plants dwarf and stocky. If in a nice growing temperature, they will soon be ready to be placed in 2-inch pots, which should be clean and liberally drained, as in all stages Primulas dislike sour or stagnant soil about their roots. When the young plants reach this stage, if a close frame can be given them where gentle warmth can be maintained, so much the better. The frame should be kept close until root-action commences and the plants have taken to the new soil, when they can be gradually hardened and grown under cooler conditions. The hardening process must be done carefully, or a check may be occasioned and the plants stand still for a long time. They should be arranged as close to the glass as practicable, over a moist cool bottom.
As the season advances and all fear of frost is past, they can be grown in cold frames with abundance of air; and when the nights become sufficiently warm, air can be left on all night.
Potting should be attended to from time to time as the plants require it, until they are placed in 5- and 6-inch pots, which is large enough for all ordinary purposes; but if larger plants are required, 7-inch pots can be used. Care must be taken that the plants never become pot-bound while in small pots, or rapid progress is considerably impeded. When potting, the plants should be placed sufficiently low in the pots to keep them firm at the collar. Many cultivators are careful not to bury the collar when potting, for fear of the plants damping off. They are in consequence tumbling in every direction if moved, and liable to be injured unless small stakes are placed round them to keep them upright and steady. There is no fear of the plants damping when deep potting is practised. Primulas delight in a light rich soil composed of rich fibry loam, leaf-mould, a little well-prepared cow-dung, and a small percentage of broken charcoal, with plenty of coarse sand to keep the whole porous.
Watering is an important item in growing Primulas, and the water-pot should be used judiciously. They should never suffer for the want of it, or be saturated : an intermediate state appears to suit them best. For some time after the operation of potting, water should be very carefully applied. Stimulants are necessary when the pots are well filled with roots, and nothing acts quicker upon the plants than clear soot-water. Primulas do not care for much water over their foliage, and slight dewing only is necessary on very warm afternoons.
The plants cannot endure strong sunshine, and must be shaded during bright weather: at the same time the shading must not be so heavy as to exclude light. They will do well in cold frames until the approach of frost, and in any house during winter where frost is excluded. Shelves close to the glass are capital places for them. The earliest batch can be allowed to come into bloom any time in early autumn according to circumstances, and will continue for months in a temperature of 45° to 50° according to the external temperature.
The double varieties supersede the single ones for cutting purposes; and for bouquet-making they are admirably adapted, and last a long time in a cut state. Wm. Bardney.