Primulas grown inare exotic-relatives of our native primrose and cowslip. They are mainly winter flowering and are ideal plants for the amateur gardener as they do not require much heat. The three most popular species are Primula obconica, P. malacoides and P. sinensis (including the form stellata). An attractive hybrid for the greenhouse is P. kewensis.
Primula obconica has the longest season of flowering and often blooms intermittently throughout the year, but it is as a winter- and early spring-flowering plant that it is chiefly valued. Colours available are pink, salmon, red. Blue and white. The leaves can cause a rash with some people.
Primula malacoides has smaller, more numerousborne in elegant sprays. One whorl of flowers above another. The colour range of this primula used to be limited to shades of pink, rose and carmine. But more recently other colours such as lilac, violet-blue and salmon-scarlet have been added. This is certainly one of the most valuable winter- and spring-flowering plants for the amateur.
Primula sinensis has larger flowers and a greater range of colour and flower forms, one type with more star-shaped flowers being distinguished as P. sinensis stellata. These flowers are borne in winter and early spring. Another form with fringed flowers is sometimes called P. sinensis fimhriata. In all its forms, P. sinensis is a little more difficult to grow than either P. obconica or P. malacoides.
Primula kewensis is a hybrid with yellow flowers which appear in spring.
The cultivation of P. Obconica, P. malacoides and P. sinensis is similar but there are differences in sowing time. Seed of P. obconica and P. sinensis is best sown in March or early April; P. malacoides in late May or June. It is very fine and needs only a light covering of the seed compost in which it is to be germinated. A temperature of 16°C. (60°F.) is needed for quick germination and it must not fall below 13°C. (55°F.) in any case. It is wise to cover the seed pots or pans with both glass and paper until germination takes place but then the seedlings must have full light to keep them sturdy. The compost must also be kept moist throughout.
Primula kewensis is treated similarly with the sowings in this case being made in February or March to give a longer growing season. Once established, this hybrid prefers lower temperatures than the other primulas, and needs only enough heat to keep frost out of the greenhouse.
As soon as the seedlings can be handled they should be pricked out into seed boxes. 3 to 4 in. apart, or potted separately in 3-1/2-in. pots using the John Innes No. 1 Potting Compost. I think possibly the best method is to prick out first into boxes, shading these from strong sunshine for the first few days and then pot up separately into 3-1/2-in. pots as soon as the leaves touch. From these pots they are moved on to 5-in. pots when ready, which should be before the middle of August. Use John Innes No. 2 Potting Compost for this move.
From June to the end of September they can be housed in a garden frame the greenhouse is apt to get too hot and dry – with shading provided from the sun during the hottest part of the day. It helps to plunge the pots to their rims in sand, ashes or peat to prevent them drying out too quickly. If they are left in the greenhouse the glass should be permanently shaded and the paths and staging damped down each day to maintain a moist atmosphere.
The early flowers should be picked off to encourage the plants to build up their strength.
As soon as the pots are well filled with roots, feeding with liquid or soluble fertiliser should start, and should be repeated every week or 10 days up to and during the time of flowering. This is particularly important with P. malacoides.
By the end of September the plants should be back in the greenhouse again. They require a cool temperature and are best if kept on a shelf near the glass.
Once back in the greenhouse. Very careful watering is needed. Apply the water direct to thefrom the spout of the watering-can. Primulas need plenty of water but not on their leaves in autumn and winter where it is likely to encourage grey mould, a disease to which they are rather subject, especially in a cold, damp atmosphere. Grey mould is less likely to appear at temperatures above 13°C. (55°F.). but perfectly good primulas can be grown at a minimum of 7°C. (45°F.) if care is taken to prevent damp. If trouble should develop, pick off any leaves that show signs of decay. If yellowing of the leaves occurs sprinkle calcined sulphate of iron around the base of the affected plant.