Primrose and Polyanthus - Getting the best Out Of Your Plants

: Howard Drury; Fact Sheets; 06 April 2022; Hits: 1168

Primrose and Polyanthus Trial For Commercial growers at Ball Colegrave

Primroses and Polyanthus

By Howard Drury

Introduction

Primroses and Polyanthus make very good pot plants and have become very popular over recent years, mainly due to improved seed breeding techniques, which have given us larger flowers in an improved colour range. Newer dwarf strains of Polyanthus are much better suited in their habit to pot culture, with an extensive range of bright, cheerful colours. The exciting hybrid Primroses retain the ideal pot form, coupled with the true brilliance of colours only previously found in the Pacific strain of Polyanthus. Primroses and Polyanthus are good subjects for cool rooms, they should never be subject to high temperatures, as this will only produce weak spindly growth at the expense of flowers. After flowering, plants may be planted out in the garden to flower in subsequent years, although some authorities suggest that these hybrid strains are less hardy than traditional varieties normally grown outdoors.

Seed Sowing and Germination

Seed may be sown in April and May for the largest plants, or in June for smaller plants. Seed of the newest strains is relatively expensive, and it is important that good germination is achieved. If you follow the procedure outlined below, you should have good results.

Sowing Medium

Use a compost with a high peat content and additional Perlite, water the compost thoroughly before sowing the seed as evenly as possible on the surface of the compost which must be level. Some people suggest lightly pressing the seed into the surface using a firmer board, but beware that the seed doesn't stick to the board. From experience, I have found that it is better to sow the seed on the surface of the compost and then to soak the container in a bowl containing 1"‑2" of water in order that the capillary action and natural drainage afterwards draws the seed into close contact with the compost. NEVER water the container from above using a watering can. Place the seed container into a clear polythene bag, and then vary the treatment according to the season and the prevailing temperatures.

Spring Sowings

Place the seed tray in a propagator that has an accurate temperature control and set thermostat in order that a bottom temperature of 60‑65F is maintained. It is important that the temperature does not exceed 68F, otherwise seed will not germinate.

Summer Sowings

Seed trays may be placed in the coolest part of shaded greenhouse or, alternatively, in a cold-frame if the prevailing temperature in the greenhouse is liable to exceed 68F at soil level. DO REMEMBER THAT ALL PRIMULAS ARE INTOLERANT OF HIGH TEMPERATURES WHEN GERMINATING, and that this is the most common cause of failure with most sowings.

During germination protect the seed tray from direct sunlight which can cause over heating of the compost and rapid drying out. Seed will normally start to chit (sprout) in 7‑10 days (failure to do so usually indicates too high temperatures) after which time the plastic can be removed and finely sifted compost sprinkled evenly over the surface to anchor sprouting seedlings. Replace seed tray in polythene or plastic bag and maintain 60‑65F. As soon as the first seed leaves appear, remove the seed tray from the plastic bag and take care to prevent the drying of the soil surface. At all times keep the tray shaded from direct sunlight and lower the temperature to 55‑60F.

Transplanting

Traditionally seedlings were transplanted into frames or open ground beds for growing on prior to potting, however, today commercial growers prick out directly into small containers or seed trays, and grow on in cool greenhouses. Home gardeners may do likewise and even consider using small Jiffy peat pots after the initial transplanting into seed trays with a maximum of 40 seedlings per tray. Shading is advisable but only for the first few days after transplanting, then more light can be given in order to produce short, sturdy plants, after which plants may be grown in the cold frame.

Potting

Potting normally takes place during August and September and either a peat based or a John Innes seed compost with extra peat may be used. It is important that a low nutrient level is maintained and that the compost is kept very slightly acid. Higher pH levels can lead to chlorosis (yellowing) of the foliage and by using a slightly acid compost the effects of watering, for those with hard or limy water, are minimised. Plants are normally potted into 3½" pots earlier sowings may require 4" pots and later sowings may only justify 3" pots and these plants are useful for including in bowl arrangements etc. Plastic pots are widely used and providing care is taken with winter watering, few problems should occur, however plastic pots are less suitable for plunging when compared with clay pots, but this technique of burying the pots up to their rim is not frequently practised nowadays.

Summer Care

Where clay pots are employed, plants may be plunged up to their rims in peat within a cold frame. This helps reduce the need for watering as the plants are able to draw the moisture from the damp peat through the pot as they require it, and this is an ideal method for growing Primroses. Plants can be grown on in full sun in the open with the frame lights off, shade only being provided during extremely hot periods. When the plants are growing away nicely, regular weekly applications of a well-balanced liquid fertilizer may be applied. Spraying as a precaution against pests and diseases should also be carried out every 7‑10 days.

In the autumn, the plants in the plunge beds can be covered with the cold frame lights to exclude hard frosts and heavy rains. They should always be freely ventilated except in the most severe weather, when the lights should be closed and additional protection provided with matting etc. Watering may not be necessary from early November until plants are brought into the greenhouse for forcing on, or when plants are extremely dry.

Plants from later sowings are often grown on entirely in the greenhouse, but this does require more critical management in every way, but with care the results can be quite outstanding. Ventilation must be given freely, particularly during the summer months, otherwise excessive foliage will be produced. Watering is also restricted, keeping the plants a little on the dry side, especially from September to November, but do not allow plants to dry out too severely or harm will be done. Shading should be avoided unless absolutely necessary and low nutrient levels encouraged but weekly liquid feeds low in nitrogen will encourage good rosette formation. Autumn and winter management is much easier than for crops grown in cold frames, by now higher temperatures are no longer a problem and greenhouses can be kept just frost‑free at reasonable costs. As the earliest plants begin to show colour the temperature may be raised to 50F, but high temperatures will cause the foliage of the plants, not yet in flower, to grow too much.

Many home gardeners will still prefer to prick out seedlings into trays and grow these on in cool greenhouses or cold frames and then bed out plants in blocks with plants some 9" apart until the autumn when plants are potted up. This does cause a set back to the plant but it does mean that the home gardener can let nature take her course and produce a sturdy, cool grown plant, avoid choosing shady sites, feed only if necessary and then pot in September as described above.

Forcing Frame Grown Plants

Plants may be brought into the greenhouse for forcing with a temperature of 50F, by now plants are likely to be rather dry, and it may be necessary to stand pots on individual saucers to ensure each plant is adequately watered whilst avoiding any excess wetting of the foliage and growing point of the plant. Given these conditions the plants should quickly develop large heads of bloom, avoid forcing plants too early, wait until the first buds are well formed otherwise considerable foliage will be produced at the expense of flowers.

Primroses may be brought into flower at slightly lower temperatures than Polyanthus, although this will take slightly longer. Some authorities suggest that given high temperatures, primroses will produce Polyanthus‑like flowers.

Plants grown in beds outside and potted in September may be given protection in the cold frame until flower buds develop or, alternatively, brought into a very cool greenhouse immediately after potting, the latter is necessary otherwise excess foliage will again be produced.

Polyanthus in Pots

The culture of polyanthus in pots is the same as for primroses with only slight variations as follows:‑

i) Polyanthus are not very successful from late June sowings and are best started in March, April or May.

ii) Polyanthus require more feeding than primroses.

iii) Polyanthus are more susceptible to frost damage.

iv) Polyanthus cannot be forced into flower at such high temperatures without the loss of quality and weak stems.

The lower the average temperature that can be maintained, the shorter the polyanthus flower stem will be, this is very important when growing the newer dwarf Royal Pacific Polyanthus, otherwise untypical long stemmed flowers will appear. Do not forget that polyanthus may also be grown in beds in the cool greenhouse in order to provide a welcome source of cut flower, at a time of year when colour and flowers are limited.

Bringing Plants Indoors

Both primroses and polyanthus, when showing colour, may be brought into the home to provide a short term display of colour but it is important to place them in cool, wet lit positions. Failure to do so will result in pale drawn foliage and few subsequent flowers, unheated rooms are ideal for these plants.

After Flowering

In theory, these plants may be hardened off in late April and May and planted out to go on giving displays in subsequent years. In practice, this is not so simple due to climatic variations and the hardiness of the variety chosen as there tends to be within the commercial growers, a range of primroses and polyanthus that make ideal flowering pot plants. An entirely different group of primroses and polyanthus, being much hardier, sturdier and later flowering, are available for planting in the garden. Few seedsmen will specifically list varieties for pot work or planting out, and careful reading of the description in the catalogues is often necessary.

Primroses and Polyanthus In The Garden

Seed of suitable varieties may be sown as described above, pricked out and grown on in seed trays to be planted out in beds or the cold frame, until autumn when they may be transplanted to their final flowering positions in the garden (Fact Sheets 56 & 126)

Pest,s Diseases and Other Problems

Aphids:‑ commonly seen on Polyanthus and Primroses and easily controlled with a systemic insecticide. Please see Fact Sheet 24.

Red Spider Mite:‑ causes severe retarding of growth and often unobserved, not easily controlled. Please see Fact Sheet 25.

Caterpillars:‑ are commonly seen during the autumn and are best controlled physically, resorting to chemicals if necessary.

Vine Weevil:‑ a major pest in established plants, please see Fact Sheet 128.

Botrytis:‑ often caused by high temperatures and overwatering, can only be controlled by correcting the cultural conditions or spraying with a suitable fungicide.

Plants Grown too Warm is the biggest complaint, move plants to cooler positions remembering they are outdoor plants

Plants become short-lived especially in certain composts - plants are better planted in garden soil in the ground rather than potted in containers and potting composts

Primula sickness - this occurs when primrose and polyanthus are replanted on the same site year after year. Three are no chemical treatments. The best solution is to change the soil for soil the has not grown primroses or polyanthus before.

Sources of Supply

- All the major seed firms supply a wide range of varieties direct by post as follows:-

Dobies, Long Road, Paignton, Devon TQ4 7SX. (Online and catalogue only)

Mr. Fothergills Seeds. 8 St.Leger Drive, Newmarket, CB8 7DT. (online catalogues and in garden centres)

Marshalls Seeds. Alconbury Hill, Huntingdon, PE28 4HY, UK

Suttons ,Woodview Road, Paignton, Devon TQ4 7NG. (online, catalogue and in garden centres)

Thompson & Morgan Ltd. Poplar Lane, Ipswich, Suffolk, IP8 3BU.

Unwins Seeds Ltd., Now part of Marshalls (online catalogue and in garden centres)

Some Varieties that may be available to home gardeners

Primrose Europa mixed

Polyanthus Crescendo mixed

Primrose Forza mixed

Primrose Charisma mixed

Primrose Danova mixed