Potting Compost Mixtures

It is not necessary to make a different compost (mixture of soils) for each kind of plant since one general compost will serve for the majority of greenhouse plants. That known as the John Innes Potting Compost (or JIP.1 for short) is prepared as follows:

7 parts by loose bulk of medium loam.

3 parts by loose bulk of good peat.

2 parts by loose bulk of coarse silver sand.

To each bushel of this mixture add oz. of ground limestone or chalk and 4 oz. of base fertilizer prepared from 2 parts by weight of superphosphate (16% phosphoric acid); 2 parts by weight of hoof and horn meal (12.75% nitrogen) and also 1 part by weight of sulphate of potash.

For many strong-growing greenhouse plants these quantities of fertilizers and chalk may be doubled when they are moved into pots larger than 4-1- in. in diameter, and trebled for 8-in. pots or larger. These stronger composts are often called JIP.2 and JIP.3 respectively.

The limestone or chalk may be omitted for plants known to dislike lime, e.g. azaleas, rhododendrons, and ericas.

Ingredients should be passed through a sieve only for the smaller plants. For final potting of chrysanthemums, etc., use it pulled and chopped to pieces so that the largest fragtments are about as big as a pullet’s egg. Soil should be moist but not wet, and must be at the same temperature as the greenhouse. It is an advantage if loam is first sterilized by steam or electricity. Alternatively, proprietary peat-based or pure peat composts are available.

Pots must be clean. New earthenware pots should be soaked to take the kiln dryness out of them. Drainage holes may be covered with crocks (pieces of broken pot) or special wire ‘stoppers’, but no such extra drainage should be used if pots are to stand on a capillary bench.

Most plants are potted just as they start to grow. Firm potting is necessary as a rule, but not for ferns, which grow best in soil only moderately consolidated, nor for plants grown in all-peat potting composts. A potting stick made from an old broom handle is used to press down the soil.

When repotting plants, do not disturb the roots unnecessarily. Tap them out of the old pot, remove drainage crocks from the bottom, and then repot.

It is seldom wise to pot direct from small into large pots. As a rule, one size increase is sufficient at first, two sizes later, e.g. plants in 3-in. pots will be moved to 4- or 4-1-in. pots, then to 5t or 6tin., then to 7- or 8tin. Water rather sparingly for a few days, and shade from strong sunlight.

Blueing Hydrangeas, Blue flowers on hydrangeas are obtained by growing pink varieties in a rather acid compost (pH 4.0-6.) treated with alum. Good results have been obtained with the following mixture:

3 parts by bulk of acid loam.

1 part by bulk of oak leaf-mould.

To each 1 cwt. add 21 lb. of aluminium sulphate.

Pot plants in this mixture as soon as they come from the propagating frame.

Outdoor plants can be ‘blued’ by top dressing the soil in February with aluminium sulphate at approximately lb. per stem, but it is practically impossible to get any result on highly alkaline soils. Markedly acid soils produce blue flowers’ without treatment.

Bulbs in Fibre, A special compost made with 6 parts by bulk of peat, 2 parts of oyster shell, and 1 part of crushed charcoal is used when bulbs are grown in bowls- without drainage holes. This compost will not become sour, but it contains little or no nutriment and the bulbs lose quality in consequence. It is essential to moisten this fibre before bulbs are planted in it as, when dry, water tends to run off it and not soak in. Subsequently, water must be given occasionally to keep the fibre moist. Bulbs in fibre must be kept in a cool, dark place for at least eight weeks to form roots.

Bulbs in Soil. All bulbs may also be grown in ordinary compost in boxes or pots if these are drained in the ordinary way. In particular, early narcissi, early tulips, hyacinths, and Iris tingitana are frequently so grown and forced into flower between December and March. Bulbs should be potted in August or September. They may be almost shoulder to shoulder and just covered with soil. They are then placed outdoors in a cool, sheltered place and covered with 4 in. of sand or ashes. They must remain in this plunge bed for at least eight weeks before they are introduced to a greenhouse. Even then temperature should not exceed 600 at first. Later, when flower stems appear, it may be raised to as much as 75° to rush bulbs into flower.

Plunging Plants. During the summer months many greenhouse plants, e.g. azaleas, deutzias, genistas, etc., are better in the open in a sunny, sheltered position. To prevent rapid drying out of the soil, the pots are plunged to their rims in a bed of ashes, sand, or peat.

Resting Plants. All plants have a season of rest when they make little or no growth, but this is much more marked with some kinds, notably those with bulbous or tuberous roots. Water supply must be adjusted accordingly and as a rule temperature can be reduced considerably. This can often be effected by moving plants to a different part of the house.

Starting Plants. When growth is about to recommence (or earlier if it is desired to hurry the plant), temperature is raised and water supply gradually increased. With begonias and gloxinias it is best to arrange the tubers almost shoulder to shoulder in shallow boxes filled with damp peat and then transfer them from these to pots filled with ordinary potting soil when they have made two or three leaves each.

Pruning Greenhouse Plants. Shrubby, half-shrubby, and climbing plants often need to be pruned to prevent them from becoming straggly or occupying too much space. This is usually done in February or March unless plants are then in full growth or flower, when it is deferred until after this. An exception is made for regal and show pelargoniums, which are pruned in early August. Method of pruning will depend upon requirements and type of growth. Climbers can, as a rule, have some of the oldest growths removed if overcrowded, and others shortened a little. Most shrubby plants can be cut back severely if it is desired to restrict size, but hard cutting must be avoided with hydrangeas, as it limits flowering.

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