Pricking Out, Potting and Hardening Off
After germinating the seeds, the next stage is pricking out, the transfer of the seedlings into more spacious growing conditions where they can develop fully in a suitably nutritious compost.
After germination the seeds should be given as much light as possible to keep them sturdy. As soon as they can be handled conveniently, which is usually just as they are about to produce their first true leaves as distinct from their seed leaves, they must be transplanted into other boxes.
The boxes used are the standard seed trays. They are prepared exactly as for seed sowing and as a rule the same seed compost is used but, occasionally, for strong growing plants, a richer compost is used such as the John Innes No. 1 Potting Compost.
The seedlings should be very carefully lifted, one at a time, from the seed box. A wooden label makes a good tool for this and it is important that the compost in which the seedlings have been growing is in the right condition, nicely moist right through, neither too wet nor too dry, otherwise the roots will be damaged. Hold each seedling by one leaf and transfer it immediately to the box in which it is to be pricked out. Make a small hole for it with a wooden dibber, drop the roots straight down into this and press the soil gently around them with the dibber. The seedlings should be spaced approximately 2 in. apart in straight rows, except for those which are to remain in the boxes until planting out time which should be set 3 to 4 in. apart.
When the box has been filled with seedlings it should be watered to settle the soil around the roots. The watering-can must have a fine rose to prevent the soil being washed away and the roots disturbed. Give enough water to soak right through the container.
It is important to remember that the plants at this stage are unable to take up all the moisture they require and shading from the sun is necessary for the first few days after pricking out until they have made fresh roots. Sheets of newspaper placed above the seedlings will provide shade and spraying overhead on warm days will help to prevent flagging. The shading must be removed as soon as the plants have re-established themselves for they now need the maximum amount of light, except in the case of begonias and other plants which scorch easily and need shade during the hottest part of the day.
Bedding plants can be moved out to a frame once they have become established, but those which are to be moved on into pots should receive this attention before they become overcrowded – in fact as soon as the leaves of adjoining plants are touching. Pricking out is the half-way stage between the development of seedlings and potting or planting out.
This is an operation that can be mastered very quickly. One should always aim to grow a large plant in a small pot rather than the reverse. This is why the moves to larger pots are usually only modest increases: the progression is in easy stages. Plants from boxes or trays are normally potted into 3 or 3-½-in. pots. When established in these they are moved on to 4- or 5-in. pots and, if it is necessary to pot them on again, use the 6- or 7-in. size. Large plants such as chrysanthemums orwill eventually need 8- or 9-in. pots.
The reason why only modest increases in pot size are made is that where large amounts of compost are involved it is liable to become wet and sour before the plant can fill the pot with a network of roots. This results in sickly plants which never develop satisfactorily. It is usual for John Innes No. 1 Potting Compost to be used when potting plants in small pots and John Innes No. 2 and No. 3 Composts for the larger sizes.
Plants must never be removed from pots or boxes when the soil is dry. It is always advisable to give them a thorough soaking an hour or two before such moves. When removing the plants care must be taken to keep the root ball intact.
Plastic pots, which have moreholes in the base than the traditional clay pots, need not be provided with a layer of broken crock before filling with compost. But clay pots need crocks to prevent the drainage holes from becoming blocked. Place a little soil in the bottom and press moderately firmly with the finger tips. The plant is then centred in the pot and the space between the root ball and the side of the pot filled in with compost trickled from the hand. This is lightly pushed down and then firmed with the fingers. This process is repeated until the firmed compost is within 1 to ½ in. from the pot’s rim (depending on the size of the pot). This space is left for the purpose of watering.
A potting stick or rammer is often necessary when potting on plants into 7-, 8-, 9- or 10-in. pots. This can be made from a broom handle by cutting a piece about 12 in. long and shaping a wedge at one end. The firming done with the fingers in the case of smaller pots is done much more effectively with the wedge end of this simple tool, and potting is completed by firming the surface with the blunt end of the rammer. The soil must be really firm when potting into such large containers.
On the whole it is difficult to generalise about the firmness of the soil, but plants which need light potting include calceolarias, double-flowered and winter-flowering begonias, gloxinias and streptocarpus. Plants like pelargoniums, fuchsias, impatiens and hydrangeas can be firmed adequately with the fingers., chrysanthemums, carnations and tomatoes in their final pots, and the majority of woody plants need firm potting with a rammer.
If the soil used for potting is reasonably moist, as it should be, and the plants are watered an hour or two before potting, then there is no need to water the plant immediately after this operation is completed. The plants can be left unwatered for two days in summer and three to four days in the wetter, duller months. A rose should be used on the can for the first watering to avoid soil disturbance.
The remarks I have just made refer to conventional composts and not the soilless kinds, which need different treatment. A rammer should never be used with these for they do not need to be made so firm, and as I have already mentioned, they are not really suitable forbecause they do not provide sufficient weight to keep the pots upright.
All pots must be thoroughly cleaned before used. Clay pots need soaking and scrubbing. Plastic pots are easily cleaned with a damp cloth. Remember, too, that new clay pots must be thoroughly soaked in a tank of water for an hour or so before use otherwise they will draw moisture from the soil to the plant’s disadvantage. In addition, it becomes very difficult to remove plants from pots which have not been soaked.
The term hardening off refers to the gradual acclimatisation of plants grown in heated structures to outdoor conditions. In this instance ‘gradual’ is the operative word for this is a process which must not be rushed. A too rapid transfer from warm to much cooler conditions would give plants a very severe check or indeed kill them outright.
Let us consider seedlings andraised in a warm propagating frame first. When these are ready for hardening oil it is best if the frame can be left open for a day or so before moving the young plants into the proper. This cannot be done, however, if the propagating frame also includes seedlings or cuttings which are not sufficiently advanced to be subjected to such treatment. Once out on the staging it will be necessary to move the plants gradually to the coolest part of the house.
Those plants intended eventually for outdoor cultivation will be moved on to a garden frame where, over a period of ten days or so, thewill be progressively advanced from nil on the first day or so to the complete removal of the frame light. During this time watch the plants carefully to make sure they are not suffering from too much exposure. This will be shown by markings forming on the foliage and a general blueing and cessation of growth. If cold winds develop during the hardening off process, the frame light should be so propped open that the wind does not reach the plants. All plants abhor draughts and none more so than those at this delicate stage of their development.
If space is at a premium, the plants can often be removed from the frame rather earlier (say, after about a week) and stood under a sheltered wall of the greenhouse, garage or home.