Seed Sowing: Potting Off and Potting On
The basic terms used in potting are `potting off’, when young rooted cuttings or seedlings are moved from trays into pots, and ‘potting on’, when the more advanced plants are transferred to bigger pots.
Nowadays plastic pots are generally used in preference to clay, but whichever type you have ensure that they are clean and dry before using them.
As soon as cuttings have developed a good root system they should be carefully lifted from their trays and put into individual pots about 7.5-9 cm (3 – 3-½ in) in diameter. When seedlings are large
enough to handle easily they can be treated in the same way (as an alternative to pricking out into trays).
For this first potting, use a fairly weak compost, such as J.I. No 1, or an equivalent soilless type consisting of loam, peat, coarse sand, John Innes base fertilizer and ground chalk.
Allowing for drainage
Drainage material is not necessary in plastic pots as the holes are devised so that the compost does not leak. Furthermore, there is a trend towards using less
drainage material in the bottom of small clay pots. When there are some drainage holes provided, place a few crocks (pieces of broken clay pots or stones) over the drainage holes and cover with a thin layer of roughage such as coarse peat or partially-rotted leaf mould. If you are using soilless compost, crocks or drainage materials are not normally necessary. Place a layer of compost over the drainage material and firm lightly with your fingers.
Transferring the plants
Hold the rooted cuttings or seedlings in the centre of the pot, with the roots well spread out, and trickle compost all around until it is slightly higher than the rim of the pot. Give the pot a sharp tap on the bench to settle the compost well down and lightly firm all round with your fingers. Make sure the compost is pushed right down to the bottom.
Some soilless composts, however, require little or no firming, so check the manufacturer’s instructions first.
Remember to leave about 13mm (½ in) between the surface of the soil and the rim of the pot to allow room for watering.
After potting off, water the plants thoroughly, using a fine rose on the watering can, to settle them in further. Then they can be returned to the bench.
Plants need potting on to prevent them becoming ‘pot-bound’ (when the roots are packed very tightly in the pot). If this happens the plants will suffer from lack of food, growth will be poor and they will dry out very rapidly and require frequent watering.
However, it is worthwhile noting that some plants, such as pelargoniums, are more floriferous (bear more flowers) when slightly pot-bound.
Plants should be moved to the next size of pot, for instance from a 9cm (3-½ in) to a 13cm (5 in), from a 13cm (5 in) to a 15cm (6 in) and so on. The reason for moving only to the next size pot is that plants dislike a large volume of soil around their roots because they cannot absorb water from all of it and, therefore, it is liable to remain wet. This can result in root rot and the possible death of the plant. Small moves allow plants to put out new roots quickly.
- place crocks and drainage material in bottom of larger pot, then carefully remove plant from old pot;
half-fill new po wiht compost;
hold plant in centre of pot, add more compost to within 2cm (1/2 in) of rim;
firm all round plant.
Composts and drainage
Richer composts (those containing more plant foods) are generally used for potting on. If you prefer the John Innes type, then use No 2, which contains twice as much fertilizer and chalk as No 1. Some plants (for example chrysanthemums, tomatoes and strawberries for fruiting under glass) like an even richer compost, such as J.I. No 3 — particularly when they are moved into their final size of pot).
Drainage material, as described under potting off, is generally advisable when using soil composts in pots that are 13cm (5 in) or larger. A layer of crocks about 2-3cm (1 in) deep should be sufficient, plus roughage.
Repotting the plant
Remove the plant from its pot by turning it upside-down and tapping the rim of the pot on the edge of a bench. The rootball should then slide out intact. On no account disturb this ball of roots and soil, but remove old crocks, if any, from the base. Scrape off any moss or weeds on the surface with an old wooden plant label or similar object.
Place enough soil in the new pot so that, when the plant is placed on it, the top of the rootball is about 13mm (1/2 in) from the top of the pot. This will allow for a light covering of compost with room for subsequent watering. Firm the compost lightly with your fingers and then stand the plant in the centre of the new pot. Trickle fresh compost all round the rootball until you reach the top of the pot. Give the pot a sharp tap on the bench to get the compost well down and firm it all round.
If you are using soilless composts, follow the maker’s instructions for firming. You will probably need to add more compost to reach the desired height. Finally, water in the plants, using a fine rose on the watering can.
Potting on is best done when plants are growing actively in spring and summer — or in the autumn, although growth will then be slowing down. Plants potted in spring and summer will quickly root into the new compost because of the warmer weather.