The plants can be bought as pot or ground tubers, a tuber being the thickened storage root which can be stored over the winter in a frost-free place and planted out the following spring. The ground tubers are not easy to obtain, but the pot tubers can be bought from a nurseryman who specializes in dahlias. Pot tubers are raised by keeping rootedgrowing in 3- or 5-in. pots for a season, then harvesting and retailing the resulting compact tubers in the following dormant season.
If the gardener has no, rooted cuttings can be bought in May or June, each plant being grown separately in a3-in. pot and planted out.
There are also three ways in which a gardener can raise his own plants, from seed, from striking cuttings and by division of mother ground tubers.
Plants of the single Coltness Gem type which can be obtained in mixture or in strains coming reasonably true to colour, and other mixed bedding types, such as Unwin’s Dwarf Hybrids, which have a range of variously coloured semi-double blooms, can be grown from seed. Seeds saved from named varieties produce very few plants which even approach the form or colour of the parent plant, but it is interesting to grow on and test out seedlings from good varieties in the hope that a seedling worth perpetuating will result.
Sow all types of seed in late March or April in a pot, pan or box containing John Innes seed compost, or one of the recently introduced composts ofand sand or peat and vermiculite, which are blended with a balanced base and can be obtained from a nurseryman. These new composts can be stored in a dry state for any length of time, and only require a thorough soaking before use — the peat and vermiculite mixture soaks more easily than the peat and sand mixture.
Distribute the seed thinly on the levelled surface of the well-moistened compost and cover the seeds with a further layer of compost or sand about 1 in. deep. No water need be added after the seed has been sown.
Cover the containers with paper to keep them moist and to prevent them from becoming overheated if they stand in direct sunlight.
If the pots, pans or boxes stand in a greenhouse, place them in a polythene bag before covering them with paper, to preserve the moisture and encourage germination.seeds germinate rapidly, though sometimes erratically. Therefore keep a close watch over them and remove the paper as soon as there is any sign of growth.
Raise the seed in a steady temperature of 60 to 65° F. (16 to 18° C.) to ensure an even germination. If the greenhouse or frame is not normally heated to this temperature, a useful propagating unit can be constructed with a wooden-sided frame and polythene-covered lights over the top. Bury mains or low-voltage heating wires in the middle of a layer of sand 2 to 3 in. deep on the bottom of the frame. These will make it possible to maintain an ideal temperature for the germination of all seeds and for the striking of cuttings.
Prick off the seedlings as soon as they can be handled, and, to ensure good plants, put 6 rows of 5 plants into each seed box containing any of the recommended potting composts.
Once the seedlings are established and there is no danger of frost, remove the trays to a cold frame to harden off the plants.
If there is no heat in the greenhouse, sow the seeds in April. Plants from the seedlings will flower slightly later than those grown from seed which is sown in March.
Cuttings are obtained from the previous year’s ground or pot tubers, which have been stored through the winter in a frost-free place. At about the beginning of March take the old tubers out of store, clean away any old soil and cut out any diseased parts.
Place the tubers in a suitable container, such as an old kipper box, on a layer of light soil, peat or compost. Cover them with the soil, peat or compost to just below the level of the crown, and water gently so that they are kept moist but not too wet.
Put the box in a greenhouse which is heated to a temperature of 60 to 65° F. (16 to 18° C.) to induce immediate growth. Although the tubers are easier to handle in boxes, they can also be placed on a greenhouse bench and covered directly with soil.
After a time shoots begin to grow from the crowns of the tubers. When the shoots reach a length of 3 to 4 in., take cuttings by severing the shoots just below the bottom pair of leaves. Trim each cutting, leaving only the top pair of leaves and the central growing point, and sever the cutting again with a sharp knife or razor blade immediately below a node Further shoots will develop from the dormant buds on the tubers at the base of the first shoots and provide an ample supply of fresh cuttings.
To encourage the rooting of the prepared cuttings dip them in water to a depth of J in. and then into one of the proprietary hormone rooting powders. Shake off any surplus powder before inserting the cuttings in the rooting medium.
The rooting medium can be either a mixture of equal parts peat and sand, the peat and vermiculite mixture already mentioned, or just sand or vermiculite. Put the chosen rooting medium in a container which will accommodate the cuttings — a 3-in. pot for example will take four — and place each cutting in a hole made against the side of the pot with a small stick or pencil. Firm the compost up to the cutting with the stick.
During propagation make sure that each batch of cuttings is correctly labelled with the varietal names and the date the cuttings were made. Then place the cuttings in a propagating frame and keep them moist. Avoid excessive moisture and condensation, which can cause severe damping off and the consequent failure of the cuttings, by allowing the cuttings a little, especially at night.
The cuttings do not usually take more than 2 to 3 weeks to root, although the time taken does vary according to the variety and the conditions for growth. When the young leaves start to grow again, which is a reliable sign that rooting has taken place, pot the rooted cuttings singly in 3-in. pots, using either John Innes No. 1 potting compost, peat-sand or peat-vermiculite compost.
Peat pots are invaluable for young plants such as dahlias. If the rooted cuttings are potted into them the plants in their pots can be planted out into the soil with a minimum of root disturbance, and the pots eventually disintegrate. Peat pots can also be used for striking cuttings, since they can be inserted singly in Jiffy-strips or in small-sized pots filled with the vermiculite-peat compost, and each plant, when rooted, can be potted on into a 3-in. Peat or clay pot.
After potting shade the plants for about two days and then keep them in a well-ventilated greenhouse or frame until the end of April, when they should be removed to a cold frame and hardened off.
The lights need not be used unless there is likely to be a frost, when it will probably also be necessary to cover the frame with matting or sacks.
From the time the plants are potted until they are planted out they must be kept well watered, but never allowed to become waterlogged.
The third method of propagation is by division of the tubers. Each division must have an eye (an alternative name for a bud) attached, because if a fang of the tuber is removed without a bud no amount of care will make it grow. If it is difficult to see the buds, place the tuber in a warm spot in peat or soil until the buds just begin to develop.
Use a sharp knife to cut through and separate as many sections of the tuber with a bud as possible, and in the case of those varieties which produce very small and thin tubers take care not to damage the eyes. It is advisable to pot or box the divided tubers and grow them on for planting out inas green plants at the end of May.