Growing Dahlias – Propagating Dahlias and Dahlia Bulbs

propagating dahlias

Dahlias can be multiplied in three different ways : by taking cuttings, by the simple division of their roots or by growing them from seed.

If you require many new plants, exactly like the parent plant, then taking dahlia cuttings is the best method, but division of the roots allows gardeners who don’t own a green house, to increase their stock with ease.

Growing dahlias from seed is a much used method for propagation, as the resulting varieties give a great range of differing colours as opposed to an exact match.

Propagating Dahlias by Taking Cuttings

For the average amateur, when propagating dahlias, one dahlia tuber of each variety will produce an adequate number of cuttings. The remaining tubers can be left in store until planting time, or can be started into growth about one month before planting them out. The number of cuttings that can be obtained from any one root varies between five and ten.

When the shoots have grown to about 3 in. in length, which takes roughly a month, each one should be cut off together with a small portion of the old stem, which is called a ‘heel’. I feel that cuttings taken with a heel form better tubers, and more of them, throughout the growing season, than those that are cut above the root and just below a leaf joint. If cuttings are taken without a heel this usually allows more shoots to develop in their place, but normally far too many are produced for most amateur gardeners.

To get at the base of the shoots scrape the peat away from the tubers.

Prepare the cuttings with a very sharp knife or razor blade, in order to obtain a clean, smooth cut, rather than a tom one. Trim the bases of the cuttings neatly, to remove any ragged edges, and remove the bottom leaves, also the next pair if necessary, to give a good length of stem for insertion in the rooting mixture. When the cuttings have been prepared dip the bases in water and then into a hormone rooting powder. This will assist in the rapid growth of roots.

There is a reasonable range of suitable rooting mediums for dahlia cuttings, but the one I use is made up of equal parts loam, peat and coarse sand, the parts being by bulk. Some people use the John Innes Seed Compost, which is certainly suitable. This can either be bought ready mixed from horticultural nurseries, or be mixed at home. It may be made up as follows: 2 parts sterilised loam, 1 part peat, and 1 part coarse sand (parts by bulk). To each bushel of this mixture add: ¾ oz. of ground chalk or limestone, and 14 oz. of super-phosphate. Mix all the ingredients thoroughly. Other rooting mediums are pure coarse sand, and vermiculite. If you are rooting them in either of these two mediums they must be potted up as soon as roots have formed as these materials contain no plant foods, and the young plants will starve.

The cuttings can be inserted in the chosen soil with a wooden dibber (a small stick with a tapered end), either around the edge of a pot, or in seed boxes. Insert the cuttings about to 1 in. deep, and firm them well. Make sure that the containers are well drained beforehand by placing a layer of broken flower pots in the bottom, over which a layer of coarse peat or leaves should be placed.

After watering the pots or boxes of cuttings they should be placed on the greenhouse staging and covered with sheets of newspaper for a few days to protect them from the sun, and to prevent flagging. The temperature of the greenhouse should be maintained between 13° to 16° C. (55° to 60° F.) with a moist atmosphere.

If a closed propagating frame is available in the greenhouse the cuttings may be placed in this, where they are less likely to flag and will root much quicker. Again, pots or boxes can be used, or they can be planted directly into the bed of the frame, using any of the rooting mediums which I have recommended.

By far the quickest way of rooting dahlia cuttings is under a mist propagator. This system prevents flagging completely, and the cuttings will root in no time at all. I am not suggesting that this is an essential item in dahlia cultivation, but if you are lucky enough to possess one of these, then by all means root your dahlias under it. One important point I should mention is to keep each variety separate, and label each one.

Potting Rooted Dahlia Cuttings

When the dahlia cuttings are seen to be making new growth this is a sign that they have formed roots. If they are being rooted in pots it is a simple matter to invert one of the pots, tap the edge of it on the side of a bench, and so expose the root ball. If a mass of white roots has formed then it is time to pot the young plants.

For this potting I use 3 1/2  in. pots, and place one plant in each. Again, it is advisable to add some drainage material before potting, I.e. a layer of broken flower pots, plus a layer of peat or leaf-mould. The soil I use is the John Innes No. 1 Potting Compost. As with the seed compost, it can either be mixed at home, or bought in from a garden nursery. For those who like to do their own mixing the formula is as follows: 7 parts good, sterilised medium loam, 3 parts peat, and 2 parts coarse sand (parts by bulk). To each bushel of the mixture add 3/4 oz. of ground chalk or limestone, and 4 oz. of J. I. Base Fertiliser (obtainable from garden nurseries).

The young plants should be potted fairly lightly, firming the soil with the fingers, and then placed on the greenhouse bench, and watered well. If the weather is sunny they may need shading for a few days until they are established. A daily syringe with clear water will help to prevent flagging. Ventilate the house during fine spells.

If the cuttings were rooted early in the year, say February or early March, they may need to be repotted in 5 in. pots otherwise they could become short of food. Use the same compost as for the first potting. April-rooted cuttings should not need this further potting before planting-out time.

Pinching Out Dahlia Plants

When the young plants have made three or four pairs of leaves, pinch out the growing tip to encourage side shoots to grow. You will then have a good, bushy plant when the time comes to plant out, rather than a single-stemmed plant. The later-rooted cuttings will probably need to be planted out before pinching them, but after a week or so in the garden the growing tips can be removed.

Hardening Off Dahlia Plants

Before the young plants are placed out in the garden they must gradually be acclimatised to lower temperatures. If they are taken straight out of a greenhouse with a temperature around 13° C. (55° F.), into the garden, they would suffer a considerable check to growth. Therefore, the hardening-off process begins in early May, about two or three weeks before planting out, in the southern half of the country. In the Midlands and the North the time to start hardening off would be proportionately later.

The pots are placed in a cold frame, and if a frost threatens the frame lights should be covered with layers of sacking or straw. Give them plenty of ventilation, increasing the amount gradually until the lights are left off completely, both during the daytime and at night. From the middle of May they may be placed under the wall of a greenhouse or the dwelling house. They will be sufficiently protected here until planting time. All the time they are in pots do not forget to check them regularly in case they need watering.

Propagating Dahlias by Division

The propagation of dahlias is not restricted to those gardeners who own a greenhouse. Another method is to split up any large roots into a number of smaller ones. The dormant tubers can be divided just before planting them out; they are not boxed up in peat first to start them into growth. When doing this job it is important to note that each division should consist of a stem with a tuber or tubers attached. It is from the base of each stem that the new shoots will arise  so if there is no stem present then a new plant will not form. The tuberous roots may be either pulled apart with the hands or cut cleanly with a knife. Some of the large roots can be split into as many as four, five or six separate plants.

Those with a heated greenhouse may prefer to start the tuberous roots into growth and divide them when the shoots have developed. By this method you can be sure that each division contains one or more growths and has every chance of success. Box the tubers up during April in peat, or a mixture of equal parts peat and sand. The tubers can then be split up just before planting them in the garden, when all fear of damage from frost has passed.

30. September 2010 by Dave Pinkney
Categories: Bulbous Plants, Plants & Trees | Tags: , , | Comments Off on Growing Dahlias – Propagating Dahlias and Dahlia Bulbs


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