How to grow Dahlia
Commemorating Andreas Dahl, a Swedish botanist who was a pupil of Linnaeus (Compositae). Half-hardy, tuberous-rooted perennials from Mexico, first introduced into Britain in 1789 by Lord Bute.
Species cultivated (Few of the following original species are available, although they may occasionally be seen in botanic gardens and the like). D. coccinea; 4 feet, scarlet, September, the parent of the single dahlia. D. coronata, 4 feet, fragrant scarlet flowers on long stems, autumn. D. excelsa, 15-20 feet, purplish-pink flowers, summer. D. gracilis, 5 feet, scarlet-orange flowers, September. D. juarezii, 3 feet, parent of the cactus dahlias, flowers scarlet, late August and September. D. merckii, 3 feet, lilac and yellow flowers, October (together with D. variabilis the parent of most modern double dahlias). D variabilis, 4 feet, (syns. D. pinnata, D. rosea, D. superflua), variable flower colours, even a green form was suspected at the end of the nineteenth century. The parent of show, fancy and pompon dahlias.
Cultivation Nowadays dahlias are comparatively easy to grow. They tolerate all soils between the moderately acid and alkaline and for ordinary garden purposes need little or no specialized attention, yet will flower profusely In their evolution they have produced multiple types and hundreds of thousands of varieties simply because they are a cross-pollinated plant. This means that it is possible to produce unusual and original cultivars by raising plants from seeds, which is an additional asset. Furthermore, with correct culture, plants will flower continuously from July until the first autumn frosts, providing a colourful display over a range of several months.
Soil preparation This begins in winter or early spring by digging of the site, at the same time incorporating plenty of bulky organic materials such as peat, leaf-mould, spent hops, vegetable compost, or well-rotted horse, pig or cow manure, but not poultry manure which encourages too much growth at the expense of flowers. Put any of these into the top foot of soil, because dahlias make a mass of fibrous roots in this region. The organic materials can be mixed into the planting holes if only a few tubers or plants are grown, or if dahlias follow spring bedding plants, but generally it is better to dig them in the ground overall.
A fortnight before planting, topdress the ground with a general granular fertilizer containing a higher amount of potash in comparison with the nitrogen and phosphate content Root crop fertilizers have this analysis, and potato fertilizers are very good for the purpose. This application will provide the extra plant food needed during growth, the organic materials previously supplied mainly providing humus for improving the soil conditions and water retention.
Type of stock The choice of stock will depend on the purpose which the plants are to fulfil Dormant tubers are best for a generaly garden display, for they flower earlier than dahlia plants and produce more flowers over the season as a whole. If you want extremely early flowers, for instance blooming in May, you can plant tubers in pots, or even in the greenhouse border, in February. If you have cloches, you can plant tubers out of doors in April and they will start to flower during early July. Remove the cloches in mid-June. In both these instances the best flowers will be over before the growing season has finished. For the best results over the whole season, plant dormant tubers during the first half of May out of doors. They will not usually need protection, because by the time the shoots emerge above ground level it is likely that the threat of any late spring frosts will be past. Nevertheless keep sacks, pots, polythene bags or other materials handy in case of occasional night frost at this time. Flowering will start in late July and early August.
There are two types of tuber, one being the ground root, a large bulky root resulting from growing a dahlia out of doors without restricting the roots. If replanted from year to year, the number of tubers tends to increase to excess, too many poor quality flowers result, and vigour and tuber formation decrease. Division every second year into several portions is advisable; each portion containing several growth buds, or eyes, and having at least one complete healthy tuber to start them into growth. (At this point, it may be noted that, unlike the potato, dahlia eyes are not on each individual tuber, but are congregated at the base of the old stems). An easy way to judge how many portions a root can be divided into is to put it in the greenhouse for a fortnight or three weeks. Spray overhead with water every second day until the shoots are about inch long. Do not bury the tuber in any material as this will encourage unwanted root growth. With a small hacksaw cut the root into portions according to where the emerging shoots are grouped, or lever it apart with an old screwdriver.
The other type of dahlia tuber, the pot-grown or pot tuber, is often sold in general garden shops and multiple stores in the spring. It is produced from cuttings struck in early spring and grown in pots all through the season so that the roots are restricted and the tuber forms into a neat rounded mass. Although pot tubers are easy to store and transport, forming very good stock for the garden, they are not so good as ground roots for producing cuttings, generally having insufficient bulk to be divided. Pot tubers become ground roots after a season of growth out of doors and their planting times are the same as for ground roots. Before actual planting, chip away some of the wax coating if present to allow moisture to swell the tuber. All tubers can be planted until mid-June.
The dahlia plant itself, which provides a type of stock commonly sold by dahlia nurseries, is formed by rooting dahlia cuttings. Plants grown from cuttings flower later than those grown from tubers, though if you need early flowers before mid-August, it is a good idea to specify on the order sheet ‘April Delivery’. If you have a greenhouse or frame, you can then pot the plants into 5 inch pots and they will grow into fine bushy specimens by planting out time. This is standard technique for large and giant-flowered varieties.
It is not difficult to keep Dahlias from one 6 years to the next if simple precautions are observed. The Dahlia is a native of the warm Mexican climate: therefore, in cooler areas the tubers must be lifted in the autumn, dried and stored in a dry, frost-free place.
1 Dahlia foliage is very sensitive to frost and will be blackened by the first frost.
2 Immediately after this occurs, cut down the plants to within 9 inches of ground level.
3 Remove the tops and lift the plants with a fork, taking care not to spear the tubers in the process.
4 Shake off the soil and stand the tubers upside-down in a dry, airy place to drain away the surplus sap.
5 Once the tubers are thoroughly dry, they can be stored in any dry frost-free place.
6 Storing them in a bit of dry peat is not necessary, but it will help to exclude frost and absorb excess moisture.
7 Tubers can be stored outside in a deep frame, if they are close together against a 10 sheltered wall and protected with leaves and mats against frost.
8 Dahlias are usually propagated from cuttings made from shoots that arise from the old stems. These arise from the tubers, which are stood close together on a greenhouse bench and covered lightly with soil or peat to retain moisture.
9 Maintain a temperature of 60°F (16°C) and take cuttings when the shoots are 2 to 3 inches long.
10 With a sharp knife, remove lower leaves.
11 Make a basal cut below a joint.
12 Insert the cuttings firmly with a dibber around the edge of a 31 inch pot of sandy compost.
13 Each cutting should be clearly labelled with the name of the variety and watered in. Then stand the pots in a close frame in the propagating house. Hormone rooting powders usually increase the number of cuttings that root while reducing the time. The inclusion of a fungicide helps reduce loss.
You cannot plant unprotected dahlia plants out of doors until late May, or even safer, the first ten days in June. With cloches or in sheltered situations, free from late spring frosts, you can plant out in late April or early May. In the north and in Scotland, mid-June.
Planting Out This stage is best tackled by taking out a hole in the ground with a small spade. Stakes should be inserted at this time to avoid damage to the tubers which would occur if they were put in later. The hole should be wide enough to prevent cramping and deep enough to allow the upper surfaces of the tubers to be about 2 inches below ground level. Replace the earth on top, shaking the tuber to settle it round the root as you proceed, firming it in by gentle treading. This applies to both ground and pot tubers. Planting distances are 2 feet apart for pompons, 2i feet for ball dahlias and all others, except the large and giant decoratives, such as cactus and semi-cactus, which should be 3 feet apart.
Keep the soil watered periodically to swell the tubers and to start the shoots into growth. Shoots should emerge above the soil within five weeks; if not, dig up the tuber and inspect it for decay and slug damage. Slug pellets applied above soil level round the root when planting both tubers and plants are an advisable precaution. Dahlia plants are placed in a hole taken out with a trowel and their roots set so that the potting soil is just below ground level. Bituminised paper, or fibre pots, should be carefully removed from the plants before planting out. (With peat pots especially, make sure to keep the soil moist enough to encourage the roots to penetrate into the open ground, since failure to do this is a frequent cause of stunted, poorly growing plants). Again, it is important to plant to a stake, previously driven in, thus avoiding damage to the roots.
Moreover, arranging the stakes in a desired pattern can be a useful guide to design.
Summer Care The main requirement is copious watering, not a lot of feeding.
Provided that you have prepared the soil as suggested, all that will be needed during the growing period will be two topdressings of sulphate of potash, each at the rate of f oz per square yard. One should be given at the first sign of the petal colour opening from the bud, to improve stem strength and flower colour; the other during early September to improve tuber formation. Monthly feeds of liquid manures made from seaweed extracts are also very good and give excellent results even if used for foliar feeding. The dahlia makes a lot of leaves in August and even in very wet weather the soil may remain dry round the roots.
The need to water very frequently can be largely avoided even in the hottest weather if a thick mulch of straw is provided at the roots in mid-July. This keeps down weeds as well as encouraging better root growth.
Tubers will need at least one strong stake, but dahlia plants are better if they are supported by a triangle of three canes or stakes. Such plants have to carry all the weight of stems, leaves and flowers on one main stem or ‘leg’, so are very prone to wind damage. Tubers on the other hand, push out rigid shoots from below soil level and are much less likely to be broken by the wind in the early stages of growth. These shoots should be tied to the stake every 18 inches, whereas the dahlia plant needs tying every 6 inches for additional protection. A good average length for dahlia supports is 5 feet; these are knocked in to the ground to a depth of 1 foot. Avoid having the stakes higher than the blooms because the wind will knock the flowers against them.
Ground tubers can be left to produce flowers on the tips of their main stems.
Allow about eight main stems per division to emerge, and cut off any others below soil level carefully with a knife. Large and giant-flowered varieties should be allowed to produce about five stems only.
Pot tubers, unless they produce sufficient main shoots from below soil level, will have to be treated like green plants. The leading growth tip of the plant is pinched out, or ‘stopped’, about a month after planting out, usually when about six pairs of leaves have developed. This encourages sideshoots to be pro duced so plenty of flowers come into
bloom as a start; otherwise, if not stopped in advance, dahlia plants pro duce one central flower only at first.
Take notice which are the strongest emerging sideshoots after stopping, and
when they are 3 inches long, remove the excess ones by snapping them out from their joints with the main stem. Retain five shoots only, however, with large and giant-flowered varieties. The technique with the pot tuber is to select initially the strongest main shoot, similar to that of the dahlia plant as the central growing stem, removing the others. This main shoot will be stopped and the sideshoots selected in exactly the same way. A ground tuber is not usually stopped, the flowers being borne on the terminal, or crown buds of each stem. It can, however, be treated like a pot tuber or green plant as far as shoot growth is concerned, but by stopping and selecting one main stem, the flowers, through having to be produced on sideshoots, will be about three weeks later than on the tips of the main stems. Pompon varieties need no de-shooting or disbudding.
Disbudding should be done to all other types when the flower buds are about the size of a pea. Allow the main, centrally placed, largest bud to remain and flower on each shoot, removing the others, together with the fresh secondary shoots which will emerge from each leaf joint on the stem as the flowers mature. Leave just one, fairly low down, on each stem to produce the successive flower, again disbudding and de-shooting. This technique is adopted throughout the flowering period and is the only way to achieve a long flowering season combined with good quality flowers with long stems for cutting.
Left to their own devices, dahlias produce a mass of buds and flowers and soon become uncontrollable, their very tiny, poor blossoms often becoming single by the end of the season. If you need small-flowered dahlias, grow special small-flowered varieties.
Lifting and Storing Ideally this is done once frost has blackened the foliage. If, however, the autumn continues without frost, it does no harm to lift dahlias in late October and early November. Only in the mildest of places, in very sheltered situations or during unusually gentle winters can dahlias be left out of doors in the ground all winter. They can be put into a clamp in the same way as potatoes, but the disadvantage here is that they may be killed if the weather becomes very severe. Furthermore, you cannot examine them for signs of rotting or put them in the greenhouse to take cuttings.
To lift dahlia roots, first cut off the stems just above soil level. Then lift by prising in a circle with a broad-tined fork, working well away from the stems. After lifting the roots clear of the soil, pick off as much adhering earth as possible. Then place the roots upside down in a well-ventilated greenhouse, frame or shed for at least a fortnight. During this period they will lose excess moisture and by the time the remaining soil becomes dust dry, they will be ready to be put into winter storage. There they should be covered with sacks or straw at night if frost threatens. Only in very wet autumns should artificial heat be used, never exceeding 70°F (21°C).
Before placing them in store, retrim the stems as low as possible, without actually cutting into the tubers. Retie the labels on one of the tubers, because in store the stems will become paper dry and will actually drop off. Most dahlia roots need no covering in store, and in fact, a frequent cause of loss during the winter is covering them up, putting them away in a cupboard and forgetting about them until the spring. Lay them on racks in a frost-proof shed, cellar, or in a greenhouse which can be kept frost free. Straw bales provide good frost protection.
Very tiny tubers, however, should be covered in boxes or pots with materiall such as garden soil or sand. During the winter, sprinkle the surface with water very occasionally if it gets dust dry, but avoid giving sufficient water to start the tubers into growth. A good temperature to aim at in store is 40-50°F (4-10°C); failing that, it should never fall below 34°F (1°C) nor exceed 50°F (10°C). If you have to store them in a warm place, shrivelling is likely, so all tubers must then be covered with sand or soil in boxes, but keep the boxes separated and put only one layer of tubers in each box. Avoid any store that is subject to drips or draughts, or is so airtight that it encourages fungus rot.
Every month inspect the tubers and if any parts are rotting, cut them out with a sharp knife. Dry the surfaces left with a rag and smear on captan or zineb to prevent further rot. Occasionally fumigate with smoke pellets to deal with aphids which may have hatched out in store or bulb flies which sometimes attack the roots.
Pests and diseases As a general precaution, always spray dahlias with insecticides every three weeks during the season of growth, including those growing in the greenhouse and frame.
Sometimes the soil becomes infected with verticillium wilt, when the stock must be burnt and a fresh growing site found. Cauliflower-like outgrowths, due to crown gall, also mean that affected stock must be destroyed, but it is slow 3 to spread and healthy stock can still be grown in the same ground.
A common leaf disease, especially in humid summers, is dahlia leaf spot, causing light green ringed spots which later turn brown. In this event, treat the leaves with zineb.
Plants are sometimes attacked by virus diseases, of which light green patches or yellowing bands up the veins and perhaps dark green blisters on the leaves are symptoms. A more certain sign is dwarfing of the plant, which becomes very close-jointed and bushy, producing small flowers. Burn stock affected in this way, for there is no cure at present.
Common pests are blackflies in early summer, often migrating from broad beans, greenflies during summer and autumn, thrips and capsid bugs from time to time.
A difficult pest to control is the red spider mite which may attack some plants in dry seasons, causing yellow mottling. Frequent syringeing under the leaves with water and spraying with malathion every ten days is the control routine to follow.
Earwigs are often a nuisance, eating holes in leaves and flowers. These can be controlled if you provide upturned pots, loosely filled with woodwool, straw, hay, etc., and placed on top of the canes or stakes; these should be emptied into boiling water or paraffin. Also puff DDT on to young flower buds.
Wasps sometimes make damaging attacks on dahlia flower stems and it is usually necessary to destroy the nest completely.
Propagation The preparation for growing from seed is a simple matter. Remove the petals as they fade and take the seed pods indoors before the frost, later extracting the seed and placing it in envelopes. The seed is sown in boxes in mid-March, and the seedlings are potted off in May and planted out in June. The best breeding, however, is done by crossing selected varieties by hand, and covering the blooms with old nylon stockings to prevent chance pollination by bees and other insects. It should be remembered that dahlias do not come true to type or variety from seed, though dwarf bedding types, such as `Coltness Gem’ or `Unwins Hybrids’ are commonly grown in this way as they come reasonably true.
Years ago dahlia shoots were grafted on to tubers to produce plants, but only research into virus control now employs this technique. Nowadays dahlias are commonly propagated from cuttings. Tubers are packed close together in boxes of soil in February, put on the greenhouse bench with bottom heat of about 60°F (16°C) and watered. When the shoots, produced after some three weeks, are about 2k inches long, they are cut off close to the tuber just below a leaf joint, and after removing the lower leaves, they are inserted into holes round the edge of 3 inch pots. The holes are made by inserting a pencil-sized dibber 1} inches deep into the rooting medium in the pots, commonly sand or a mixture of equal parts of peat and sand. Five cuttings are placed in each pot. The pots are then placed over bottom heat from soil-warming wires, boiler pipes, or paraffin or electrical heating. The temperature should be about 60°F (16°C) round the pots. Cover the pots by suspending polythene sheeting above them in the daytime, plus brown paper if the sun shines, and spray them gently with water morning and night, removing the covers over night. Do not make the mistake of overwatering the pots during the rooting period, or rotting may take place. Add water to the pot only when the sand surface dries out and then dip it in a bucket of water with a finger over the drainage hole until bubbles cease to rise. Otherwise, rely on overhead spraying on the cuttings themselves.
After two or three weeks, when new tip growth is evident, the cuttings will have rooted and can be potted off individually in ordinary potting compost. For the first ten days afterwards, keep them in a warm part of the greenhouse, but for the rest of the time until planting out they grow much better if kept cool. Certainly they should be ready to be put into a cold frame three weeks after potting off.
The division of tubers described earlier is the other method of propagation.
Types of dahlias On January 1st 1966, a new system of dahlia classification came into being. As far as Britain is concerned the National Dahlia Society is the authority for domestic classification. Periodically the society issues a classified list of varieties showing the type or size to which any named variety belongs There are now ten main groups, some being subdivided into sizes according to flower diameter. These include single-flowered, anemone-flowered, collerette, paeony-flowered and miscellaneous (containing such types as orchid-flowered). As far as the gardener is concerned the most popular groups are the decorative dahlia, with flat broad petals; the cactus dahlia with petals that roll backwards to form a quill; semi-cactus dahlias, which have part only of their petal length rolled; pompon dahlias, like drumsticks, their flowers having blunt, tubular petals, under 2 inches in diameter; and the new group of ball dahlias which comprise all the previously known groups of medium and large pompons and the similar, but larger, double show varieties, plus any globular shaped varieties which were previously small or miniature decoratives.
Size groups are: pompons one size only; ball dahlias are divided into miniature balls, 2-4 inches, and balls over 4 inches, decorative, cactus and semi-cactus dahlias are each divided into five groups ; miniature, under 4 inches, small, 4-6 inches, medium 6-8 inches, large-flowered 8-10 inches, giant-flowered over 10 inches. Bedding dahlias are put where their flower shape designates them.
Exhibiting Cultural technique varies little from that described. Cuttings are mostly used for propagation purposes ; they flower during late August and the first half of September when most dahlia shows are held. Tubers of the large-flowered and giant varieties are started into growth in the greenhouse in mid-January, cuttings being taken for rooting during early March; plants, when put in the frame, later on, should be put into 5 inch pots by early May. All other varieties are started off in mid-February, the best plants being obtained from cuttings rooted during the end of March and the first three weeks in April. Those taken before this period will usually flower much too early for the shows. For show work, it is much better to grow at least six plants of each good variety, so restricting the number of varieties to the capacity of the outdoor space available to grow them in. When garden plants are grown, for display, distances should be 2 feet aprt for pompons, 2i feet apart for ball dahlias and all others, except the large and giant decoratives, cactus and semi-cactus which should be placed 3 feet apart. Many exhibitors mulch the giant varieties with manure in July; for the others, a straw mulch is used. During flowering it is common practice to protect the flowers of the large and giant varieties either with cones of builder’s bituminized brown paper, or even by erecting metal uprights to support a roof made of corrugated vinyl clear plastic sheeting, giving the effect of an open-sided greenhouse.
Always cut the flowers the evening before and stand them in a cool, dark place in water over night. Large and giant blooms must have a 2-foot cane tied along the stem when it is cut to prevent the bloom toppling over in transit. Common methods of transport include oil or distemper drums with holes drilled round the edge to which the individual blooms can be tied; milk crates with one bloom in each corner resting in a water-filled bottle, or old butcher’s liver tins especially for pompons. It is always advisable to carry flowers to a show in water.
The best way to pick up showing techniques is to join a dahlia society if there is one in your locality, or if not, to contact the National Dahlia Society.