Dahlias Brighten Flower Garden in Summer and Autumn

New Zealand Journal of Agriculture, Volume 87, Issue 4, 15 October 1953, Page 379

Dahlias Brighten Flower Garden in Summer and Autumn

NO flower garden seems complete unless * ' dahlias, with their bright showiness during summer and autumn months, are present. These plants are easily grown and propagated and give a good display both in borders and as cut flowers. In this article for the flower gardener R. R. White, Horticultural Instructor, Department of Agriculture, Dunedin, discusses some of the problems involved in the growing of dahlias. Routine flower garden work for November is dealt with by H. P. Thomas, Horticultural Instructor, Department of Agriculture, Wanganui.

THE dahlia was so named to commemorate the work of the Swedish botanist Dr. Dahl. The genus contains a number of species indigenous to the sandy plains of Mexico. The modern dahlia bears little resemblance to the parent species used by hybridists. The first three species used were D. variabilis, which has purple or red flowers, D. coccinea, a scarlet, and D. mercki, lilac and yellow. Another, D. juarezi, which was introduced in 1872, gave rise to the cactus varieties. In the early nineteenth century the tubers were tried as vegetables in Paris and some were sold on the city markets of Britain as Jerusalem artichokes. Though rich in nutritive matter, they were never grown as a market garden crop. The dahlia was adopted in the latter half of the nineteenth century as a show

flower, but the stems were weak and the flowers hung downwards. It was not until 1913 under the auspices of the National Dahlia Society and the Royal Horticultural Society that the first trial was conducted to select varieties suitable for garden flowers. From then on the dahlia became extremely popular for garden use until today it is one of the most popular flowers grown. Classification For those wishing to exhibit dahlias a classification list was produced jointly by the Dahlia Society and the Royal Horticultural Society Committee. As the dahlia is a member of the Compositae, it has two types of florets: Ray florets, namely the outer ones which have coloured petals, and disc florets, which form the central disc and have no petals. It is on these features that the classification is based. 1. Single dahlias: Those not exceeding 4in. in diameter and with a single outer ring of ray florets and a central disc. 2. Star dahlias: These are small and with two or three rows of nearly pointed ray florets not or scarcely overlapping at their more or less recurved margins and forming a cupshaped flower with a central disc. 3. Anemone-flowered dahlias: These have an outer ring of flattened ray florets surrounding a dense group of tubular florets which are longer than the disc florets in single dahlias. 4. Collarette dahlias: These have one or more rings of flat ray florets with a ring of small florets (with collar) approximately half the length of the ray florets. 5. Peony-flowered dahlias: These have flowers consisting of two or more rows of more or less flattened ray florets with central discs. 6. Formal decorative dahlias: These have fully double flowers showing no discs. All the ray florets are regularly arranged, the margins of the florets are usually slightly incurved, more or less flattened towards the tips which may be broadly pointed or rounded. 7. Informal decorative dahlias: These have fully double flowers showing no discs. The ray florets, which are not regularly arranged, are broad, with margins very slightly rolled backwards, more or less flat or slightly twisted. 8. Double show dahlias: These have fully double flowers over 3in. in diameter, almost globular with central florets, like the outer but smaller; florets have margins incurved, tubular, short, and blunt at the mouth. 9. Pompon dahlias: These have flowers like those of double show dahlias but smaller. 10. Cactus dahlias: These have flowers fully double, with the margins of the florets rolled backwards for not less than three-quarters of their length. 11. Semi-cactus dahlias: These have fully double flowers, with florets broad at the bases, and margins rolled backwards or twisted for less than threequarters of their length. 12. Miscellaneous dahlias: Any which do not fall into the foregoing sections. 13. Dwarf bedding dahlias: Plants usually do not exceed 24in. in height.

The flowers may belong to any of the foregoing sections. Propagation Seed Sowing Dahlias, particularly the small bedding types, can be raised from seed. During July or August seed can be sown in a box of good seed mixture consisting of 2 parts of loam, 1 part of sand, and 1 part of peat or leafmould to which has been added IJoz. of superphosphate and f-oz. of lime per bushel of mixture. The seedlings when about lin. high can be pricked off into a box containing a suitable pricking off mixture at a spacing of about 3in. each way. Alternatively the seedlings can be planted direct into small pots. The seed sowing method must be used when a new variety is required incorporating various characteristics of two suitable parents. Cross pollination to raise new varieties can be done as follows: The central disc florets are removed with a pair of forceps, the ray florets with the female parts of the flower being left intact. The pollen is placed on the pistils of these flowers. The flower can be covered with a paper bag and labelled to indicate the cross. The pollen bearing flowers should be picked 2 days before pollination and stood in deep water to ensure that the pollen is not dried up. Division of Tubers The clumps of tubers can be broken up, but this method usually restricts the number of new clumps, as many eyes and tubers are . damaged. The best method is to place the clump in a box or in some warm spot so that it starts into growth. When shoots begin to appear the tubers can be separated with a long sharp knife, each one being cut off with one or more shoots. These can be planted out in their flowering positions in the border, care being taken to see that the young shoots are not damaged. Cuttings Where a clump of tubers is too small for suitable division the best method of propagation is by cuttings. The plants should be started into growth and when the shoots are 3in. to 4in. above the surface they should be cut off with a sharp knife and trimmed to make a cutting. The cutting can then be inserted in a compost containing sharp sand and left until it has rooted. Rooted cuttings should not be left in the compost longer than necessary, as they tend to make rapid root growth which may be damaged during lifting, and other cuttings nearby may be disturbed. The rooted cuttings can be potted up into 4in. pots, a suitable potting compost being used. The potted plants can then be grown on until they are ready for planting out. Preparation of the Soil As dahlias are gross feeders, it is necessary to give the soil adequate preparation before they are planted out. . Dahlias do well in most soils provided that there is plenty of humus or compost worked in. They like a sunny situation which is not too exposed to winds. A dressing of superphosphate, sulphate of ammonia, and sulphate of potash can be applied

together at the rates of 20z., Ijoz., and loz. per square yard respectively. Planting out Before planting is done it is best to plan the area to be planted to give the best arrangement for flowering, namely the taller varieties at the back and the colours grouped together. The area can then be staked to give the planting positions. The holes to accommodate the plants should be larger than the rooting systems, whether the plants are rooted cuttings, divided tubers, or clumps. The soil should be firmed around the plant. For the taller varieties requiring staking it is best to put the stakes in before the tubers are set so that there will be no damage. After the area is planted it should be given a good soaking, preferably with a fine. rosed hose. Cultivation Cultivation consists of hoeing to keep down weeds and also to keep the soil loose to reduce loss of moisture. Care must be taken not to hoe too deeply or the tubers may be damaged.

Staking and Tying If the position is at all exposed, staking is necessary to support the plants, especially the taller growing varieties. A good sturdy stake is required and if this is placed near the centre of the plant, the tying can be done so that the stake is hidden. The stake should be driven into the ground so that its top will be about 6in. below the maximum height of the fully grown plant. Tying can be done with raffia or some other suitable material, but care must be taken to ensure that stems are not tied too tightly to the stakes or the binding may cut into the stems as they grow. Labels may be placed on long stakes so that they show above the growth, if desired, but it is usually preferable to tie them on the supporting stake lower down, tie them loosely on the stem, or insert them in the ground at the base of the plant. Thinning and Disbudding Thinning of the stems is not necessary, except where the grower wishes to produce show blooms. Disbudding, however, can be done to give a stronger main bud. As dahlia stems are rather tough and do not break easily and cleanly, this operation is best carried out with the use of a sharp knife or secateurs. Old flower heads should be removed as soon as they begin to fade to maintain the vigour of the plant and keep it tidy. Watering During periods of dry weather watering may be necessary. Regular hosing can be carried out and a good drenching every 3 or 4 days is preferable to a daily watering, as the latter system tends to produce too much soft growth. Mulching the ground will help to retain the moisture in the soil. Liquid manuring can also be carried out if the plants require further feeding during the growing season. This is best carried out after watering. Post-flowering Care As with all plants that have perennating organs the longer growth is maintained the greater, will be the amount of food stored for the next season. Therefore dahlias should be allowed to grow for as long as possible and it is usual to leave them until the first frosts occur. When foliage has died it should be cut off to within about 4in. . of ground level. . This material if disease free can be placed on the compost heap. However, should it be suspected of having any virus or fungus disease that will carry over, it should be burnt. In order - that named varieties will not be lost labels should be tied to the stems of each clump of tubers. Wooden or plastic labels are best and should be tied on with string or wire. Lifting and Storing Lifting is best done with a spade and the soil should be shaken off the clump. Careful handling will ensure that there are not too many tubers knocked off the clump during this process.

The clumps can then be stored in a cool, dry shed where there is adequate ventilation. Any parts showing damage or rots should be cut out

and the cut dusted with flowers of sulphur. If a suitable shed is not available, the clumps can be successfully stored out of doors. A mixture of beach sand and sawdust can be used, with a larger proportion of the sand. This can be mixed and a layer about 3in. deep laid on the soil surface. The clumps should be placed together in rows and covered with the sandsawdust mixture to form mounds. The labels used in the border can be used to identify the groups in the mounds. For this overwinter storing the. driest spot in the garden should be used. After winter a close watch should be kept on these stored dahlias to see that they do not break into growth too soon, because of the possibility of damage by late frosts. Diseases and Pests Although dahlias are strong growing and have fewer diseases than many other types of plants, the diseases which do affect them can cause a great amount of trouble to the gardener. The best disease preventive is to start with disease free stock at all times. Virus diseases are the most serious and there are a number which attack dahlias. Symptoms vary from a stunting effect to a mosaic effect on the leaves. Virus diseases can be transmitted by aphides and thrips, and in some cases by knife blades or on the hands, so it is necessary that in a group the diseased plants should be handled last. Although virus diseases cannot be cured by sprays, dipping, or any other known treatments, the spread of these diseases can be controlled somewhat by the following methods: — 1. Destroy completely by burning all obviously infected plants as soon as they are observed. 2. Purchase only virus free plants. 3. Use for propagation only those plants which show no symptoms of infection. 4. Control aphides and thrips by spraying with nicotine sulphate 40 per cent. 1 in 600 or 4/5 fl. oz. to 4 gallons of water plus a spreader, which is

effective only when temperatures are high and the weather calm, or H.E.T.P. 1 in 1200 or 2/5 fl. oz. to 4 gallons plus spreader, which is effective at lower temperatures than nicotine sulphate. Note: The directions on the container should be followed and all prescribed precautions taken when these materials are used. Dahlia Wilt Dahlia wilt (Verticillium dahliae') has a wide host range including stone fruits, berry fruits, solanaceous plants, and garden plants. It causes wilting and the leaves of badly affected plants hang as if suffering from drought. The only method of control is to remove the affected plants and burn them. Dahlia Smut

Dahlia smut (Entyloma dahliae) is a fungus which first shows as light yellow-green spots on the leaves, which later turn greyish brown, the edges becoming dark brown. The dead tissue soon becomes brittle and falls out, giving a shot-hole effect. The fungus overwinters on dead leaves or stumps. Early planting helps as a control and Bordeaux 3:4:50 should be applied' when the disease first appears. All dead leaves should be burnt.

Stem Rot Stem rot (Sclerotinia sclerotiorum) causes, rotting of the stems. Sunken water-soaked regions appear at ground level on the stem, which soon collapses. Under moist, shady conditions the stem may become completely encircled with a white fluffy mycelium and black sclerotia develop inside the stem. Control measures consist of having good drainage and not having the plants overcrowded. Diseased plants should have infected parts cut out and destroyed. Leaf Miner Leaf miner tunnels in the leaves. A 2 per cent. D.D.T. spray applied early in the season or 1 : 600 nicotine sulphate at weekly intervals would most likely control the egg-laying adult.

Looper Caterpillar Looper caterpillar, which is common in spring, grows to about lj-in. long. It is light green, almost hairless, with white and darker green stripes on the back. The caterpillars eat from the lower surfaces of the leaves, leaving only a thin skin of upper epidermis, which ultimately dies and falls out, leaving holes. When moving the caterpillars arch their bodies and proceed in a series of loops. They can be controlled by spraying with a mixture of 21b. of arsenate of lead powder to 100 gallons of water. Dusting with D.D.T. or derris is also effective. Earwigs Earwigs can be troublesome and are controlled either by trapping them or by spraying them with D.D.T. Garden Work for November With the approach of warmer weather renewed interest will be taken in flower garden activities and plans will no doubt be formulated for a more successful year than ever before. As in all gardening work the gardener must look ahead, and with most of the seed sowing for the raising of annual bedding plants now finished, the work of prime importance during November will be sowing seeds of biennial and perennial plants for continued displays in the future. If this year’s spring display of wallflowers, sweet william, forget-me-nots, and the like are not yet removed they should be taken out immediately to make way for the planting of annuals for a summer and autumn display. Ground recently cleared should receive a dressing of well-rotted manure, or organic material during digging, before any summer bedding is done. Where this material is not available a fertiliser should be applied of 3 parts of blood and bone, 2 parts of superphosphate, and 1 part of muriate or sulphate of potash at 3oz. to the square yard.

If the soil is dry, it should be watered thoroughly the day before setting out the plants. Care should be taken to see that the consolidation of the soil is not excessive during planting; a . length of board as a walk can be used. The necessary hardening off of boxed plants and the setting out of such plants as petunias, zinnias, salvias, asters, marigolds, stocks, and phlox should now be carried out without delay. For late summer flowering planting should be continued as space and plants become available of such reliable subjects as calendulas, hybrid wallflowers, paris or tree daisies, hardy calceolarias, gaillardias, dianthus,. • coreopsis, and Salvia horminum (Clary) . Sowings Now for Future Display Sowing should be continued of hardy annuals in places where they are to grow and flower, but they should be sown thinly, and later thinned out for best results. There are a few varieties of annuals, however, which are grown for border edgings or mass effect and these may be left unthinned; for example, Virginia stock, evening-scented stock, and linum. Biennials Biennials commonly grown include wallflowers, Canterbury bells, sweet william, myosotis, Bellis perennis (double daisies), East Lothian and Brompton stocks, and lunaria (honesty). Botanically many of these plants are not strictly biennials (some are perennials), but because they can be readily raised from seed and give their best display in the first flowering season, the usual gardening practice is to treat them as biennials. Biennials are plants which develop into strong young plants the. first season, winter well, and then grow on to maturity and flower during the next spring and early summer. After the first season’s flowering they are better discarded. Many gardeners retain such plants as wallflowers for a number of years, but they become very woody and although they bloom each spring,- the display is not comparable with that made in their first season. Success lies in sowing seeds of biennials early; November is an opportune time. The seed may be sown in the ground in frames or outdoors in soil moderately rich and broken up to a fine tilth before the drills are prepared. Sowing in boxes as carried out when raising annual plants can also be done and is particularly advisable where there is likely to be trouble from pests, weather, or weeds. Perennials . In addition many perennial plants may be raised from seed sown in November. Some of these which are easily raised from seed are Lupinus polyphyllus, Coreopsis grandiflora, Chrysanthemum leucanthemum, aquilegia, delphinium, hollyhock, anchusa, mecanopsis, statice, gaillardia, and geums.

The seed should be sown in pots or boxes of sterilised soil, comprising two

parts of turfy loam, one part of leafmould, and one part of clean sharp sand, and then placed in a glasshouse, without artificial heat, in a garden frame, or on a veranda. Where such facilities are not available sowing in the open ground could be done if great care is taken and a watch is kept .on possible losses through weed and pest invasion, or extremes of moisture or dryness. If a good watering is given to the boxes several hours before sowing, further watering should be unnecessary until after germination has taken place. In all cases it is most important to raise perennials from a good strain of seed which should be as fresh as possible. As some seeds are often slow in germinating, it may be necessary to leave the boxes or bed undisturbed for as long as 6 weeks in some cases before the young seedlings appear. Rockery Plants Seeds from plants in the rockery may be saved as soon as ripe and may be planted immediately where new plants are required. They should be sown in pots or pans of gritty soil and left either uncovered or covered with the finest possible layer of sand. Among the rock plants which may be raised from seed sown in November are aubrietia, Silene compactus, campanula (various), alyssum, arabis, cheiranthus, cerastium, and primula. Glasshouse or Border Display For a bright display in the glasshouse during next winter a small early sowing of the following seeds may be made toward the end of November, where early plants are desired: Cineraria, Primula obconica, P. kewensis, and P. malacoides. ' Small seeds need very little covering of soil. A light dusting of finely sifted soil is said to give more even germination than no covering or a heavy one. Water the soil several hours before sowing and - further watering, if necessary, should be done only by standing the container in a dish of water, which allows a gradual intake from below. Finally it is advisable to place the container in a warm, shady corner and cover it with a piece of glass and paper to prevent any drying out before germination. Germination of primulas is generally irregular and it may take from 3 to 6 weeks. As soon as the seedlings can be handled safely they should be pricked off 2in. apart each way into boxes. Gladioli Earlier plantings of gladioli will now be growing strongly and if staking has not already been done, this work should be carried out as soon as possible. Insert each' stake 2 to 3in. behind the plants and tie each spike loosely as growth progresses. Support is necessary to.keep the flower spikes straight as well as to avoid damage by excessive wind or rain.

Chrysanthemums Chrysanthemums not yet in their permanent quarters should be planted without delay. A strong, stout stake should be provided for each plant and care should be taken to be sure the plant is securely but not tightly anchored to it from the outset. Stopping, or pinching back, excess top growth of any one-stemmed plants to

encourage branching may be necessary at this stage of their growth if no natural break has occurred. . Dahlias Dahlia planting should soon be completed. In colder districts where early planting is unwise this work should be pushed on, and in the milder districts a succession of flowering is possible by a further planting at present, if young plants are available. Earlier plantings will now be growing strongly and where young plants have not been used, it may be found that excessive growth is coming away from old tubers. Thinning of shoots in such cases would be wise and it is best to remove these by gently pulling them away from the tubers with the fingers, as cutting them may only serve to encourage further growth from lower buds that remain. Early staking is necessary and if possible the supporting stakes should be a little smaller than the ultimate height of the plants. The ties should be reasonably loose around the plants, but fixed firmly to the stakes to avoid them slipping downward. Bulbs and Perennials Early flowering subjects such as hyacinths, narcissi, and tulips may now be lifted carefully if their place is needed for summer bedding. They should, however, be removed and replanted in an odd corner of the garden where they may die down naturally. Spring perennials like polyanthus, primroses, and aubrietia can also be lifted to make way for the summer flowers. These plants may be divided up where necessary and set out in a reserved part of the garden to grow on during the summer. Pruning Flowering Shrubs Pruning of those shrubs which flower in early summer after some growth has been made should be done after they have flowered. The old flowering shoots and weak wood should be thinned out, and the young growth allowed to develop and ripen; for example, on philadelphus, deutzia, and diervilla (Weigela). Remove dead blooms of rhododendron and azaleas as soon as they are noticed to prevent seed pods forming. Mulching * Mulching of camellias, rhododendrons, azaleas, and ericaceous plants during November with well-rotted compost, spent hops, sawdust, or leafmould helps to conserve moisture and maintain the soil conditions which these subjects seem to prefer. It is necessary, however, to see that the soil around the plants is in a moist state before the materials are applied. Hedges Hedges should be given their first clip of the season as soon as early growth makes them untidy. The soft clippings, if healthy, will make a valuable addition to the compost heap.

Roses To conserve the vitality of roses suckers or basal growths must .be removed as soon as they are seen. Discriminating disbudding must be practised if large well-formed blooms are wanted. They should be sprayed in advance for possible mildew or aphis infection.