Avoid consumption by animals – Keep away from pets
Active during: Summer
Common Name: Dahlia
Type: Tuberous root
About 30 species from Mexico, Central America and Colombia. Some of these species were employed in hybridization in the late 1700’s forming the ancestors for today’s hybrids. There now exist tens of thousands of cultivars.
Dahlias are stout, erect, herbaceous plants with woody, hollow stems. The plants range in height from just 20 cm to nearly 2 meters, depending on type and cultivar. Flower-size varies from 1 cm across to 20 cm across. The blooms come in many different shapes from small ball-like poms to huge dinner-plate decoratives. Dahlias offer a very wide range of colours.
Dahlias are invaluable in the mixed perennial border. They provide colour and are a constant source of cut flowers at a time when there are few other blooms available. Dahlias fit into the gardening cycle admirably, as they are best planted in late Spring, just when the Winter and Spring-flowering bulbs have finished blooming. They can therefore be planted in the spots vacated by the Spring bulbs. Dahlias may be lifted in late Autumn. The space thus made available for planting Winter and Spring flowering bulbs. Dahlias are best planted as tubers. These are available from gardening Retailers from late Spring into early Summer. Plant Dahlias in full sun or where they will receive some shade during the hottest part of the day. In very hot districts some relief from the afternoon sun will benefit flowering and lessen the water requirements. Dahlias do well in almost any soil, as long as drainage is good. Sandy soil with a rapid drainage and evaporation rate will obviously require more frequent watering than clay soil with a slow drainage and evaporation rate. Dahlias prefer fast-draining sandy soil to boggy clay and will bloom better in it. Ideally the soil should be on the rich side. Sometimes tubers are available from as early as late Winter. Do not be tempted to buy these. They have been imported from the northern hemisphere where they were lifted at the end of the northern Summer. Consequently they have spent the better part of eight months in cold storage and although they may show sprouts quite early on, they are generally lacking in vigor because of their long enforced storage. It is wiser to wait for locally-grown tubers, which arrive in the shops only in late Spring. These have not been subjected to lengthy storage and have all the necessary food reserves to give you healthy, strong-growing plants.
Dahlias should never be planted before the soil has warmed in late springtime. To achieve the best results, plant late. They suffer if they are planted too early in Summer and therefore start flowering when they still have several hot months to contend with. They should only start flowering in mid-Summer to late-Summer and will then provide their best show in late Autumn and early Winter, right until frost cuts them down. Plant tall growing specimens towards the back of the flowerbed or border, and short specimens in the foreground. Space tall-growing varieties about 40cm apart and the small edging varieties 15cm apart. Nevertheless, the planting-hole for all Dahlias should be the same: about the size of a medium casserole or saucepan, both in diameter and in depth. Place the dug up soil beside the hole. Mix well-matured manure and compost together with some of the soil removed from the hole, in a 50/50 proportion. Do not use fresh manure as it will burn or rot your Dahlias. Place a 5 cm layer of the soil mixture in the bottom of the hole. Above this layer, put in a 5 cm layer of sand. Place the tuber on the sand with the individual fingers spread out and the short, dry stalk pointing upwards. Purchased tubers should not be dissected or divided because sprouts form only on the base of the old stem, not on individual tubers. Fill in the remaining 5 cm or so of the hole with the remaining dug up soil. If a tuber already shows a sprout, be careful not to damage this when filling in the hole. Provide a sturdy one-meter long stake right next to the tuber of tall varieties to support them as they grow. If you have an inadequate water supply, make the planting hole a little deeper and fill in the hole in such a way that you create a shallow depression in the soil surface above the tubers. This will ensure that no water is wasted in run-off. Only severe frost will kill dormant tubers left in the ground. Dahlias are especially easy to grow in southern Africa. As long as they receive regular, deep-watering, little else is needed to make them produce a mass of superb blooms over a long period. Immediately after planting, the soil should be soaked right down to beyond the bottom of the planting holes. After this initial soaking keep the soil moist at root level by watering deeply twice a week, or whenever the soil around the tubers starts to dry. Do not let the soil dry out between watering and do not allow the foliage to wilt. This will cause stress and loss of vigour. Patience is needed after planting, as it may take six to eight weeks before any growth becomes visible above ground level. If you plant early in the season it might be even longer than eight weeks before you see signs of life. During all this time the soil must be kept moist (but not water-logged) at tuber level or the young underground shoots making their way to the surface may die for lack of water. Once this has happened the tubers themselves will die and rot. Towards flowering-time the plants will need even more frequent watering, as their rate of transpiration will increase because of the increased size of the plants and hotter weather. A mulch applied on top of the soil at planting time will help retain moisture and keep the soil cool. This mulch is best made up of compost containing old manure. Dahlias are gross feeders and will benefit from such nutrition. Feed the plants every few weeks, refer to the section on feeding for guidance. An application of super-phosphate in mid-Summer and a top dressing of well-decomposed manure will be most beneficial. This will ensure that you harvest lovely fat tubers in early winter. The tall-growing varieties should be tied loosely to stakes as they grow, to prevent the increasingly heavy stems from breaking. Use raffia or other string. First tie one end of the string tightly around the stake. Then tie a loose knot around the Dahlia stem at an axil in such a way that the knot will not tighten around the stem. The plants can be encouraged to fill out and become more bushy and less tall by topping off the main shoot once the plant almost reaches maximum height. Simply bend the top of the central shoot until it snaps off. This operation will encourage the side shoots to grow outwards and should also be carried out with the low growing varieties, as it encourages all types of dahlias to produce more blooms. When the first flower buds appear towards mid-summer you will notice that each flowering stem holds three buds, the central one of each being bigger and taller than the two ancillary buds. If you want fewer but larger blooms on taller stems, pinch out the two secondary buds on each stem, leaving only the main bloom to develop. The length of usable stem for the vase may be further increased by also cutting off the next two side shoots further down the stem. All of this cutting back, and pinching out will not harm your dahlia plants in the slightest. On the contrary, it will encourage more bushy growth and an increase in stalk and bud formation throughout summer and autumn. When cutting dahlias for the vase, make sure that you choose blooms not quite fully open. The centre of the bloom should still be in bud, with only the outer petals open. The bloom will open fully within a day or two. Avoid picking blooms of which the centres are already fully opened up or overblown. These will not last in the vase. It is best to cut your blooms early in the morning. Refer to the section on cut flowers to achieve long vase life.
Keep your Dahlias neat and clean by removing all yellowing foliage from time to time. If you wish to leave your Dahlias where they are and overwinter them in the ground, you may do so. Our winters are not sufficiently severe for frost to penetrate deeply into the soil and so kill the tubers, except in the very coldest districts. The dormant Dahlias will tolerate being watered along with your Winter growing plants provided that drainage is good and the tubers do not become waterlogged. If you choose to lift your dahlias, the right time is at the onset of winter. Either discard them or store them during Winter for replanting in the next Spring to give them their needed dormancy period. Winter and Spring flowering bulbs may then be planted in the vacated spaces. To store and save Dahlias for the next season’s planting can be very rewarding and far easier than most gardeners think. If the plants have been well-watered and fed during their entire growing period you will be amazed at how much bigger the tubers have grown. You will now be able to split them up and so multiply them. Start the lifting and storing process by cutting off the main stems about 5 cm above ground level, using secateurs. Then carefully dig up each tuber using a large garden fork. Do not break off individual tubers. Transfer the name-tags (if used) from the stake to the neck of the tuber so that you will know next planting season what colour or variety you are planting where. Any tubers damaged or pierced by the digging fork, should be discarded. To remove the soil from between the roots of the dug up tubers it is best to hold them one by one in a bucket of water and agitate them gently until clean. They should be allowed to air-dry for three or four days in the shade or in a garden shed, but never in full sunlight, as the skins will burn and the tubers will then shrivel up. Very large clumps of tubers may be trimmed by cutting off excess tubers. Tubers with broken necks should be discarded. Large plants may now be divided by cutting right through the stalk from the top downwards so that you are left with two equal halves. Remember that the eyes for new growth will only form on the bases of the stalks and nowhere else. Dust the clumps with a fungicide and pack them into cardboard boxes in-between layers of dry sawdust, vermiculite, sand, peat-moss, wood shavings or shredded paper. Start with a layer of dry bedding material in the bottom of the box, pack in a layer of tubers and repeat the layering process until the box is full. Store the boxes in a dry, dark, frost-free place which does not get warmer than about 20ºC at any time. Your garage or tool shed is probably ideal. The tubers will rest for about four months. Replant them when all danger of frost is past. If you wish further to increase your stocks of a particular Dahlia plant, the best method is to take cuttings in Spring. Refer the section on Propagation for the correct method. Virus may stunt your plants. Virus infected plants should be removed altogether as they cannot be brought back to health. Virus diseases are spread by aphids and thrips. These sucking insects transmit the disease by feeding on an infected plant (not necessarily a Dahlia) and then feeding on your Dahlia and so transmitting the disease through sap transfer. If you have planted your Dahlias too closely, and especially during wet weather spells, powdery mildew or smut disease may necessitate spraying or dusting with a fungicide. Remove some of the lower growing foliage if it becomes overly dense.