Dahlia tubers should be lifted when either the blooms are no longer enjoyable or when the plants have been frosted. They should be cut down to 15 cm, or thereabouts, from soil level and carefully dug up. Leaving a spread of stems, evenly cut, provides a stable base for them to be stood upside down to drain. Take care not to damage the crown as it is from this point that next year's shoots emerge.
Tubers should be stood upside down in order to allow moisture in the stems to drain away. When the sap has drained trim to stem to 1 or 2 cm above the crown. Do not put into storage while any moisture remains in the stem! The stem will rot and kill the nodes from which the new growth will sprout. They should be treated with yellow sulphur and put in wooden boxes, packed with peat, vermiculite, wood shavings or sand to stop tubers drying out.
Keep in a frost-free place. Most garden sheds and garages are not frostproof. They should not be kept in too warm a place as they will shrivel, or a cold damp situation where they will rot. The ideal conditions are those that a few of our customers tell us about. They store them in the wine cellar and cover with some 'spare' oriental carpets. Not everyone has a wine cellar! Replicating those conditions is the objective.
It is advisable to examine tubers several times in the winter and if, any trace of mildew is found, the infected part should be cut away. Dust the wound with yellow sulphur.
After winter storage (which these days is just a few weeks due to the lateness of frosts) the tubers can be started off to produce cuttings. To obtain maximum growth from the new plants, the tubers should be encouraged into early growth ideally about mid February to late February.
Set the tubers on a light bed of moist peat/light compost and then cover the tubers almost completely with the same, leaving only their crowns exposed. A tomato tray or seed tray is ideal for this although if you have a large number of tubers, setting them down on the bench in the greenhouse works well. Place the boxes somewhere with a little warmth to start the tubers growing, such as a conservatory or warm greenhouse. With the moisture from the peat and an average temperature of around 15 degrees C, (we aim to maintain a temperature range of 15C to 25C) growth will start within two or three weeks. Eyes will appear on the crown of the tuber, and these eyes will produce the shoots that can be used for cuttings. Keep the compost moist while the tuber is growing, but be careful not to overdo it. The compost should not become soggy. Use a watering can with a very fine rose to give you good control over the amount of water you are supplying.
When the shoots are sturdy with two or three leaf joints they are ready to use as cuttings. Use a sharp knife to cut away the shoots a little above their base, where they join the crown. Take care not to cut the crown itself, which would prevent further shoots forming. Trim neatly immediately below a leaf joint, dip in rooting compound such as Seradix or Strike and plant in either seed trays or pots filled with a gritty seed compost (John Innes Seed) or an equal mixture of peat and sand or seed compost. They may look a little droopy for 7-10 days, this is quite normal! Three weeks on and roots will begin to form. Once the cutting is clearly rooted and showing signs of growth, prick out and pot up in 9 cm pots into a well draining potting compost such as John Innes No 1. We strongly recommend that all purpose or multi purpose composts are not used as they may contain materials that will kill the delicate rooted cutting. Larger flowered cultivars especially would benefit from being potted on into 125 mm pots at the end of April. Plant out in mid to late May after the danger of frosts has gone.
Your own local micro climate will lead you to adjust the timings and dates we suggest here but we find these work for us here in our gardens on the High Weald in West Sussex. Normally Dahlias are planted 75-90cm (2ft 6in -3ft) apart, except for bedders at 40cm(15ins) apart. They thrive in most soils, but like moisture and good drainage. Do not plant before frost danger is past, for us, generally about the third or fourth week in May. In the winter dig in compost or manure on the site where they are to grow.
Use a good quality compost as this will retain moisture to help plant growth, and secondly, when the tuber is lifted in autumn, compost falls away, leaving a clean tuber. Make a hole 20 - 23 cm square and 20 - 23 cms deep and put in half a bucket of compost, and insert a stake or strong cane in the centre of the hole.
Water the plants in their pots well some hours before you plant out. Remove pot (unless using biodegradable pots like those used by us), but do not disturb the root ball.
With a hand trowel, plant next to the stake firming well. Plant so that the final soil level is just below the lowest leaves - and water in. Three days later hoe in around the plant 60g (2oz.) Fish, Blood and Bone or similar and water in again.
Remember to label each plant.
Stopping is the removal of the growing tip when about four or five pairs of leaves have been formed. Nip out the tip just above the pair of leaves, but avoid squashing the stem. This causes strong side shoots to develop at the leaf joints, forming the framework of the plant on which strong flowering stems will grow.
Dis-budding is the removal of all buds that appear in the leaf joints below the primary bud.
Retain the primary bud but remove side shoots from the two or three upper leaf axils on each stem. This can get a bit subjective as the habit will vary from one cultivar to another, the objective is to promote as long a flowering stem as possible whilst leaving sufficient leaf axils for future blooms. Disbudding is performed to encourage growth to the primary bud which will result in specimen blooms.
Here is some advice that may help keep your cut flower dahlias much longer than you possibly expected:
Air moves rapidly into the water-conducting tissues of the flower stems and plugs the cells creating air pockets. To prevent immerse the stems in hot water and trim to the length required at a 45 degree angle with a sharp knife. Cutting at an angle enables the stem to stand on a point, allowing water to be in contact with the cut surface. Cutting under water ensures that air does not enter the stems. Never use secateurs or floristry scissors to cut the stems as they will crush the vascular system in the stems and block the water uptake. Be sure to strip any foliage that will be under water in the vessel you will be using. Bacteria will form if foliage is in the vase water, and cause premature demise of your beautiful blooms. Discard any decaying leaves or petals because gases and bacteria form on wilted or decaying flowers. Be sure to clean the inside of the vase you will be using with a bleach solution. This process destroys any bacteria that may be present. Use fairly hot water as it will move into the flower stems more effectively than cold water (kinetic energy). Allow the vase of flowers to cool slowly, this process is called "hardening" which ensures maximum water uptake. In this one brief period while the water is cooling, the fresh stems, leaves and flowers take up almost as much water as in the remainder of their life. The main objective in this process is to get water and nutrients as quickly as possible to the blooms.
As an alternative to flower food using 'cheap' lemonade instead of water can add to the vase life, not sugar free! However, if you use water and if your tap water is high in salts or fluorides, consider purchasing and keeping distilled water on hand for your blooms. Chlorine, however, in tap water is not a problem since it also acts as a natural disinfectant. Please note that using a home made concoction might not be as effective as professional flower food because they don't contain the complex mixture of preservatives and nutrients your blooms requires to last for several days. Adding aspirin, wine, bleach, or pennies to cut flower blooms will not help to keep the blooms fresh longer.IF possible trim the stems as described above every 1-2 days. Be sure to rinse the stems,cut the bottoms approximately one-quarter inch and return the blooms to a clean vase. Flowers that are going limp are not drinking well need to be recut. Always discard wilted blooms. Be sure to keep your flower blooms away from hot or cold air drafts, sunlight, and hot spots. They should not be kept near ripening fruit since the fruit emits ethylene gas which shortens the vase life of the blooms.
Enjoy your dahlias!!