Growing dazzling dahlias

Dahlias have shaken off their gaudy image, to make a dazzling comeback, says Alan Titchmarsh as he explains how to grow them


Tools for the job

 

Few flowers can equal the sheer flamboyance of the dahlia. However, their unabashed self-confidence was their ultimate downfall as simple forms and natural elegance became the order of the day. Dahlias offered such a huge range of brilliant colours, in single, double, spiky and pom-pom forms, many with fluted and curled petals, that it is not surprising a few of the more refined examples reasserted themselves in contemporary schemes. 'Bishop of Llandaff' led the way with its lovely semi-double, vermillion-red flowers, set-off against compact mounds of bronze-purple leaves…soon followed by new varieties, such as the softly reassuring burnt-orange flowers of 'David Howard', again with purple foliage, or the spellbinding 'Honka’ with its disarmingly elegant, orange-eyed, spiky yellow stars. Today, there is an ever-increasing range to choose from, so I’ve picked a few of my favourites to get you started.

Ten varieties to try

How to grow dahlias

Despite having a reputation for being drama queens, dahlias are very easy to grow. However, like many things in life, the more you put in the more you get out. Dahlias are prolific flowerers over many months, so need a lot of sustenance to keep performing at their best. Well-fed dahlias will produce more flowers with brighter colours and stronger stems. Prepare the planting site now by digging thoroughly and incorporating lashings of organic matter, such as well-rotted manure or garden compost. A base-dressing of blood, bone and fishmeal will also be appreciated especially on poorer soils.

Planting The golden rule to remember with dahlias is that they hate frost. So, pot-grown specimens cannot be planted outside until the threat of frost has past. However, the planting of dormant tubers can start in April in the south because the vulnerable shoots won’t break surface until mid-May, after the last frost. Bear in mind that you will have to delay planting by a few weeks if you live in the north or if your garden is in a frost pocket. If an unseasonably late frost is forecast after dahlia shoots are peeping through the surface, make sure you protect them with newspaper, upturned pots or garden fleece. Pot-grown dahlias can be planted out after being acclimatised to the harsher conditions outdoors (known as ‘hardening off’) and need to be planted at the same depth as they are in the pot. Water thoroughly in the pot and after planting, to make sure they do not run short. Dormant tubers should be planted about 15cm deep and marked with a cane or label, so you know where they are located. Believe me, with six weeks to wait before shoots appear above ground, they are easily mislaid! Rooted cuttings that were taken from forced tubers indoors earlier in the spring (see ‘Save money on plants’, below), can also be planted out after the last frost.

Save money on plants If you want a lot of dahlias, buying them as plants or tubers can prove to be expensive. However, you can save a lot of money by buying one or two dahlia tubers now and forcing them into growth, in a greenhouse or conservatory, to produce cuttings. You can expect to get 10 to 15 plants from each tuber which, with luck, will all flower this year.

  • Once the shoots are about 8cm long, remove them with a sharp knife and trim to the leaf-joint nearest the base.
  • Insert them around the edge of a small pot filled with moist cuttings compost, after dipping the cut end in hormone rooting compound.
  • Cover the pot of cuttings with a clear polythene bag held in place with a rubber band and place it in a warm, well-lit spot out of direct sunlight, or in a propagator.
  • A fortnight later the cuttings should have rooted and can be potted up into individual pots. Maintain a minimum night temperature of 10oC at all times.

Looking after dahlias Dahlias require very little routine care once planted. Taller varieties and those with very substantial flowers or weak stems are best staked before they come into bloom. Use canes for individual stems or proprietary linking stakes for clumps. You can encourage plants to bush out by removing the growing tip (known as ‘stopping’) when the plants have reached about knee-high. This will encourage side-shoots to grow and flower. On the other hand, if you want extra-large flowers, for indoor arrangements or for impressing your gardening friends at your local horticultural show, keep the main bud on each stem and remove the sidebuds instead. Dahlias are shallow-rooting plants and really appreciate an early summer mulch of well-rotted manure or garden compost. The mulch will help keep down weeds, prevent the surface soil drying out and will provide much needed nutrients. However, make sure the mulch is kept away from the dahlia stems. To get the best display of flowers, dahlias will respond to a high-potash liquid feed, such as tomato fertiliser, given at weekly intervals, during the summer. The main problems that can befall your dahlias are summer storms that can cause unsupported plants to collapse and sodden flowerbuds to rot before they open. Fortunately, both are easily solved by judicial staking and snipping off affected buds. The main pest is the earwig, which can shred flowers. Earwigs can be trapped in straw-stuffed, upturned pots at the top of canes pushed in amongst the dahlias. The earwigs use them as convenient hiding places. Check the traps daily and dispose of any earwigs found. With good weather, you should be enjoying a dazzling crop of blooms by July, with a continuous display until October.

Lifting and storing If you have a sheltered spot with well-drained soil, you should be able to keep dahlias successfully outside by leaving them in the ground overwinter, provided you protect the soil with an insulating mulch. For most of us, however, the only guaranteed way of ensuring our dahlias survive is to dig up and store the tubers indoors. If storage space is limited, lift one or two examples of each variety as insurance against potential winter losses. You can then use these tubers to propagate new stock if the winter is particularly severe (‘Save money on plants’, above).

How to save and store dahlia tubers

  • Once the foliage has been blackened by frost, trim the tops off then dig up the tubers carefully – trying to avoid spearing or slicing through them with garden tools.
  • Clearly label each tuber.
  • Leave the tubers to dry and clear off any remaining soil.
  • Use a sharp knife to remove wispy roots and damaged tubers and cut the main stems down to 10-15cm using secateurs.
  • Place the tubers upside down in trays to drain any moisture from the stems.
  • Dust tubers with garden fungicide and pack into boxes filled with shredded newspaper, dry compost or vermiculite.
  • Keep in a dry, cool but frost-free place until it is safe to plant them out the following spring.