Dahlias - Planting and Aftercare
Thursday, 14 March 2019 | Catherine@shrublands
Dahlias are spectacular plants which add height and drama to the garden in summer. Their flowers come in various forms, sizes and a huge array of colours. It is quite possible to have Dahlias in flower from mid-summer to the first frosts especially if you cut the flowers for the house or deadhead regularly.
Dahlias like a fertile, well-drained soil and reasonable moisture levels. They are better suited to a soil with more substance such as a loam or loamy-clay. They struggle on very light sandy soils lacking in humus and moisture retention. They can be grown to great effect in large pots if you think you do not have suitable soil (please read the section on growing in containers below)
They are tuberous plants and the fleshy roots can be prone to frost damage in cold areas so it is advisable to start them off in pots in March-April and plant them out in May. In autumn when the first frost has knocked back the foliage lift and store the tubers somewhere frost free over winter such as a greenhouse or shed. Repeat the process the following spring. If you live in a milder part of the UK and your soil is free draining you may be able to get away with just applying a good thick mulch but be aware you may loose them if you experience a hard winter.
Dahlias need lots of sun and prefer at least 6-8 hours of direct sun per day when they are in full growth. They tend to produce a lot of foliage and many are quite tall. For this reason they need a fairly sheltered spot, they are not well suited to very exposed positions. You can try shorter or dwarf varieties if your garden is a little on the windy side.
Planting the tubers
The best way to plant Dahlias is to start them off in pots and keep them somewhere protected until around May time when they can be planted out. Use a pot just big enough to accommodate the tuber and some compost all around it. Plant with the eyes of the shoots uppermost. If you look you will see the remains of the old stem where the tubers join together. Around here is where the eyes of the new shoots will form. The stump should sit at the surface or just poking through. Give the pot a tap and a shake to help the compost work its way in to the gaps so there is good contact between the tuber and the soil and water in.
Ideally you should keep your Dahlias in a greenhouse but anywhere that will give a bit of protection against frost that is naturally well lit and airy will do fine. A porch, cold frame, a garden shed with large windows or a cool windowsill would be ok. To begin with don’t over-water keep the compost just damp but as they start to grow you will need to gradually increase the watering but again keep the compost just damp.
You can plant your growing Dahlias out once the risk of severe frost has past usually around May, later in colder areas. Dig a good sized hole and fork the soil over really well, the tubers will send out an extensive root system. It is a good idea to dig in plenty of organic matter such as homemade compost or bought soil improver or well-rotted manure. This will help feed the hungry plants and improve moisture retention around the roots. When planting your Dahlias out mix a handful of Blood Fish and Bone or Growmore in to the planting hole. Make sure you mix it really well with the soil you are going to backfill with.
Many Dahlias grow to 3-5ft tall so place some tall plant supports around them before they get too tall this will support the growth saving you the job of trying to stake them after the fact.
Make sure you keep the area weed free so the tubers can grow away without competition.
Dahlias are bushy plants and produce a lot more growth than you might think from the dormant tuber you receive. Taller Dahlias should be planted about 3-4ft apart. Smaller varieties which grow to 3ft or less can be planted 2-3ft apart.
Pinching out and deadheading
You want to encourage your Dahlias to produce side shoots. This will promote sturdier growth and more flowers. Do this by pinching out the tip of the main shoot to the first set of leaves in June/July time. Also the larger the plant the more you need to restrict the side shoots. Remove all but 4-5 of the side shoots leaving the strongest ones. This will concentrate the plants energies into producing flowers rather than excess foliage.
If you want to maximise the flowering period then deadheading is really important. It stops the plants wasting energy on setting seed and producing more flowers instead. Flower buds that have yet to open a rounded whereas spent flower buds are pointed. Remove the bud with the stem to just above the first set of leaves. Cutting the flowers regularly to enjoy indoors will also help to keep the plants producing more for you.
Watering and feeding
When in full growth Dahlias can be thirsty plants. You may have to water them during very dry summers. Do this once or twice a week or more if they seem to need it but make sure you give them a thorough soak at base of the plants so the water will get to where it's needed, the roots. If you are growing them in pots they will need watering more frequently, every couple of days or maybe every day at the height of the season.
Dahlias best suited to growing in pots are the shorter varieties which may not need any staking at all or if they do then not too much. As previously mentioned Dahlias are hungry and thirsty plants so make sure you use a decent compost. A loam based compost such as John Innes No.2 or No.3 is ideal. You can mix this with your own compost or a multi purpose compost if you wish, up to 50:50. Incorporate some slow release fertiliser too following the manufacturers instructions.
You will want to use a good sized pot and the deeper the better. Even for the most compact Dahlia you will want a pot which is a minimum of 35cm (14 inches) in depth or around 20 Litres in volume.
At the height of summer you may need to water Dahlias in pots every day or every other day. If you are watering very frequently this can lead to a leaching of nutrients from the compost so apply a liquid feed once a week from when they start flowering. A high potash feed such as tomato food is just fine.
Finally sit back and enjoy the dazzling flowering display! It may seem like Dahlias are difficult to grow but they are not as long as their requirements are met. There are few other perennials which flower for so long and make as much impact so we think they are worth a little effort!