This month I want to look at dahlias in depth. For a long time out of fashion these plants are now very much loved. I don't tend to place much value on what may or may not be fashionable but they are such a versatile, hard-working and impactful plant that I wouldn't want the garden to be without them. At this time of year and through the autumn they bring a lot of colour, texture and vibrancy to the garden.
Dahlias come in an extensive range of colours and flower styles, which can be fun to work with. Various shapes are possible ranging from the Single and Cartwheel open discs that are very good for pollinators through to the more textured forms of the Anemone and
Waterlily varieties, and onto the structural types of the Cactus, Ball and Pompom flowers. So-called Dinner plate dahlias with large flower heads are becoming more popular, but these can be harder to work into a coherent display and require more extensive staking. There is a wealth of different colours and colour mixes to choose from, so at least one dahlia should find a home in any given display.
Dahlias are bulbous plants, heavy feeders, and frost tender. It is important that they are planted in well drained and fertile soil in order that the tubers don't rot, and the growing plants are able to get sufficient nutrition.
Plant the tubers deep to at least the depth of a spade, some well-rotted compost could be added to the hole but make sure it is not too wet. Once backfilled, mark the hole with a
stick and a top dress of balanced fertiliser or a mulch of compost could be added on top to improve the soil fertility.
Though frost tender we leave the tubers in the ground throughout the winter. It has been standard practise to remove the tubers each winter, dust them down and store them in a frost-free environment. However we moved a few years ago to keeping them stored underground. For this reason it is important to plant them deep as those 15cm/6" of soil act as an effective thermal blanket over the tubers. We also add a mulch of compost or manure to the surface of the soil at the start of the winter, which adds yet another layer of warmth. This way the plants are able to spread and establish themselves well in their planting areas.
Using the marker stick from the planting as a guide the dahlias now need some form of staking. This is usually done with a group of encircling twiggy stakes wound together to
form a kind of up-turned basket. The flower stems grow through the support, in time disguising it, and the sticks help to hold the weight of the flowers. Some dahlias grow in a much more upright way and will benefit from more vertical staking such as garden canes to keep the flower heads aloft.
Given good growing conditions dahlias will shower you with many flowers so keep dead-heading through the summer and autumn, and they'll only stop come the first frosts.
There are two effective ways to propagate dahlias. For clones of those you already have, basel cuttings are the best way to go. In early spring lay the dahlia tubers on top of a tray/pot of compost somewhere light and warm indoors. Keep the compost moist allowing the plants to grow roots and new shoots but leave the top part of the plant exposed.
As new shoots grow, when about 5cm tall use a sharp knife to sever the shoot with a slither of tuber attached and place into individual pots of compost. The new shoots need that
piece of tuber/food storage to establish so cut carefully. By mid-summer the roots should have formed and the cuttings can be planted out, but we tend to leave ours in the greenhouse until the following spring when the plants have strongly established.
If you are looking to have your own unique plants, growing from seed is a good option. The 'children' plants will be something of a lottery depending on which particular flowers cross-pollinated, and you can get some interesting throw-backs to previous generations of crosses. Simply leave some flowers to run to seed and then harvest the seed heads when they have turned brown.
If kept dry dahlia seed can be kept and sown in spring, but I have always found most germination success by sowing the seeds fresh in the autumn. It also gives an extra six months of growth through the winter, but they must be kept in light and frost-free conditions. By the summer they should have started developing their own small tubers, but much like the basel cuttings I would recommend to save planting them in their final positions until the following spring.
It's a shame that dahlias don't have scent, but they have so many other wonderful attributes that I would very much recommend growing them.
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