Last Updated on 19/11/2022 by Barney
If you’re looking for an easy-to-grow flower with a long flowering season then growing Dahlias should be at the top of your list. Flowering from mid-summer right up until the first autumn frosts I cannot think of a better value for money plant.
Dahlias come in a variety of shapes and sizes, making them perfect for any garden. In this guide, we will teach you everything you need to know about growing dahlias. From planting to pruning, we have you covered. Anyone can grow dahlias!
I’ll be completely honest; I was never that keen on them until now I always thought they were too flouncy with their showy blooms. However, years of growing them for clients and more recently becoming a Dahlia addict I am truly in love!
There are 1000s of varieties and a whole myriad of colours, shapes & tones that will draw you into the dark side, ‘dahlia-itis’ I call it.
So, without further ado continue reading for my easy steps to grow dahlias.
Growing Dahlias in the Flower Border
#1 Choosing & buying your Dahlias
The hardest part when growing dahlias is coming to terms with the fact that you just simply don’t have the space to plant them all.
My advice when choosing Dahlias is to start with flower form. Do you want a flower that is huge and flouncy or delicate and formal? Cut flowers for the vase or superb pollinators for bees & butterflies?
Dahlia varieties are placed into the following groups, Single flowered, Anemone, Pompom, Single orchid, Collarette, Cactus Dahlias, Waterlily, Semi cactus, Peony, Decorative & Miscellaneous. Further information on the classifications can be found at the Dahlia Society
Within these groups lies 1000s of hybrids. You can now see why choosing a variety (s) to grow is the hardest part!
There are small, 45cm high plants that are perfect for front of border planting & attractive displays in planters and tall, 1.2m varieties that look fabulous in large planters or back of border planting.
Dahlias can be bought as Dahlia tubers or ready potted plants.
Dahlia tubers are the underground storage organs of the plant (just think what a daffodil bulb is to a daffodil) and can be divided yearly to produce new (free) plants, more on that to follow below.
Tubers are typically available for purchase from garden centres & on-line suppliers in the spring. The advantage of buying on-line rather than garden centres is the sheer variety available.
Pot grown Dahlia plants are available to purchase in Spring and Summer.
#2 Planting & Soil Preparation (borders & planters)
Plant Dahlia tubers in late Spring when the risk of frost has passed. Sometime towards the end of May is usually fine. However, if you have a greenhouse you can start the tubers off undercover in early Spring.
Once you have purchased your Dahlias you will need to find the best spot in the garden to plant them.
Dahlias love full sun and a rich soil. They will fail to flower if planted in a shaded position.
To grow Dahlias pick any place in the garden that receives a minimum of 6-8 hours of direct sunlight.
Planting in the ground
Once you have decided on the best location I recommend a sprinkling of organic fertiliser such as blood, fish & bone. This will get the tubers off to a great start by promoting a healthy root system. Healthy roots = strong plants.
Alternately (or do both) dig in some well rotted manure with a garden fork. This works great at providing nutrients, enhancing poor soil and holding onto moisture in the Summer.
Prepare the planting hole by digging to the depth of your tuber. The biggest mistake many newbies to Dahlias make is planting the tuber too deeply.
There should be a central stem on the tuber, this was the old above ground growth. Make sure that this central stem is slightly protruding above the soil surface.
It might not be apparent which way up to plant the tubers and your worry is that you plant the tubers upside down. Look out for the old cut stem and if that is not apparent then the tubers ‘eyes’. These are small nodules at the top of a tuber where the new shoots will sprout from.
Once you have that sorted simply backfill and give a good watering.
I have learnt from experience that now is a good time to place any plant supports, especially if growing large varieties. Staking & supporting them after they have become formed bushy plants, you run the risk of damaging the plants.
Planting in pots and planters
At a minimum you will need a pot 30cm in diameter by 40cm deep. If growing a large variety then a larger pot will be necessary.
Good drainage is important to grow Dahlias in pots as the tubers will rot if sitting in wet sodden compost. Ensure your pot has 2 – 3 drainage holes covered with crocks.
Fill a pot with a either a good quality peat free multipurpose potting compost or preferably a loam based compost such as John Innes No2 , leaving enough space for the tuber. I always mix in a granular slow-release fertiliser at this point.
As with planting in the ground, the ‘eyes’ on the tuber face upwards, this is where the new growth will appear. Another good indication of which way up the tuber should be is more often than not an old stem is visible.
Simply place the tuber into your pot, fill the remaining space up to and over the eyes with compost ensuring the compost is in contact with all of the tuber. A few taps of the planter should help with this.
Give a good drench of water after planting and put in place any necessary supports. I find the tomato – peony supports are best for keeping the larger varieties upright.
#3 Water Dahlias Less with Mulching
Dahlias are thirsty plants and to grow Dahlias successfully they are going to enjoy a really good drink, especially in the hot summer months (don’t we all).
You can cut down on having to pop out every morning or evening to water, by applying a thick mulch around the base of the Dahlia.
A couple of inches of organic matter or well-rotted manure will really help in retaining moisture in the soil.
In pots where the compost will dry out quicker consider a mulch of slate chippings or gravel around the plant.
#4 Pinching Out (not to be missed)
Pinching out is an important step. Pinching out Dahlias produces bushier plants and more flowers than Dahlias that have not ‘pinched out’.
When your plants are about 45cm high, simply take a clean, sharp pair of secateurs and snip the main stem down to 2 or 3 sets of leaves.
Dahlia stems are hollow so perform this on a dry day. Perform this on a rainy day or when rain is forecasted and the risk is rain entering the hollow stems and rotting the plant away from the tuber.
It is also a good idea when pinching out to remove any leaves that are touching the ground. This creates better air circulation around the plant and removes the chance of slugs and ear wigs using them as a highway up to the good stuff.
When buying ready grown Dahlia plants this will not be necessary.
#5 Watering & Feeding
Dahlia plants need ample watering and feeding throughout the growing season to ensure healthy plants with masses of flowers.
One good watering every other day is normally fine except in the height of summer where you will want to water every day if you do not have a deep mulch around your plants.
To ensure a great display of flowers start feeding in July when the first flower buds appear until early September. Use something like Tomato right which is high in potassium. Apply every 2 weeks for best results.
Dahlias are fabulous at repeat flowering if you regularly deadhead.
When the booms start to fade and drop petals simply follow the stem down to a set of leaves and make the cut above with a sharp pair of secateurs.
#7 Winter Care
Being a tender perennial, once the first frosts have been Dahlias will look very sorry for themselves.
To overwinter dahlias there are a couple of options.
Option 1 – if planted in the ground apply a deep mulch of organic matter to protect the tubers from the cold.
Option 2 – if your soil tends to become waterlogged and / or you live in an area with prolonged cold snaps you should lift the tubers and store for next year.
To store them for winter simply cut all the old foliage back to the ground. Carefully dig the tubers with garden fork and store in pots or crates filled with dry compost or sharp sand .
The tubers are dormant and the pots or crates need to be stored in a dry, frost free place away harsh frosts or freezing temperatures. A garden shed, greenhouse or garage is perfect.
Dahlia Quick Questions & Answers
It’s a Wrap from Me
Dahlias are wonderful plants to grow, and they’re easy too! If you haven’t tried growing dahlias before, now is the time. They make great additions to any garden, and with a little bit of care, you can enjoy their beautiful blooms for many seasons. So go on, grow some dahlias! You won’t regret it.