Dahlias are a dazzling delight to grow for all gardeners. Thanks to their cheery, bright colours and impressive impact when planted en masse, you’ll love the sight of dahlias in bloom so much that once you start growing them, it’s almost impossible to stop! From watering, growing and overwintering, read on for our complete guide to dahlias.
When to plant dahlias?
Due to the tenderness of dahlia bulbs, they won’t survive a frost. You can start them off in early spring so long as they’re under cover. Once all chances of frost have passed, usually in late May or June, you can then look to move or plant your dahlias outside.
How to grow dahlias
There’s a few options when it comes to learning how to grow dahlias. You can either plant your dahlias from tubers in the ground or in planters; grow dahlias from seed (not recommended for beginners); or plant dahlia plants.
Growing dahlias in the ground
Once all chances of frosts are over, you can plant dahlia bulbs directly in the ground. Always check the guidance of each variety when planting, you’ll typically find this on the back of the packet or label. To plant your tubers in the ground, ensure the tubers are approx. 30 inches (75 cm) apart and plant them just below the soil level. Mulch over the top to ensure that the tuber and its future growth is kept moist and protected. You could also add a horticultural fleece to act as further protection from the elements.
Once the dahlia sprout has grown three sets of leaves, it’s best to pinch out the tips of the main shoot, down to the highest pair of leaves. Pinching out your dahlia stems at this stage will help them grow stronger and produce more blooms when in flower.
Growing dahlias in pots
If you’d like to plant out your dahlias sooner rather than later, tubers can be planted out in pots from as early as March or April. You must ensure the pots are kept under cover and are well protected from any cold temperatures. Once the weather warms up and your plants are ready, you can move your dahlias outside and plant them into the ground around late May and June.
To plant dahlias in pots, fill a 2-3L planter with good quality compost. In the middle of the pot, place your tuber with the main stem facing upwards, then cover over with more compost. Lightly water the pot, add in a label so you can remember which variety you have planted, and place it safely in a warm spot such as a greenhouse.
You shouldn’t need to water again until new shoots begin appearing, and even then you should water it only lightly. Once a few weeks have passed, you’ll begin to see the shoot poking through the compost. Once the stem has grown three pairs of leaves, you can pinch out the stem using your fingers down to the highest pair of leaves. This will help your plant to grow strong and produce more blooms when it’s in flower.
If you decide to keep your potted dahlia in its pot then do make sure you choose a compact variety as some can grow very tall and become tricky to support. Choose a large container to allow for optimum growth and support that growth with good quality compost and a slow release fertiliser. If you’re planting in early summer, you can leave the pot outdoors and you won’t need to keep it as sheltered as you would if you were planting in March or April.
Planting Dahlia plants
It’s possible to buy dahlias in pots as fully grown plants or as large bedding plug plants. To plant out a 2-3L pot dahlia plant, begin by digging a hole in a sheltered, sunny position. The hole should be 30cm deep and 30 cm square. To prepare the hole, add plenty of organic matter. Gently place the plant in the hole and fill in the soil around the root ball. If you have more than one tuber to plant, ensure that they are spaced at least 75 cm apart. Once they get going, dahlias can grow very quickly so it’s wise to place a stake in to support their growth. To prevent breakages, you can tie the heavier stems to the plant support or stake.
How to deadhead dahlias
Regular deadheading is important for getting the most out of your dahlias. In fact, picking more flowers in general for cut flower bouquets will actually encourage more to grow so you’ll have a bountiful flower patch all season long.
To deadhead dahlias, follow the stem down to the first pair of leaves that you encounter, and cut the stem. For cut flowers, always cut with a nice sharp pair of floristry scissors at a 45 degree angle. Cutting the stem at this angle on healthy flowers will help them absorb more moisture when you put them in a vase of water for beautiful displays.
When do dahlias bloom?
Dahlias typically bloom in July and can continue to bloom until November when the first frost arrives. Thanks to their typically long flowering periods, you’ll have stunning floral displays both in the garden and in your cut flower vases indoors.
How to look after dahlias?
Dahlias typically prefer moist soil. In hot climates, water dahlias in pots every day. If your dahlias are in the ground, water them once a week with a large dose of water (a sprinkle is not enough!).
Use a slow release fertiliser for dahlias in pots to aid growth and keep the plant healthy, absorbing nutrients.
To get the most out of your dahlias, we recommend picking them regularly and deadheading often – this will promote more growth and leave you with more flowers than if you just left them alone.
Dahlias can grow quickly with some varieties having heavier flower heads. To support their growth, it’s wise to place a stake alongside the plant. To prevent stem breakages, particularly in windy weather, tie the heavier stems to the plant support or stake.
What is the best way to overwinter dahlias?
If you choose to leave your dahlias in the ground overwinter, you may risk losing them if we’re hit by particularly hard temperatures over the colder months. If we face a mild winter, you may be able to keep your dahlias surviving by covering them over with a thick layer of mulch.
To do this, allow the dahlias to blacken in autumn before cutting them down to a few inches above the ground. Ensure that each dahlia is labelled then generously cover it over with buckets of mulch. Giving the dahlias a nice thick blanket of mulch will aid their chances of survival through the colder months. Once the frosts are over, you can clear this away in spring.
The alternative and most preferred option is to lift dahlias out of the ground. Firstly, cut each plant down to around 6 inches once the tops of your plants have been caught by frost in autumn. Then dig each plant up gently and tap off excess soil from the tuber and roots. Put each tuber upside down in a box layered with newspaper to dry off for a few weeks. While it may be tempting to wash off your tubers, don’t! Getting them wet at this stage may spoil them and cause them to rot. Once a few weeks have passed, wrap the tubers in newspaper or store them in a storage box surrounded by dry sand or dry compost – this will prevent them from drying out too much. Store your box of tubers in a shed, garage, attic or wherever you can find that’s a cool, dry sheltered spot. We recommend that you keep a close eye on them over the winter and check none are rotting. If you find rot in one of the tubers, separate it as soon as possible as you don’t want it to spoil the rest of your precious varieties.
When to lift dahlias?
The optimal time to lift dahlias is in the autumn after the first frost has come and the plants have blackened. In the UK, this is usually around November time.