Q: I love dahlias for their vibrant colors and intricate forms. This year, I would like to plant them in pots to brighten up my outdoor seating space. Are there any container-growing tips I need to know to keep them looking their best?
A: Dahlias are a great choice for a sunny container garden – and there's such a wide variety to choose from. Growing them this way is similar to how you'd grow them in the ground – however, there are a few things to bear in mind for the best display possible.
How and when to plant dahlias in pots
Planting dahlia tubers in pots should be done in mid-to-late spring, when risks of frosts have passed. You can plant them a little earlier if starting them in a frost-free greenhouse – when temperatures rise, harden the plants off for a week or so before moving them outdoors permanently.
'Set one to a 12in container,' instructs John Negus, a gardening expert. He advises covering the base with drainage material – crocks or stones – and filling it with loam-based potting compost. The tops of the tubers should poke through the soil's surface.
If you're growing a tall variety of dahlia, add a stake to the compost to help support it as it grows. Then, lightly water the compost, and wait for green shoots to appear. Once they do, continue to water lightly – not too much at first as this can cause the tubers to rot.
When the plants are about 12in high, remove the growing tips. This will reward you with bushier plants and more blooms.
If you're buying pot-grown plants from a garden center in summer, plant these straightaway, at the same level that they were planted originally.
John has been a garden journalist for over 50 years and regularly answers readers' questions in Amateur Gardening magazine. He has also written four books and has delivered many talks over the years on horticulture.
How to care for dahlias in pots
When strong new shoots form, John says to start liquid-feeding the plants on a weekly basis with a high-potash tomato fertilizer – such as Espoma Organic Tomato-Tone from Amazon. 'Continue until late summer.'
Once they have established in their pot, dahlias are thirsty plants. Ensure you give them plenty to drink, especially during periods of drought. However, don't overdo it – a common container gardening mistake – as waterlogged soil can damage the tubers. Putting the pots on pot feet, such as these UFelice pot risers from Amazon, will help any excess water to drain away.
Dahlias make beautiful cut flowers. But, if you're leaving some blooms to enjoy in your garden, deadhead them once they've gone over to encourage more. Cut just above the top set of leaves on each stem.
Winterizing dahlias in pots
In colder regions, you will need to take steps to winterize your dahlias, as they are not hardy plants.
In the fall, you can cut the stems back and lift the tubers to dry off before storing, says John. 'Remove as much compost as possible and then either hang them upside down or sit them somewhere warmish for a few days to dry out.' They can then be stored in trays of sand that are kept somewhere frost-free.
'Alternatively, if you have the space, you could put the pots somewhere frost-free, and leave the tubers undisturbed, letting the stems die back naturally,' he says. 'Keep them somewhere warmish and light in the first instance to give the compost a chance to dry a bit, but not enough to encourage the stems into growth. Once the stems have died back, the pots can be put somewhere dark and cool but it must be above freezing. If there is a chance of them getting frosted, wrap them in newspaper, fleece, or a blanket to keep them a bit warmer.' You can move the pots back outside when the weather warms.
What pests can be a problem for dahlias grown in pots?
'Guard against earwigs damaging flowers by smearing Vaseline around the stem base,' recommends gardening expert John Negus. Aphids can also be a nuisance: you can rub them away with your fingers or use a pesticide on more severe attacks.
Look out for slugs, too, although they are not as problematic for dahlias grown in pots as those in the ground. There are many natural ways to tackle them and prevent them from devouring the foliage.
What are the best varieties of dahlias for pots?
Choose compact dahlia varieties for your container display – the Royal Horticultural Society recommends those that grow to around 2ft.
The 'Mystic Dreamer' dahlia from Burpee, is a good example, with strikingly dark foliage and pink, single blooms. Other suitable options include the bright red 'Ellen Houston', and 'Totally Tangerine' which has orange, statement centers.
Without a doubt, dahlias are one of the best summer bulbs for a spectacular show, suiting cottage-garden schemes and modern backyards alike. And, by planting them in pots, you can enjoy them in any-sized space – even a compact balcony.
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The garden was always a big part of Holly's life growing up, as was the surrounding New Forest where she lived. Her appreciation for the great outdoors has only grown since then; over the years, she's been an allotment keeper, a professional gardener, and a botanical illustrator. Having worked for Gardeningetc.com for two years, Holly now regularly writes about plants and outdoor living for Homes & Gardens.
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