Growing dahlia FAQs - The Laundry Garden dahlia guides

Helen Derrin

Jenny Williams from The Laundry Garden

Dahlias have been growing in The Laundry Garden since 2017, in the borders and now also in the cutting garden. They are one of my favourite flowers for adding colour and bringing joy into the garden.

Using my knowledge of eight years, I'm going to share my top growing tips. I've written the questions people most often ask to get your dahlias looking the best they can this season.

growing dahlia questions answered by experts
growing dahlia questions answered by experts

Where do dahlias grow best? And how much sun do they need?

Dahlias come from Mexico and Central America, grown by the Aztecs for food in mountainous regions. These plants require a lot of sunlight, at least six hours daily. Plant them in nutrient-rich, well-draining soil or compost.

They hate sitting in clay soil or any type of soil that holds moisture to them all the time, as they are prone to rotting. So, if you don't have free-draining soil, dig your tubers up and store it over winter.

What do I do with my dahlia tubers when they arrive?

If you're not ready to plant your tubers when they arrive, unpack them carefully and try not to disturb or damage the tubers. Then, keep them in a cool, dark, frost free and most importantly dry place until you are ready to plant them. You also need to store them so they're not touching, this will avoid any issues with mould and rot.

When to plant dahlias in pots?

When March or early April finally comes around, now is the time to pot up your dahlias. Wait until late April or even early May to plant them directly in the ground. This is when the ground is much warmer, and the danger of frost has passed.

When the weather is right and you are ready to pot them up, before you do anything else, go out and buy some labels. If you buy different types of dahlias, once they're out of the package, you won't know which is which.

This is particularly important if you're planning to work your dahlias into different colour schemes. So, learn from my mistakes. Write your label before you remove the tuber from the compostable packaging.

Starting your dahlia tubers off in pots is a good idea to give them a head start and some early protection from slugs. At this critical stage, when the shoots are just emerging, they are at their tastiest. This is generally my favourite method.

Choose a free-draining compost as you don't want anything to hold moisture against the tuber. Part fill your pot with compost and place your whole tuber beneath the soil. Bury the whole tuber up to the stalk. Keep the little stalk of the plant’s previous year’s growth (usually indicated by a cut edge) to be slightly poking out.

when to plant dahlias in the ground
when to plant dahlias in the ground

When & how do I plant my dahlia tubers directly in the ground?

If the frosts have passed then you can plant your tubers straight into the ground where you want them to grow. don't forget those occasional late May frosts we get here in the UK!

Prepare your soil, by clearing any weeds and digging in some well rotted manure or compost. This is a good time to add some nitrogen to the soil to feed the leaf growth.

I always use blood, fish & bone or chicken pellets, but your compost or rotted manure will also have some goodness in there. Be careful not to go overboard as too much nitrogen will cause too many leaves instead of flowers. You want slow and steady growth to create strong stems and prolific dahlia flowers.

When planting your dahlias, make sure you give them plenty of space, I would say around 30-50 cm apart. They thrive best in an open, sunny position with at least six hours of sunlight.

How to protect dahlia tubers from slugs & snails?

Protect your tuber ready for when those tasty shoots start emerging. Particularly if you're planting straight in the ground,

If you aren't already aware, dahlias are manna from heaven to slugs. Just accept that you'll generally have a sacrificial dahlia along the way. However, with a bit of preparation, there are some great methods you can use.

Copper rings - slugs hate copper. Place these copper rings on the soil around your tuber to create a slug proof wall of defence.

Sluggo is my go to organic solution. Scatter a small amount around each dahlia tuber. Sluggo is a naturally occurring iron compound which is harmless to all wildlife, but hated by slugs and snails. When it breaks down, it even adds nutrients to the soil.

Shell on Earth is another option some people use. I haven't tried it but I know a lot of people recommend it. The gritty hard, hard-edged structure of the shells makes it impossible for slugs and snails to ooze their way over.

Wool pellets are another great and organic method to keep the slugs away without harming them. I often use these when I pot up as they swell and protect the leaves as they emerge. Similarly to the shell option, slugs and snails cannot clamber over wool. I ask our local farmers for a bag of wool clippings, which also do the job just as well.

Do you feed dahlias?

The compost you use for the first stage of potting or planting out is enough to get them started. You only start feeding them once your tubers have a bit of growth on them and when you're planting them in the ground.

During the growing season, give them a general-purpose plant food to promote growth. When flowering, feed them with a potassium-rich feed such as a tomato plant feed and seaweed to promote flowering.

Give your tubers a light water when you first plant them, but do not water again until you see the shoots emerging. This avoids the potential risk of causing the tubers to rot. As soon as they start producing green shoots, water them regularly as the root system will be thirsty.

How to pinch out dahlias

Pinching out the growing tip of dahlias encourages more side shoots and, therefore more flowers. You can more than double the amount of flowers that you'll get by pinching out.

Wait until your young plants are around 8-15 cm tall, and then take a pair of snips and snip down to the bottom leaf growth. By doing this, you will get multiple side shoots for every stem that you’ve pinched out. More stems = more flowers.

Most importantly, don't be afraid of pinching out. Dahlias are incredibly tough and resilient plants that bounce back from a good chop. They will deliver flowers in abundance because of pinching out.

growing dahlias tips, hardening off dahlias
growing dahlias tips, hardening off dahlias

Can I plant my dahlias straight out from the greenhouse?

No, definitely not. The sudden shock of going from their warm and cosy starter home to outside can cause your young plant to sulk or even die. The ideal approach is to spend a week bringing them out in the daytime and then putting them away at night. This process will help them survive in cooler temperatures.

This process is known as 'hardening off'.

When do I plant out my young dahlia plants?

Once you pinch out the young shoots and harden them off, you can plant out your dahlias.

Prepare your soil the same as if you were planting them out directly. See question 4 for more details.

Do my dahlias need support, and when do I put this in?

Flowering dahlia plants are top-heavy, and most varieties need structure. This prevents them from collapsing with the weight of their blooms.

You can use various methods depending on where you plant them. Whichever option you choose, put your staking system in first and your plants after. I've learnt the hard way, stabbing and killing some of my precious plants.

Jute Netting is one of the methods we use in The Laundry Garden. It gives you the opportunity to create sculptural shapes with your planting. Jute netting is great for small or medium-height varieties.

This method is excellent for cut flower dahlias. Put some sticks in your bed about 50cm high and stretch the netting across, tying it securely in place. The netting allows the dahlias to grow through.

Plant supports, such as this tall mound support, are great for individual plants in the border or to support them in pots. We use them in our giant dahlia trough, and they work brilliantly

Finally, you can also create a crosshatch of bamboo sticks, hazel, birch, sycamore or other brushwood branches. They will provide a strong system for the dahlias to grow through.

are dahlias poisonous to dogs?
are dahlias poisonous to dogs?

Are dahlias poisonous to dogs?

Dahlias (their tubers, stems, leaves and flowers) are all mildly poisonous to dogs. But to have any noticeable impact on the dog, they would have to eat a huge number of them.

Unless your dog is distressed or extremely hungry, then he or she is unlikely to leap into your flower beds and eat your dahlias. Or jump on the table and chomp on your vase of flowers. If for whatever reason a dog eats a few flowers or stalks, it might cause a mild stomach upset.

For added reassurance, we have three dogs, who spend the whole time in our dahlia-filled garden. They have absolutely no interest in eating them.

Best dahlia varieties for pots

Growing dahlias in pots or containers is a fantastic choice. However, not all types of dahlias are suitable. Choose the smaller varieties such as any of the Bishop varieties. Dahlia ‘Bishop of Canterbury’ or Dahlia Bishop of Oxford are my favourites.

Bishop dahlias are a compact form with prolific flowers, making them the best dahlia variety for containers.

Do remember, however, that dahlias are hungry plants whose tubers can double in size when growing. Make sure you use a generous pot and plenty of good quality compost. Give them a water when you first plant them.

If you have a large trough or container, then have fun and try any of your favourites. Make sure you stake them well, and water and feed them.

I have mould on my tubers in storage. What can I do?

Stay calm; there is still hope. Give the tubers a squeeze and if there’s no squishiness, then simply scrape the mould off and wipe with kitchen paper or similar. Do check your storage set-up, as it sounds like some dampness is getting in. Cut away soft tubers or parts that don't look or feel right.

My dahlia tubers have started shooting in storage, what do I do?

don't worry; this can sometimes happen as the weather warms up. If it's the end of Feb onwards, then you are probably best to pot them up. Get them started in your greenhouse or on a cool windowsill out of direct sunlight. If it’s before February, then pot them up, but keep them in a dark, cool and frost-free place to slow down any further growth.

My tubers have started rotting. What can I do?

Again don't panic, they are still savable. Get a clean, sharp knife and cut out the rotting section. Feel the rest of the tuber to ensure it’s firm and visually looks OK. From there, you should be good to put them back in storage (though do check why they are rotting in the first place) or continue to get them potted up.

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