When to Plant Dahlias: A Seasonal Guide for Garden Enthusiasts

When to Plant Dahlias: A Seasonal Guide for Garden Enthusiasts

Lesley Ann Sandbach

Spring's arrival announces a season of renewal and growth, a time that I, like many garden enthusiasts, eagerly wait for. There's a special place in my heart for dahlias, those stunning blooms that add a splash of colour to my garden each summer and make wonderful cutting flowers for the house.

As the days grow warmer, the excitement of planting dahlia tubers builds, promising a season filled with vibrant flowers.

In this blog, I'm excited to share my insights and experiences on when to plant dahlias. Whether you're a seasoned garden designer, a landscaper, an estate owner, or simply someone who cherishes historic homes and their gardens, this blog is for you. 

So, let's put on our gardening gloves and delve into the world of dahlias according to garden trends, exploring when to plant them for the most colourful and impressive show.

The Dahlia Diaries: Why Time is Everything

As I learn more about dahlias, I become more interested in growing them. It was during one particularly challenging spring that I realised the importance of timing in planting these beautiful flowers. In a rush of garden enthusiasm that year, as soon as the snow melted, I planted my dahlia tubers into the soil in early April.

Oh, how quickly things went wrong. The soil was still frozen solid, just inches below the surface. No matter how much I watered, those tubers couldn't break dormancy with the frigid temperatures lingering underground. Within a few weeks, every one had rotted to a mushy mess. My dahlia dreams were dashed before they'd even had a chance to sprout.

Growing large dahlias through a 3-ring support with grid

Determined to avoid repeating my mistake, I paid close attention to soil temperatures the following season and waited patiently until the middle of May to plant. The ground had thoroughly warmed up by then. This minor adjustment made a significant difference, with the tubers thriving and blooming in an array of stunning colours.

Dahlias require precise timing for planting. Their tubers need warm soil to grow but planting them too early can lead to rot before they even sprout. Since that initial setback, I've been meticulously checking soil temperature charts, ensuring I plant at the ideal time for my climate. This attention to detail has been key to enjoying the full potential of these beautiful flowers season after season.

Counting Petals: When to Plant Dahlias

Spring is the perfect time for planting my dahlias. Now that the nighttime temperatures are consistently above freezing, the ground has warmed up enough for dahlia tubers to grow. I like to get my dahlias in the ground around mid-late  May, although I always bring on a few of the previous year's stored tubers in pots in the greenhouse.

  • I start by getting my planting beds ready. I add some compost or other organic matter to lighten the soil and provide nutrients for strong plant growth. The soil should be loose and friable for the tubers to grow their roots. 
  • Next, I lay out tubers about 12-18 inches apart since dahlias grow large.  The tubers will normally have a stub of the previous year's growth together with a few 'eyes' at the top and several tubers dangling down. The top of the tuber should be about 4-6 inches below the soil surface.
  • After planting, I water them nicely to help settle the soil around the tubers and encourage sprouting. 
  • I'll keep the soil moist until sprouts emerge, which usually only takes  1-2 weeks. At this stage, beware slugs which love the juicy emerging shoots!
  • Once the plants have reached several inches tall, I'll start a regular watering and feeding schedule to support large blooms all summer.
  • Be sure not to plant dahlias too early when the ground is cold. Waiting until mid-late May ensures the tubers will not rot in wet, cold soil before being able to  sprout. If you do have room to pot up some of your tubers and keep them in a sheltered, frost-free place, they will get off to a good start. 

By following these guidelines, you can enjoy a garden with beautiful dahlia flowers in colourful varieties throughout the summer and fall.

Starting Dahlias Indoors: A Practical Approach

 I often start my dahlias indoors before transplanting them into my outdoor garden. It gives me a head start on the growing season and ensures that my dahlias have the best possible start in life.

Potting and Early Care for Tubers

When starting dahlias indoors, potting and early tuber care are crucial. 

  • I begin by selecting healthy tubers free from any signs of disease or rot. 
  • Then, I gently place them in pots filled with well-draining soil. It's essential only to cover the tubers with a thin soil layer, exposing the eyes. This allows for better root development and prevents rot.
  • Once the tubers are potted, I give them a good drink of water and place them in a warm, sunny location. I provide them with plenty of light to encourage strong growth. 
  • Regular watering is also essential to keep the soil moist but not soggy.

Transitioning from Indoor Start to Outdoor Planting

  • After a few weeks, the dahlias will sprout and develop a strong root system. This is when we begin transitioning them from indoors to outdoors. 
  • I gradually acclimatise them to the outdoor conditions by placing them in a protected area for a few hours each day. This allows them to adjust to the changes in temperature and sunlight.
  • When the danger of frost has passed, and the soil has warmed up, it's finally time to transplant the dahlias into their permanent spot in the garden. 
  • I dig a hole deep enough to accommodate the root ball and gently place the dahlia plant in the hole. I water thoroughly and support the plants, as dahlias can grow quite tall and need a little help staying upright.

Also, I have grown healthy and vibrant plants by starting dahlias indoors and following these practical steps. It may require extra effort and attention, but the reward of seeing beautiful blooms in the garden is well worth it. 

Planting Dahlias Outdoors

Planting Depth and Spacing for Optimal Growth

When planting dahlias outdoors, a few key factors must be considered to ensure optimal growth and blooming. 

  • Firstly, paying attention to your dahlias' planting depth and spacing is essential. These plants thrive in well-drained soil, so make sure to dig a hole that is about 6-8 inches deep. If your soil retains moisture, add some organic matter like compost to improve drainage.
  • Spacing is also crucial for dahlia plants. Give them enough room to grow by planting tubers at least 18-24 inches apart. This will prevent overcrowding and allow air circulation, which helps prevent diseases.

Effective Watering and Mulching Techniques

Next, we talk about effective watering and mulching techniques. 

  • Dahlias require consistent moisture to thrive, especially during their active growth period. 
  • Water deeply once or twice a week, ensuring the soil is evenly moist but not waterlogged. 
  • Mulching is also beneficial for dahlias as it helps retain moisture, suppresses weed growth, and insulates the soil during cooler months.
  • Choose an organic mulch like straw or wood chips and spread it around the base of the plants, leaving a small gap around the stem to prevent rot. 
  • Reapply the mulch as needed to maintain a 2-3 inch layer.

By following these guidelines, you'll be well on your way to enjoying a beautiful garden filled with stunning dahlias throughout the growing season.

Specific Planting Methods

When it comes to planting dahlias, you can use a few different methods depending on the type of dahlia you have. Here are some specific planting methods to consider:

When To Plant Rooted Dahlia Cuttings?

Rooted dahlia cuttings can be planted in the spring after the last frost. It's essential to wait until the soil has warmed up before planting, as cold soil can cause the cuttings to rot. 

Dahlia 'David Howard' is tall and always benefits from staking.

I like to wait until late April or early May to plant my rooted dahlia cuttings.

Best Practices for Planting Potted Dahlias

Potted dahlias can be planted at any time during the growing season. However, it's best to plant them in the spring when the soil is warm and the risk of frost has passed. 

When planting potted dahlias, be sure to dig a hole that is large enough to accommodate the entire root ball. Gently remove the plant from the pot and place it in the hole, ensuring the top of the root ball is level with the surrounding soil.

Guidelines for Sowing Dahlia Seeds

Here are some guidelines that you need to know:

  • Dahlia seeds can be sown indoors in late winter or early spring, about six to eight weeks before the last frost date. 
  • Fill a seed tray with seed compost and lightly press the seeds into the surface. 
  • Keep the soil moist and warm; the seeds should germinate within two weeks. 
  • Once the seedlings are large enough to handle, they can be transplanted into individual pots.

Timing and Techniques for Starting Dahlia Tubers

Dahlia tubers should be indoors about six to eight weeks before the last frost date. Fill a large pot with a well-draining potting mix and plant the tubers, making sure the eyes are facing up. Water the pot thoroughly and place it in a warm, sunny spot. 

As the tubers sprout, you can move them to a larger pot or plant them directly in the ground once the risk of frost has passed.

Ongoing Care and Maintenance

When planting dahlias, a few specific methods can help ensure healthier blooms and overall plant success. I have experimented with various techniques and learned a lot. 

Here are some tried and tested planting methods that I have found to be effective:

Fertilisation and Pruning for Healthier Blooms

Before planting dahlias, it is crucial to enrich the soil with organic matter, such as compost or well-rotted manure. This will provide essential nutrients for the plants.

  • Once the dahlias start to grow, regular  feeding is key. I like to use a balanced fertiliser every two to three weeks throughout the growing season.
  • Pruning is also essential for encouraging healthier blooms. I trim the plants by removing the lateral buds to promote bigger and more abundant flowers.

Pest and Disease Management Strategies

Dahlias can be prone to certain pests and diseases, but there are ways to combat these issues effectively.

  • I use natural repellents such as neem oil or beer traps to deter pests like aphids and slugs. These help protect the plants without harmful chemicals.
  • Proper hygiene practices are also crucial. Regularly remove any dead leaves or flowers to prevent the spreading of diseases like powdery mildew.
  • Additionally, I recommend inspecting the plants regularly for any signs of pests or diseases. Early detection allows for prompt action, preventing any major damage.

By following these specific planting methods, you can enjoy vibrant and healthy dahlias in your garden throughout the growing season. 

How do I support my dahlias?

One of the joys of growing dahlias is that they come in so many different sizes. Here’s my guide to keeping your dahlias upright:

  • For smaller dahlias I use a one-ring support that I pop over the tuber when I plant it out. This both marks the place where the dahlia has been planted and supports the plant as it grows

  • A larger clump or a taller growing dahlia (plants that grow up to 1m, such as ‘David Howard’) will benefit from an L1 with grid – a larger one-ring support with a rectangular grid to keep the stems upright

For really large dahlias, such as the ever-popular ‘Café au Lait’, there are two solutions: one of our large three-ring herbaceous supports will keep even the largest and most unruly dahlia in check (featured in the photograph above, the dahlia will reach the top of the support by the end of summer); I prefer to put 5 of our multi-ball stakes around the clump, put 3 layers of string around the clump to keep it in place and then weave a cat’s cradle of string between the stakes to make a grid for the stems to come through


Planting dahlias can be a rewarding experience for garden enthusiasts like me. 

Following the seasonal guide, we can ensure that our dahlias thrive and bloom beautifully. Whether we plant them in spring for early summer flowers or wait until the warmer months for a burst of colour in late summer and autumn, the key is to give them the right conditions and care. 

For added support and style, do please consider Muntons. Handmade in the Cotswolds, our plant supports provide the necessary stability and add a touch of elegance to your garden.  Smaller dahlias can be supported with 3 or 5 multi-ball stakes while larger varieties, such as the gorgeous 'Cafe au Lait' need a three-ring plant support to contain their vigorous growth and huge flower heads.

Frequently Asked Questions

What month do you plant dahlia tubers? 

Dahlia tubers can be grown in pots undercover in March or April and then planted in the garden in late May and June. Start by half-filling a 2-3 litre  pot with peat-free multi-purpose compost. Place the tuber in the pot with the central stem upwards and cover with more compost.

When should I plant my dahlias outside? 

In mid to late May, dahlias can be planted out into the garden once no more frosts are forecast. Before you do this, you may need to remove some shoots from the tuber, leaving around five. It feels harsh, but it will encourage bushy plants that produce many flowers.

Do dahlias come back every year? 

Dahlias are not frost-hardy, so when temperatures plummet, the leaves and remaining blooms collapse in a heap. However, all is not lost. The tubers that grow beneath the ground are unharmed and can live to perform again year after year,  in normal winters. However, I do ensure that I lift a few tubers once the frost has blackened the leaves and store them in a dry, frost-free place so that I can be sure of having some to pot up the following spring.

Do dahlias grow better in pots or on the ground? 

Despite a popular misconception that tubers must be grown in the ground, dahlias can do quite well in pots if cared for properly.

Is May too late to plant dahlia tubers? 

If you're not too fussy about potting your tubers, you can wait until mid-April to May to plant them directly outside. Although the danger of frost is still present, especially in colder areas of the country, it would not be harsh enough to kill the tuber.

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