Grow Huge Dahlia Blooms With These 7 Simple Steps

Learn How to Grow Big & Bold Dahlias

By Hannah Miller at DIY Gardening

Hello, my name is Hannah Miller, and I’m the co-owner and blog contributor here at DIY Gardening.

I’m also a huge fan of Dahlias, and in fact, they are my favourite late summer and autumn plant, so much so that I have a border and several pots full of them.

Dahlias can be grown bushy with lots of stems and smaller blooms, or they can be grown more compact with fewer stems but each producing a huge, delightful flower.

By following the 7 simple steps on this page, I’m confident you’ll be able to grow huge dinnerplate-size dahlias easily year after year.

Hannah Miller

Hannah Miller

Photos of Our Huge Dahlias

The dahlia photos shown below were taken in 2021:

Closeup of large yellow and orange dahlia flower
Closeup of large pink and red dahlia flower
Two large yellow and orange dahlia blooms
Dahlia bloom next to measuring tape

Step 1: Location Location Location

Dahlias love sunlight and lots of it!

The best place to plant a dahlia is in full sun, where they get at least 8 hours of sunlight a day.

Locate in a south-facing position, and you’re far more likely to see huge blooms.

Don’t forget to give your dahlias a little bit of space as they dislike crowded borders where they have to compete for sunlight.

The location is arguably the most important step to growing huge, healthy dahlias. I know because I’ve experimented with growing them in full sun, partial shade and even full shade. The ones grown in full sun were significantly bigger and healthier than the other two.

Step 2: Give Your Dahlias a Head Start

Dahlias are incredibly sensitive to frosts as their stems are very watery and brittle.

At the first sign of frost, most dahlias wilt and are finished for the season.

The dahlia tubers are just as sensitive, and for this reason, most gardeners will delay planting the tubers until all risk of frost has passed.

You can give your dahlias a head start by growing them in containers in a sheltered location until it’s safe to plant in the garden. 

I started my dahlias this year in pots that I left outside during the day and placed in a garage during the night until the cold weather had passed.

If you want to grow big dahlia flowers, give them a head start by growing them in a protected location earlier in the season.

Step 3: Prune Out Side Stems

Most gardeners prefer a bushy dahlia plant that produces lots of blooms, and they achieve this by cutting out the centre stem with secateurs or by pinching. This encourages side shoots and additional stems, each with multiple blooms.

However, as the plant’s energy is diluted, each flower will be smaller.

If you want to grow huge dahlia flowers, you’ll need to do the opposite and pinch out the side shoots on each stem, leaving just one flower per stem.

The plant will then divert as much energy as possible into that one bloom, which will grow bigger.

I usually aim for 5 or 6 stems per plant and prune the side shoots off so only one flower grows per stem.

Some gardeners enter competitions to see who can grow the largest dahlia flowers, and selective pruning is crucial to diverting energy and growth potential to the desired flowers.

Pruning dahlia side stems

Step 4: Deter Slugs

Slugs see dahlias as an all-you-eat buffet and will often travel past other edible plants just to get at the dahlia leaves.

Dahlias are at risk from slugs throughout the entire growing season but are particularly susceptible when their leaves are young and tender and close to the ground.

Slugs will devour entire leaves, and the dahlia will then struggle to create energy from the sunlight, and the blooms will be fewer and smaller.

Some form of protection or deterrence is almost certainly needed in the spring and early summer, at the least.

Our guide to slug control lists products that actually work, including organic options.

Slugs damage to dahlia leaves

Slug damage to dahlia leaves early in the summer

Step 5: Feed Those Hungry Dahlias

Dahlias are hungry plants, but a more measured approach is needed rather than throwing down as much fertiliser as you can.

We’ve already published a guide to the best feed for dahlias, and from our research and experience, we know that a two-step approach is best:

1) As soon as the dahlias break through the soil, feed regularly with a balanced fertiliser. 5-5-5 and 5-7-5 are ratios often found in commercial dahlia feeds, and these are perfect up until the buds start to form.

2) When buds start to form on the dahlia, it’s time to ditch the balanced feed and switch to a low-nitrogen fertiliser than will encourage large blooms and prevent the dahlia from becoming too leggy and bushy. Tomato feed is perfect as it’s formulated to grow huge tomatoes and will do the same for dahlia blooms. I use fertilisers with a ratio of 2-12-8 from late summer onwards and it works perfectly.

Read more about the perfect dahlia fertiliser here.

Westland's fish, blood and bone fertiliser for dahlias

I use blood fish and bone from spring until early summer. The 10kg buckets are the best value for money.

Step 6: Give Your Dahlias Some Support

Dahlias are notorious for snapping, often due to the wind but sometimes for no apparent reason.

I normally support dahlias with green plastic-coated metal stakes as they blend in with the foliage and are sturdy enough to prevent the dahlia stems from snapping. I use adjustable ties to keep the delicate dahlia stems close to the stakes, so they grow upright and vertical as much as possible.

Unfortunately, dahlia flowers that are grown deliberately to be as large as possible are heavier and require more support.

The best way to prevent the stems from snapping is to use a metal or timber frame. I suggest avoiding flimsy frames and supports and going for something sturdier and thicker as dahlias can get very heavy.

Step 7: Consistent Watering

I’ve been growing dahlias for years, and I’ve learned that they like a good flood of water once or twice a week during the summer, and if grown in pots, they should be watered daily when it’s scorching.

Light, shallow watering is problematic as it encourages shallow root development; this is far from desirable given that dahlias are very top-heavy.

Soggy, wet soil can rot the tuber, so they should never sit in waterlogged soil.

Dahlias should be grown in well-drained soil and watered deeply with soil kept moist but not sopping wet.

Discover More

Our guide to the best dahlia feed is a great read and perfect for novice gardeners. We also explain which NPK ratios are best for this hungry plant.

Our dahlia photo gallery contains images we took ourselves, which you can use on your website, blog or social media.

This guide to growing dahlias in pots is well worth reading too.

Head over to our blog for the latest content by Daniel and Hannah.

More From Hannah Miller:

This guide to growing huge dahlia flowers was created by Hannah Miller and was posted to our blog on the 9th of September, 2021.

Hannah is a keen amateur gardener, mother and the owner of one cute labradoodle plus a whole host of interesting plants.

She enjoys gardening as much as she cares about the environment and animals and likes to share her knowledge with others.

More About Hannah Miller.

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