Our 7-Step Dahlia Rescue Plan

Written by Hannah Miller. Fact checked by Daniel Woodley. Published to Blog on the 21st of September 2022.

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Are your dahlias struggling this year?

Are you seeing few or even no flowers?

Perhaps the buds look small or weak?

Let us explain what’s going on and what steps you can take to get your dahlias back to their best.

1) It’s Too Hot

This summer has been the hottest on record in the UK and many other European countries, so it should be no surprise that some plants have struggled.

While dahlias prefer a sunny spot, I’ve seen many struggle if temperatures are consistently high.

Over the years, I’ve noticed that the most sensitive dahlias are the dinnerplate type, while ball dahlias tend to perform better in hotter climates.

The issue isn’t so much the heat but a lack of water.

Dahlias are thirsty plants, and if the soil is too dry for too long, they usually respond by limiting the number and size of the blooms. In extreme temperatures, I’ve seen them fail to produce any flowers at all.

This year (2022), our dinnerplate dahlia buds struggled to form and open. It wasn’t until after the prolonged summer heatwave that we saw them bounce back. In fact, it wasn’t until September 19th that we saw the first blooms open on some of them.

Dahlia Rescue Plan Step 1: 

I suggest you make sure the dahlias are adequately watered, especially during heatwaves. Consider a drip irrigation system so you can water deeply once every couple of days during dry weather and up to once a day during rare heatwave conditions.

About the only plants that weren’t affected by the heatwave this year, were weeds!

2) Don’t Apply Too Much Nitrogen

As well as being thirsty plants, dahlias are also hungry – I’ve seen them thrive in rich organic matter but struggle in poor soil.

Unfortunately, I’ve also seen them respond to excess nitrogen in the soil by producing a ton of foliage and tiny blooms.

Based on my experience, I feel that the golden rule is to only apply a nitrogen-rich fertiliser in the spring and, from early summer onwards, switch to a low-nitrogen, high-potassium feed (such as tomato feed) that promotes bigger blooms – I’ve had really good results by doing this.

A common mistake I see is when gardeners try to boost a poorly dahlia by overdosing the soil with nitrogen, which often worsens matters.

Dahlias love compost, well-rotted manure, leafmould and similar organic material, and I’ve found that a balanced fertiliser can complement these in the spring and a low-nitrogen feed is best from early summer onwards.

Dahlia Rescue Plan Step 2

I suggest you ditch the high-nitrogen fertiliser and dig in good quality organic matter several times throughout the year instead. If the dahlias produce lots of foliage and long stems but lack blooms, use a low-nitrogen, high-potassium feed every 10 days.

3) Bugs, Slugs and Other Pests

Slugs love to munch on dahlia stems/foliage, and young dahlias will often disappear down to ground level with just a clump of stumps visible.

In my garden, I’ve seen slug damage delays dahlia growth and blooming times, so I feel having a slug plan in place is essential, especially when the stems poke through the soil in the spring.

I’ve witnessed other pests, such as spider mites and aphids, suck sap from the dahlia, and a large infestation can ruin the buds before they bloom.

Caterpillars often leave behind a network of laced leaves and chewed buds and I’ve had some issues with these in the past.

Earwigs usually destroy the petals and when I see them, they are usually in groups.

Dahlia Rescue Plan Step 3

I suggest you deploy a slug and snail control plan from the moment dahlia stems pop up through the soil.

Check the stems, foliage and buds for signs of pests and apply a bug treatment spray/plan as required.

Slug damage to dahlia leaves

4) Diseases That Prevent Flowering

In my garden, the dahlias perform best in full sun and when I give them space to breathe.

I’ve previously seen some dahlias struggle with powdery mildew and other mould-type fungi and diseases if grown in a damp spot or overcrowded. In addition, dahlias overdosed with nitrogen are more likely to suffer, as are those distressed due to irregular watering.

Dahlia Rescue Plan Step 4

I think you should grow dahlias in an ideal spot, not in deep shade or a damp area, as from my testing and experimenting, they aren’t forgiving of shade and damp. I also feel that air circulation is important and one shouldn’t crowd dahlias too close.

Also, follow best practices such as watering from the side (not overhead) and cleaning secateurs to prevent the spread of fungi and disease.

5) Pinch Out The Side Shoots

Dahlias produce buds on side shoots as well as on the main stems.

As a general rule, pinch off the side stems, and the remaining blooms will grow bigger and stronger but fewer in number. On the other hand, leave all the buds in place, and you’ll get more numerous flowers that are smaller, often tiny.

If your dahlia is struggling to bloom, or the buds are really small and underdeveloped, try pinching off some side shoots, so the plant diverts energy to the remaining buds.

Dahlias Rescue Plan Step 5

Experiment by snipping off some side shoots and poorly formed buds.

Snipping off side stems forcing energy into fewer buds and blooms

6) Deadhead to Encourage New Flowers

Dahlias respond very well to frequent deadheading and when I’ve done this, I noticed that they produce new buds quickly.

Leave the old blooms on the stems, and the dahlia will assume it’s done its job for the year and will stop producing new flowers.

Cut off these fading flowers, and the dahlia will reward you with more blooms, often up until the first frosts.

As the dahlia will divert energy to new blooms, they won’t just be more numerous but also bigger and healthier.

Dahlia Rescue Plan Step 6

I suggest you deadhead regularly so the dahlia doesn’t go dormant and instead sends energy to new buds.

7) Start With a Healthy Tuber

Dahlia tubers often rot over the winter and may not sprout the following spring, or they may grow but only weakly.

Based on my experimenting and testing, I feel that the safest option is to lift them after the frosts have blackened the foliage, then store them in newspaper or other insulation until the spring.

Those living in warmer parts of the UK can leave the tubers in the ground but should insulate them with a few inches of mulch to protect them from frosts.

Dahlia Rescue Plan Step 7

Start with a healthier tuber, and you’re likely to have fewer problems down the line. Discard weak, slickly, soft or spongy dahlia tubers. Mulch well with a few inches of soil if you plan on leaving them in the ground.

8) Bonus Tip

I usually start off the dahlia tubers indoors or under glass about a month before the last frost date, as I’ve found it helps to give them a head start.

After the last risk of frost, I acclimatise them to the outdoor temperatures and then transplant them to the border or larger container.

This headstart often leads to stronger, healthier and fuller dahlias, and I feel it will benefit those who live in colder areas the most

Dahlias Not Flowering – Summary

I have over ten years of experience with dahlias, and from what I’ve seen, the most common reason dahlias don’t flower or produce blooms late is due to a lack of water.

Nutrient problems, specifically too much nitrogen, can lead to excess foliage and few flowers.

Other causes can be:

• Bad location.
• Too much shade.
• Heatwaves.
• Lack of air circulation.
• Bugs, pests and diseases.

Should the gardener fail to deadhead and/or pinch out the side shoots, the dahlia may respond by producing very small blooms.

Dahlia Flower Photos

I’ll leave you with a few photos of dahlia flowers I’ve grown over the years and links to our dahlia growing guides:

DIY Gardening’s complete guide to growing dahlias.

This is the best fertiliser for dahlias, based on my experience and research.

Learn how to grow really big dahlia flowers.

How to lift and store dahlias for the winter.