This Fertiliser Is Perfect For Dahlias

By Daniel Woodley. Fact Checked by Hannah Miller. Published to Products on the 29th August 2021. Updated: 28th February 2023.

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Danny Woodley

Daniel Woodley

Dahlias are the fireworks of my garden, and late summer and early autumn wouldn’t be the same without these stunning plants that come in every colour and shade imaginable, from pastel blues to deep purple, crimson red and vibrant sunshine yellow.

If you want your garden to put out an explosion of colour and vibrance late in the season, then you’ll need to feed them the best ratio of nutrients at the right time.

Thankfully, I’m here to help, and on this page, I’ll explain which fertilisers worked best for me and why.

Dahlias are Hungry and Thirsty Plants

Dahlias grow quickly, have large leaves, juicy thick stems, and many will produce an abundance of large, oversized flowers.

It’s no wonder that dahlias are thirsty plants that need at least an inch of water a week and even more during dry, warm periods.

I’ve also found that in my gadren, they thrive in rich soil, ideally with lots of organic matter.

But what is the best fertiliser for dahlias?

Well, I always split my feeding into two parts; spring and summer/autumn.

Spring Feed

As dahlias grow from tubers buried in the ground, they start fresh each year and need to put on top growth and develop a root system before they can amaze us with their blooms.

Here’s how I feed my dahlias in the spring:

1) I dig in fresh compost and organic matter such as rotted manure or similar into the hole that the tubers will be placed in.

2) I always add a couple of handfuls of fish, blood and bone as it lasts just over a month and is slow-release and well-balanced.

3) I then follow up each month with another dose of fish. blood and bone until the start of summer when I switch to a different fertiliser.

My Preferred Spring Dahlia Fertiliser:

Westland's fish, blood and bone fertiliser for dahlias

Key Points:

  • Natural and organic.
  • Most fish, blood and bone fertilisers have well-balanced npk ratios and Westland has a fairly phosphorous content which is great for flowers.
  • It releases the feed gradually over 4-5 weeks.
  • Not too much nitrogen but enough to get the plant growing and the leaves big, green and healthy.
  • Fish, blood and bone feed can be used anywhere in the garden, and I use it as a general fertiliser and soil conditioner.

Summer Feed

When I see buds just starting to open, usually at the start of or in the middle of summer, I ditch the well-balanced fish, blood and bone and go for a low nitrogen feed that has plenty of potassium and phosphorus.

What’s the best fertiliser for dahlias at this time of year?

Low-nitrogen tomato feed or any fertiliser where the nitrogen is no more than half of the phosphorus level.

For example:

Most general, well-balanced feeds are around 5-5-5 or 5-7-5 or similar.

Tomato feeds and fertilisers are designed to encourage blooms and fruits so are typically 4-12-8 or 2-10-8 or similar, and this is what I look for.

Summer Dahlia Fertiliser:

Chempak #8 - low nitrogen fertiliser for dahlias

Key Points:

  • Low nitrogen so the dahlias won’t grow too leggy and straggly.
  • Potassium and phosphorous to encourage flowers, improve disease resistance, increase stem strength and boost the roots.
  • If it helps tomato vines produce large, red delicious tomatoes, it will do wonders for dahlia blooms.
  • Try Chempak #4 if you think your dahlias need more nitrogen.

Is Dahlia Fertiliser Absolutely Necessary?

There are plenty of gardeners who never use commercial fertilisers on their plants, and many go on to grow stunning dahlias, but almost all of them will feed the plants via fresh compost, rotted manure or similar.

While dahlias will grow in regular soil, they will most likely perform far below their potential with fewer blooms and weaker stems.

Dahlia fertiliser isn’t necessary, but it’s fairly inexpensive if you bulk buy and based on my experience and testing, I feel the rewards are worth the investment.

If you want to limit the number of fertilisers you buy, I recommend fish, blood and bone as it’s the perfect additive and general fertiliser with a good balance of all the key ingredients.

Photos of Our Dahlias

Below you’ll find photos of my dahlias taken here at DIY Gardening.

For the last few years, I’ve used fish, blood and bone fertiliser until the beginning of August, when I then switched to low-nitrogen tomato feed.

I think the plants are looking lush and healthy and the blooms large and colourful, but I’ll let you be the judge of that:

Large yellow/orange dahlia flower
Red dahlia flower
Pair of dahlia flowers in sunshine
Closeup of purple flower bud
Red and white dahlia flower bloom
Beautiful orange and yellow dahlia flower

More Photos

Alternative Dahlia Feeds

If you would like to try something other than fish, blood and bone or tomato feed, try one of these, dahlias respond well to these:

  • Regular compost.
  • Chicken pellets.
  • Well rotted manure.
  • Comfrey pellets.
  • Leaf compost.

7 Dahlia Growing Tips You Can’t Ignore

Whether it’s your first time growing dahlias or you just want to get better results, try my 7 tips which are based entirely on my experience and what I’ve learnt over the years:

1) Get the dahlias planted early in pots, well before the first frosts, so they have a headstart. Keep them protected from the cold and then transfer them to the garden when the risk of frost has passed. I’ve never had issues moving dahlias and I believe they transplant well.

2) I mix good quality fish, blood and bone with compost and dig in around the dahlia tubers when I plant them in the garden.

3) I usually wait until the dahlias are about a foot tall and have four sets of leaves, I then pinch out the centre stem. This encourages lots of side shoots and further stems, which will produce more blooms.

4) Don’t forget to stake or support tall dahlia varieties, I’ve had issues with dahlia stems snapping and I’ve found that frames are better than stakes but any type of support is better than nothing.

5) Want huge blooms? When the first buds start to open, ditch the balanced fertiliser and use one that has higher concentrations of potassium and phosphorous.

6) If you want to have some fun trying to grow really, really large blooms, try snipping off all the buds on a stem, except one. The plant will send all of its energy to that one bloom, and it will grow much bigger. I tried this and the bloom was huge and heavy.

7) Through my tests and experiments, I’ve found that dahlias respond well to regular deadheading and this can encourage more blooms to form, often well into autumn until the first frosts.

Bonus tip: Slugs love dahlias and will nibble the stems all the way to ground level if given the chance, I always have a slug control plan in place and it usually involves pellets and traps.

A Heads-Up About Fertiliser Ratios

The fertiliser products and ratios we’ve suggested in this article are based on my experience of growing dahlias in my soil, which could be different to yours.

The only way to determine which fertiliser is best for your garden and how much to apply is to test the soil.

Testing kits can be purchased online, and the Royal Horticultural Society offer a soil testing service by post.

More details about soil testing can be found here.

Why Trust Us? Our Experience & Product Testing Process

Daniel and Hannah, co-owners of DIY Gardening, have been growing dahlias for over 8 years and have tried numerous feeds, composts and fertilisers.

During the 2021 season, Daniel used blood, fish and bone as a base feed and Chempack’s low-nitrogen fertiliser to encourage bigger blooms once the dahlias had established.

As you can see from the photos, the results were excellent – the dahlias were healthy and produced huge blooms.

In 2022, Daniel experimented with organic alternatives such as leaf mould and composts and the results weren’t as good.

This guide was published by Daniel Woodley. Claims and important statements were fact-checked by Hannah Miller prior to publication.

Learn more about DIY Gardening’s review process here.

Author: Daniel Woodley

Daniel has over 18 years of experience in the construction, home improvement, and landscape garden industries.

He previously worked as a project manager and has experience in managing teams of tradespeople and landscape gardeners on both small and medium sized projects.

Daniel is also a keen gardener and enjoys growing unusual plants and tending to his lawn.

More About Daniel Woodley.

Daniel Woodley

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