The Best Fertiliser For Dahlias
Dahlias are the fireworks of the gardening world, 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, Daniel at DIY Gardening is here to help and on this page, he’ll explain which fertilisers work best 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.
They also 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.
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.
- 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.
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.
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.
- 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 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 soil conditioner and general fertiliser with a good balance of all the key ingredients.
Photos of Our Dahlias
Below you’ll find photos of our dahlias taken here at DIY Gardening.
This year, we’ve used fish, blood and bone fertiliser until the beginning of August, when we 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:
See more of our dahlias photos here.
Alternative Dahlia Feeds
If you would like to try something other than fish, blood and bone, try one of these, dahlias respond well to these feeds:
- 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 these 7 tips:
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.
2) Mix good quality fish, blood and bone with compost and dig in around the dahlia tubers when you plant them in the garden.
3) Wait until the dahlias are about a foot tall and have four sets of leaves, 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.
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.
7) Dahlias respond well to regular deadheading and this can encourage more blooms to form, often well into autumn until the first frosts.
A Heads-Up About Fertiliser Ratios
The fertiliser products and ratios we’ve suggested in this article are based on our experience of growing dahlias in our 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.