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Dahlias how much space?

#1 · 20th February 2022, 3:34 pm

Hi there I've got new beds built into my decking but they are quite narrow (around 30cm wide) and I don't know how much sun they will get? I really fancy a dahlia cafe au lait. Is it worth planting a tuber and simply finding out if it's the right spot? Would there be better white dahlias suited to this? Thank you

#2 · 20th February 2022, 5:35 pm

Quote from Lee Garden Ninja on 20th February 2022, 5:35 pm
Hi There,
Welcome to the Garden Ninja Gardening Forum! Great question about Dahlias which a lot of beginners choose to grow as their first steps into gardening.
Let's have a closer look at how to grow dahlias and then move on to whether or not your growing space is suitable for them.
Can I grow Dahlias in shade or full sun?
Some Dahlias will grow in dappled shade but for the most part cultivated Dahlias (ie bred versions that have been created by human involvement) do usually need full sun. Understanding where Dahlias originate from helps. Dahlias are from Mexico so as you can imagine, they require warmer growing conditions and bright sunlight to truly flourish. That's not to say some won't grow in dappled shade. Especially since some species will have originated on the edges of woodland or forest.
You may get away with Dahlias that only receive partial sun in the garden such as an east or west-facing garden - where the sun only appears for half of the day. If you have a north-facing garden though you are probably going to struggle with growing Dahlias and maybe better-picking plants that suit shady conditions more.
https://youtu.be/bDn1o0Dztcw
How to plant dahlias
Planting dahlias is a bit different from other plants as they are grown from tubers. A tuber is an underground tuberous root or storage organ. They look quite alien-like brown stubby fingers. These tubers store energy and then allow the dahlia to sprout each year. Remembering that they come from the warmth of Mexico they will need some form of winter protection. More on that below!
You want to dig a hole that's slightly larger than the tuber - this is where your 30cm bed may struggle- as you never want to crush or fold in the tuber to fit. This will damage it and probably cause the dahlia to struggle or fail to grow completely. Don't add any fertiliser the tuber should contain all the nutrients it needs. It's always better to feed once flowering with a peat-free compost mulch of comfrey tea mix.
When should I plant out dahlia tubers?
The best time is after the last frost of spring which in the UK is usually May time. Different locations will have different times for planting dahlia tubers. So apply your own local climate and weather patterns accordingly.
Planting Dahlia Step by Step Guide:
Here's the easy beginner's guide to planting dahlias.
Plant dahlia tubers in spring after the last frost (May usually in the UK)
Dig a hole slightly larger than the tuber 3-4 inches/ 7-10cm deep
Pop tuber in sitting horizontally
Backfill with the same soil - no need to add feed at this stage
Water well
Label where the tuber is and which cultivar or species of Dahlia it is
Do I need to lift my Dahlias in the winter?
Now, this is a real bone of contention amongst gardeners! Some lift their Dahlias each year once the foliage has turned black. Ie the first frost of winter. They then need to be cleaned and packed in a cool, dark and dry place. Usually a shed in apple crates. Some people dust the tubers with sand to keep them dry. Ready to be planted again the next year. However this is time-consuming and requires the dahlias don't get nibbled by mice or be allowed to rot.
The other school of thought is to simply apply a deep compost mulch each autumn to protect the tubers from frost damage which can kill them. Which is less labour intensive but does risk losing some or all of them. The key is to pick a place that's suitable for Dahlias. You probably don't want to grow them in heavy water logged clay soil which freezes solid each year,!
Let's now address the first issue which is space.
How deep should flower beds be for Dahlias?
Most Dahlias need at least 50-75cm of space when planted in the ground. This obviously depends on the variety. However, your Dahlia Cafe Au lait will grow to 1.5m each year with a spread of 75cm so thre 30cm border is simply too skinny for that. They will struggle and probably get overly congested within a year or so.
What may be better is to use containers to grow your dahlias. Using a few large containers you can grow them to their full size and it could look really spectacular. You could then overwinter the containers in a greenhouse or shed to prevent needing to lift the dahlias. Also by growing dahlias in containers, you can move them out of the way once they are finished. Rather than being left with an empty gap in your flower beds.
The advice I always give on the minimum size for flower beds is at least 70cm if not 1m for amazing borders. That's not to say you can't plant anything in there. I'd be mindful to go with a more contemporary evergreen ground cover plant like a Carex or Pachysandra to fill that space and add some green. You could also use bedding plants though it may look a bit twee!
https://youtu.be/h7DfM3xqfR4
I hope that helps with Dahlias and you certainly give them a go in containers. They are wonderful flowering specimens!
Happy Gardening.

Hi There,

Welcome to the Garden Ninja Gardening Forum! Great question about Dahlias which a lot of beginners choose to grow as their first steps into gardening.

Let's have a closer look at how to grow dahlias and then move on to whether or not your growing space is suitable for them.

Can I grow Dahlias in shade or full sun?

Some Dahlias will grow in dappled shade but for the most part cultivated Dahlias (ie bred versions that have been created by human involvement) do usually need full sun. Understanding where Dahlias originate from helps. Dahlias are from Mexico so as you can imagine, they require warmer growing conditions and bright sunlight to truly flourish. That's not to say some won't grow in dappled shade. Especially since some species will have originated on the edges of woodland or forest.

You may get away with Dahlias that only receive partial sun in the garden such as an east or west-facing garden - where the sun only appears for half of the day. If you have a north-facing garden though you are probably going to struggle with growing Dahlias and maybe better-picking plants that suit shady conditions more.

How to plant dahlias

Planting dahlias is a bit different from other plants as they are grown from tubers. A tuber is an underground tuberous root or storage organ. They look quite alien-like brown stubby fingers. These tubers store energy and then allow the dahlia to sprout each year. Remembering that they come from the warmth of Mexico they will need some form of winter protection. More on that below!

You want to dig a hole that's slightly larger than the tuber - this is where your 30cm bed may struggle- as you never want to crush or fold in the tuber to fit. This will damage it and probably cause the dahlia to struggle or fail to grow completely. Don't add any fertiliser the tuber should contain all the nutrients it needs. It's always better to feed once flowering with a peat-free compost mulch of comfrey tea mix.

When should I plant out dahlia tubers?

The best time is after the last frost of spring which in the UK is usually May time. Different locations will have different times for planting dahlia tubers. So apply your own local climate and weather patterns accordingly.

Planting Dahlia Step by Step Guide:

Here's the easy beginner's guide to planting dahlias.

Plant dahlia tubers in spring after the last frost (May usually in the UK)
Dig a hole slightly larger than the tuber 3-4 inches/ 7-10cm deep
Pop tuber in sitting horizontally
Backfill with the same soil - no need to add feed at this stage
Water well
Label where the tuber is and which cultivar or species of Dahlia it is

Do I need to lift my Dahlias in the winter?

Now, this is a real bone of contention amongst gardeners! Some lift their Dahlias each year once the foliage has turned black. Ie the first frost of winter. They then need to be cleaned and packed in a cool, dark and dry place. Usually a shed in apple crates. Some people dust the tubers with sand to keep them dry. Ready to be planted again the next year. However this is time-consuming and requires the dahlias don't get nibbled by mice or be allowed to rot.

The other school of thought is to simply apply a deep compost mulch each autumn to protect the tubers from frost damage which can kill them. Which is less labour intensive but does risk losing some or all of them. The key is to pick a place that's suitable for Dahlias. You probably don't want to grow them in heavy water logged clay soil which freezes solid each year,!

Let's now address the first issue which is space.

How deep should flower beds be for Dahlias?

Most Dahlias need at least 50-75cm of space when planted in the ground. This obviously depends on the variety. However, your Dahlia Cafe Au lait will grow to 1.5m each year with a spread of 75cm so thre 30cm border is simply too skinny for that. They will struggle and probably get overly congested within a year or so.

What may be better is to use containers to grow your dahlias. Using a few large containers you can grow them to their full size and it could look really spectacular. You could then overwinter the containers in a greenhouse or shed to prevent needing to lift the dahlias. Also by growing dahlias in containers, you can move them out of the way once they are finished. Rather than being left with an empty gap in your flower beds.

The advice I always give on the minimum size for flower beds is at least 70cm if not 1m for amazing borders. That's not to say you can't plant anything in there. I'd be mindful to go with a more contemporary evergreen ground cover plant like a Carex or Pachysandra to fill that space and add some green. You could also use bedding plants though it may look a bit twee!