Dahlia Mythbusting - Digging

Like many gardening topics, there is a lot of folklore around the right and wrong ways to grow dahlias and look after their tubers. 

One reason is because we all have different soil, and different climates, so what works for one grower might fail miserably for us - especially if they live in a different hemisphere.

However there are also certain 'rules' that aren't really rules at all, and might not be based in fact. 

We're going to interrogate some of them right now. 

Florelie Guava

I need to dig up my dahlias every year

False! Well false-ish. 

This piece of advice probably originated somewhere with a cold climate. It's true that if your ground freezes your dahlia tubers will turn to mush in the soil. This is also true if they freeze during storage - so keep them somewhere cool, but not freezing. 

In most parts of Australia we don't have to worry about freezing, so in theory you can leave your dahlias in the ground all winter long. 

There are even some people who think inground storage is the best way to store those difficult to keep/store varieties - but I haven't personally tested this.

But there is a caveat. 

If you - like me - don't have great drainage, or have a heavy, clay based soil, you might be safer digging up your clumps. 

If the dahlias are part of a mixed planting, or have another crop over the top, you might also consider if those plants need extra watering that might impact the dahlias. They can handle a normal amount of rain, but extra irrigation might cause an unnecessary risk. 

It's also a good idea to plan to dig up your dahlias and divide every two (2) or three (3) years to increase vigour and reduce overcrowding. 

If you'd like to see how some growers in cold climates overwinter their dahlias check: 

Jennie Love of Love n Fresh

Flourish Flower Farm

I Need My Dahlias to Die Back Before I Dig

Usual advice is to wait until your dahlias "die back" or go dormant before you dig and divide - but this isn't always necessary!

Kristine Albrecht of Santa Cruz Dahlias, and author of Dahlias: Seed to Bloom digs her dahlias while they are still green so she can test for virus before storage. 

In her book (linked above) she pondered the question of whether dahlias had to die back, noting that in warmer and tropical climates they don't experience a killing frost and can still dig and divide just fine. 

She asked the question of Dr Keith Hammett, a New Zealand based leading expert in dahlias, and he said "Tuber production is a consequence of day length. Frost is irrelevant. Day length is the determinant." 

Albrecht suggests waiting 90 days after the summer solstice (21 December in Australia) before digging. 

I need to Cut Back my dahlias by thirds if I don't get Frost

Nope. Like the answer directly above you don't need a frost, or to have the plants die back before digging. 

This advice is usually given: cut back the dahlias by thirds, leaving a week between each cut, to mimic the effect of a frost die back. 

However, if dahlias don't need frost, and don't need to die back, this is irrelevant!

In fact, it may be harmful. If you aren't properly sanitising between plants you could be spreading virus. 

It's also time consuming if it's unnecessary. Instead you could dig up the plant, then cut the stalk once before storing. Much simpler and more efficient.

Dahlias that Die Back Make more Eyes

I've seen this bandied around a little bit but haven't been able to find anything to back it up. 

I've also seen the similar: cutting back your dahlias encourages the eyes to pop, which makes dividing easier. 

I suspect that the two ideas may have become conflated, and that eye formation has more to do with genetics and care during the season than with cutting down the plants. 

However, I am unable to find much information - and no studies. If you come across anything please let me know!

Dahlias Need to be Grown for 120 Days

I'm not sure where this originated, but again, is not strictly true. 

More important than a strict day length count, is the period of time spent under different day lengths. 

According to this study

  • Longer day length encourages foliage and fibrous roots. 
  • Shorter day length can help with flower quality and tuberous root growth. 

It is important to note that some climates might have a season that is only 120 days long - and potentially shorter. This advice might be better for determining whether you can grow dahlias in your climate, rather than as a prescriptive rule for getting great tuber development and determining how well your tubers will store. 

A plant grown in only long day lengths, or only short day lengths, may not perform as well as one with a mix of short and long day lengths.

This '120 day' rule is better to be taken with a grain of salt in most parts of Australia where it is theoretically possible to grow in only short, or only long, day lengths if you are seeking only to meet the 120 day requirement.  

Dahlias Need a Dormancy Period

Nope. Dahlia tubers can be dug up, divided, and put on a warm bench to wake up again for cuttings right away. 

The thing is, in cooler and darker parts of Australia, your plants will struggle to grow under cold temperatures and day lengths of less than 12 hours. 

Dormancy in dahlias is usually a convenience, or a response to seasonal cycles, than a strict requirement, and can be manipulated by controlled environments and artificial light.

It has also been found that differing day lengths impact the amount and quality of flowering. This might also explain why dahlias aren't often grown year round in commercial greenhouse settings - but this is a hypothesis only.

Dahlias are Bulbs

No - dahlias are tubers. 

So what is the difference?

A bulb is an entire plant in a small package and are usually a rounded shape. Bulbs have layers of 'scales' which are actually modified leaves where they store their nutrients/energy. Roots form from the bottom from a hard 'plate'. Leaves and stems form at the top. Bulbs multiply by creating 'bulblets' or little bulbs. 

Many perennial bulbs need to die down to store enough energy to come back the following year, and require a dormancy period. 

A tuber is also a storage device for a plant. It forms 'eyes' and the plant grows and can be propagated from those eyes. 

The confusion between tubers and bulbs might be why some myths are falsely attributed to dahlias. 

A vase of dahlias

I Need to Dig and Store My Dahlias if I want to Divide Them

Nope. You can leave your dahlias in the ground and dig, divide, and replant all in one go when it is suitable in spring - so long as you have well drained soil and don't freeze (see above). You will want to do this when the soil is warm enough that it is safe to plant single tubers (which are typically less robust than clumps for staying in the cold ground). 

In fact I once knew a gentleman who dug his dahlias in spring, cut through the clump twice with a shovel, and replanted whatever remained. He grew beautiful dahlias but this is not for the fainthearted (nor recommended for unicorn varieties). 

Plants Grown From Cuttings Don't Make Tubers

Plants grown from cuttings, taken early enough, will make tubers that can be dug up, stored, and divided the following year. 

Like dahlias grown from tuber, they require time grown under long day lengths to create feeder roots, and time spent under short day lengths to allow the tubers to store nutrients and cure. 

Cuttings constrained in small pots may form smaller clumps if the roots become stunted before being planted out. Otherwise a cutting grown with enough time, and with proper nutrition and deadheading will perform similarly to a plant grown from a tuber. 

There is a caveat: some evidence suggests that for a cutting to produce eyes the following years, it must be a 'nodal' cutting. IE. the cutting must include a stem node beneath the soil or the tuber clump will be blind and unable to shoot the following year.

Triple Wren Farms tested taking and growing cuttings at different times of the year and the impact on tuber development. You can read it here


What other myths have you heard? Any other hard and fast rules you think we should look at? Anything that you do with your dahlias that you think is Quite Contrary? 

Let me know!

Back to blog