How to overwinter dahlia plants and tubers

One of the great advantages of dahlias is that if you keep deadheading them, they will continue to flower right into the fall/autumn. However, dahlias are only borderline hardy which means that the flowers and foliage will be cut back by the first frosts. 

So, at this point in the year you have to decide whether your winter dahlia care regime means lifting or overwintering the dahlia tubers in the ground.

This decision will largely be dependent upon the climate and condition where you live. Read on for help in making the choice.

How and when to overwinter dahlia plants in the ground

As a broad rule of thumb, where winter temperatures don’t go below around -5° degrees Celsius (23 degrees Fahrenheit), dahlia tubers can survive if left in the ground.

However, they will not survive if exposed to excessively wet conditions over any prolonged periods. In that case, it will not be cold but rotting of the tubers that will kill them.

You should be aware that some varieties of dahlia can be a little bit hardier than others, and the micro-conditions in your garden might vary a bit (e.g. how free draining the soil is in different borders.) So there is always a risk with overwintering dahlia in the ground in frosty and wet areas.

You might therefore want to lift the tubers of any specimens that are particularly precious to you. Although, as noted below, lifting and storing dahlia tubers is an approach that is not without its own risks.

If you think that your climate will allow you to leave your tubers in the ground, you should cut back the foliage to just above ground level and cover them with a think layer of mulch – and when I say thick, I mean thick. Start with 6 to 12 inches of garden compost, leaf mould or other organic matter.

The next step is to put a bamboo stake in the ground to mark where your dahlia tubers are wintering, but be careful not to damage the tubers themselves. It is also a good idea to add a label so you can remind yourself of the varieties you have as the shoots start to emerge in spring.

Talking of the shoots in spring, one of the big problems I always have with overwintering dahlia tubers in the ground is the slug and snail damage to these succulent shoots when they shoot up.

It is therefore a good idea to sprinkle the area around your overwintering dahlias with slug and snail pellets before the shoots emerge – in February I would suggest. I use pellets certified for organic use – normally these are ferric phosphate pellets, such as Sluggo snail and slug bait.

Keep an eye on the mulch and top it up during winter if it is blown or washed away.

How and when to dig up your dahlia tubers

Where winter temperatures are colder and/or wetter, or where otherwise desired, dahlia tubers should be lifted once the first frosts have hit. 

To do this, first cut back the damaged and withered foliage to 15 to 20cm (6 to 8 inches) above soil level with your secateurs. I use Felco number 2s.

The next step is to dig the tubers up. Clearly, you need to inflict as little damage as possible as you do this, but it is impossible to know how far from the crown of the plant the tubers and root system have spread.

The best advice is to dig round the plant in three places, about 25cm to 30cm (10 to 12 inches) from the centre of the crown.

I tend to use a garden fork for this because I find a fork is better at loosening the soil around the roots of the plant.

However, a lot of experts recommend using a spade. This is because if a spade damages the tubers, the cut will be clean and will therefore heal more easily with less risk of infection.

Once you have lifted the tubers with your fork or spade, clean off most of the loose soil.

The tubers  should, in the first instance, be stored upside down so that water can drain away from the hollow stems. This lessens the risk of the tubers rotting during storage.

At this stage, make sure you label each tuber carefully with a label that will stay fastened to the tuber over winter.

Once the tubers have dried out a little, clean off any more excess  soil that you can. It is also worth dusting the tubers with an anti-fungal powder to ward off any fungal diseases that might strike whilst they are in storage.

Winter storage for dahlia tubers

One the tubers are dry and clean, they should then be stored in a dry medium (see below for options), preferably in a slatted wooden box to allow air to circulate.

The box should be put in a dry and dark place where temperatures don’t dip below freezing.

It is worth remembering that what you are trying achieve when storing tubers is a somewhat contradictory set of goals: you want to avoid the tubers becoming dried out and shrivelled, but you don’t want them to be damp, because dampness will cause fungal growth and rotting.  Where I once went wrong in the past was to leave tubers in a frost free garage but open to the air. There was no rot, but several of them dried out completely and were not viable the following season.

The key therefore is to pack the tubers in a suitable medium – dry soil, sand, perlite, dry compost, even shredded paper – so that they remain dry but not open to the air that will actively dry them out.

Stored dahlia tubers can be then replanted once the risk of frost has receded in spring.

I usually start them off in pots in the greenhouse to get some strong growth growing before I plant them out.

Summing up on how to overwinter dahlia plants

Here are the main points to remember:

• Dahlia tubers can be overwintered in the ground where temperatures do not get below around 5 degrees Celsius (23F) and where they will not be waterlogged.
• Dahlia tubers overwintered in the ground should be covered with a thick layer of organic mulch which should be toped up during the winter.
• If lifting or digging up your dahlia tubers, take care not to damage the tubers in the process with your spade of fork.
• Clean off the excess soil and turn the lifted tubers and leave upside down for a few days to dry out.
• Further clean the dried tubers, dust with fungicide and store in a dry medium (e.g. sand or perlite) in a container that allows air to circulate.
• Store the container of tubers in a cool dry place, where temperatures remain above freezing.

What you need

More on Growing Dahlias

You can also get much more guidance on growing dahlias in these posts:

• Taking care of dahlias: the trick with deadheading
• Dahlia pinnata: a dahlia original
• How to grow dahlias: the complete guide to dahlia care
• How to overwinter dahlia plants and tubers
• All you need to know about dahlia tubers and dahlia bulbs
• When to plant dahlia tubers
• Dahlia varieties: your complete guide to all types of dahlias
• Dahlias in my garden: Six on Saturday
• Dahlia Wizard of Oz – Beautiful pink pompon dahlia
• Can you grow dahlias in raised beds?
• Do dahlias grow in Florida?
• Dahlia Wine Eyed Jill
• Can you grow dahlias in a tropical climate?
• Dahlia Ivanetti: magnificent magent dahlia
• Dahlia Night Silence – dusky pink dahlia beauty
• Visit a dahlia farm near you
• Best mulch for dahlias: a comprehensive guide
• Dahlia Islander: large, pink, loud, showy. What’s not to like?
• Heat tolerant dahlias: beat the heat with these 120 choice varieties