How To Overwinter Dahlias So They Grow Back Each Year (2024)

PERENNIALS > DAHLIAS > OVERWINTERING

By ELIZABETH WADDINGTON

Dahlias are beautiful flowers that many love to have in their gardens, but how can you overwinter them successfully?

It is all well and good being able to grow dahlias successfully in your garden, but it is important to know what to do with your dahlias over winter to make sure that you can keep hold of them.

“This last winter has seen a lot of people lose some, if not all, of their valuable dahlia collection,” shares David Hall, Manager of Halls of Heddon, a specialist plant nursery for dahlias.

“Temperatures were much colder than we would normally expect, so the sudden drop in temperatures meant the frost penetrated the ground more deeply than usual.

“Simply covering tubers with fleece, or putting a small heater on in a greenhouse, wasn’t enough when temperatures in some areas plunged to minus double figures!”

Fortunately, if you overwinter them successfully, they will grow back each year.

Read on for instructions to do so successfully, no matter where in the UK you live.

Winter Protection

In warmer and more southerly parts of the UK, dahlia tubers can frequently be overwintered successfully in your garden.¹Overwintering tender plants: lifting or mulching. (n.d.). Royal Horticultural Society. Retrieved March 14, 2023, from https://www.rhs.org.uk/prevention-protection/overwintering-tender-plants-lifting-or-mulching

This strategy may even work further north if you have a relatively free-draining soil and conditions do not become too waterlogged in winter.

With additional protection over the winter months, you might not need to dig up your dahlia tubers for storage.

In cooler and wetter parts of the UK, however, dahlia tubers must usually be lifted, cleaned, dried and stored over winter before being planted back out in the garden.

Read on for more detailed information about how to overwinter dahlias successfully in the garden, or by lifting and storing them over the winter months:

Overwintering Dahlias In The Garden

First of all, let’s take a look at how you might be able to overwinter dahlias in your garden successfully without having to lift them out of the ground.

This is a lower-maintenance option but can yield varied results.

Whether or not dahlias will overwinter successfully outdoors will very much depend on the specific details of the area where you live, and even the characteristics of the particular planting spot in your garden.

“Leaving tubers in the ground over winter is always a risk but if your ground is well drained it can be done,” says June Nash, Secretary for The National Dahlia Society.

“If you intend to leave the tubers in, plant them a bit deeper, about 6 inches below soil level.”

Bear in mind that even if you have successfully overwintered dahlias in your garden in a previous year – you could experience losses, or even lose the lot during a particularly cold winter.

1) Mulching Dahlia Tubers

Mulching dahlia tubers can certainly work in the south, and has even been known to work as far north as Scotland’s central belt in a mild year.

A thick organic mulch can help protect the tuber from freezing as long as the temperatures do not get excessively low.

“Once the plants have been frosted, cut the plants down to ground level and cover with a further 6 inches of free-draining compost for more insulation,” says June.

“In the spring this can be scraped back and left around the plant as a mulch and soil improver.”

You can use a range of organic materials to mulch dahlia tubers.

The most common options are dry mulches of autumn leaves, wood chip or bark. Straw or dried bracken are other options to consider.

“I am lucky in the fact that I live on the south coast of England on free-draining soil, so I do not need to lift my Dahlia tubers,” shares Master Horticulturist Dan Ori.

“I just add an autumn mulch over the top of cut-back Dahlias.

“This gets them through winter and, if you don’t mind the smell, about 8 to 12-month-old horse manure is perfect for this job.”

The mulch should be laid thickly, around 5cm thick or more, and it should entirely cover the area around the tuber waiting below the ground.

This should be laid in place before hard frosts threaten, and then removed before growth commences in the spring.

2) Additional Protection For Overwintering Dahlias

Where mulch is not enough to protect dahlia tubers from cold winter temperatures, additional winter protection will be required.

Cloches or other protective covers could potentially be used both to shield dahlia tubers from winter cold and from excessive wet.

Another thing to consider is that materials with high thermal mass (such as rocks or containers full of water, for example) can be placed around dahlias to catch and store the sun’s energy during the day and release it slowly at night when temperatures fall.

In essence, what this does is create a warmer microclimate around the area where the tubers are buried in the soil.

If you would like to overwinter dahlia tubers in the garden, then choosing the right site in the first place is key.

Make sure you choose a location with relatively free-draining soil, in a sheltered position that does not become a frost pocket in winter.

Lifting & Storing Dahlia Tubers Indoors

It will not be easy to overwinter dahlia tubers outdoors in all gardens. And so for more reliable results, it can often be the best policy to lift the tubers.

“All our dahlias are grown in containers to ensure the best drainage and for easy indoor storage in winter,” shares Elaine Patullo, owner of the Scottish nursery Dachshund Dahlias.

“Our pots are all stacked away into lined greenhouses over winter.”

If this sounds like a good option for you, here are some tips to help you make sure that your dahlia tubers make it through unscathed to spring:

1) Lifting Dahlia Tubers

First of all, using a garden fork, carefully pry up the dahlia tubers from below the ground before the first hard frost.

“If you don’t want to risk leaving them in, dig the tubers up when the plants have been frosted or in mid to late November if there has been no frost,” says June.

Take care not to damage the tubers as you do so.

“Handle carefully to avoid damage to the tuber and cut the stems back to a couple of inches,” June continues.

Any damaged tubers should be discarded as these are more likely to succumb to rot in storage over the winter months.

It is also usually a good idea to discard the ‘mother’ tuber (the oldest, original tuber) as this is also more likely to rot than newer parts of the plant.

2) Cleaning & Prepping For Overwintering

Once you have lifted the tubers, knock off any large clumps of soil, and wash the dahlia tubers to get rid of pests, soil and debris.

Cut off any remaining above-ground growth to a short length above each tuber, as Jack Gott, owner of JRG Dahlias explains:

“Once you have taken your dahlia tubers from the ground, remove as much of the compost as possible.

“Then, trim the hairy roots as much as possible and store them in a dry, frost-free place.”

3) Drying Tubers

Next, you need to spread out the tubers on a rack, some newspaper, or another absorbent surface to dry out and ‘cure’, or you can hang them up, as June explains:

“Turn the tubers upside down and leave for a couple of weeks to drain off, remove any remaining soil.

“Many people pierce the stems with a screwdriver or even a drill before doing this.”

They should be out of direct sunlight, in a well-ventilated location.

This should take around 2-3 weeks.

4) Storing Tubers

Once the tubers are dry, you should prepare a box for their storage.

This can be a cardboard box, or another breathable container.

This container should be filled with a just-moist medium: peat-free compost, damp sand, or damp wood shavings can all work for this purpose.

Place a thin layer of this material in the base of the box, then place in the tubers carefully before covering them over with another layer of the damp medium.

“They must now be stored anywhere that is frost-free, preferably a greenhouse,” says June.

“Keep an eye on the tubers for any signs of rotting and cut out any damage, dust with yellow sulphur powder and let it dry again.”

Store this box in a cool, dark place that remains frost-free over the winter months.

A root cellar, basem*nt or unheated garage can be a good spot.

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  Overwintering tender plants: lifting or mulching. (n.d.). Royal Horticultural Society. Retrieved March 14, 2023, from https://www.rhs.org.uk/prevention-protection/overwintering-tender-plants-lifting-or-mulching