When to plant dahlia tubers: solving the dahlia grower’s dilemma

dahlia tubers: when to plant out dahlia tubers

Best time for planting out dahlia tubers

“When should I plant out dahlia tubers?” That is the eternal dilemma of the dahlia grower in Spring.

The answer to this question will ultimately be determined by the climate and conditions where you are. The critical point to remember is that dahlias cannot really take any frost.

So, if you plant out dahlias too early – i.e. before the last frost of Spring – then any growth the plant has put on will be damaged and the plant’s development will be checked significantly.

Depending on the severity of the frost, the plant itself may not recover at all.

When to plant unsprouted dahlia tubers

You can get away with planting un-sprouted tubers a little bit before the last frost if you set them 15 to 19cm (6 to 7 inches) below the surface.

This planting depth will usually mean that the frost won’t reach them at that level and the new growth won’t emerge until after the frosts are over. You should also add a good layer of mulch when planting dahlia tubers in this way.

You can read more about how deep to plant dahlia tubers in this post.

When to plant dahlia tubers in a greenhouse

If you have a greenhouse and you want to be ultra-safe, you can wake up your dahlia tubers by potting them up and then growing them on under cover until the weather warms up. This is what I generally do, as you can see in the picture below.

The advantage of this approach is that your dahlias can get a head start and can therefore flower earlier than if they had been started in the cold soil outside.

sprouting dahlia tubers: when to plant out dahlia tubers

You can do this several weeks before the last frost if your greenhouse is going to provide sufficient protection against any hard frost that may come in that period.

But, when you have grown on the plants under cover, you need to make sure that you plant them out after any risk of a hard frost. Otherwise, if they are hit by frost after you plant them out, the lovely foliage that you have carefully nurtured will be blackened and destroyed.

When is it safe to plant our dahlia tubers?

In summary, if you are planting out unsprouted dahlia tubers, and you place them 6 to 7 inches deep, you can plant out dahlia tubers a few weeks (say 3 to 4) before the last expected frost date. If you are planting out sprouted dahlia tubers, wait until the danger of frost has passed.

As to when the right time actually is, you’ll need to judge according to where you live. I’ve planted dahlias in London and near the east coast in Scotland and I’ve not had any significant trouble planting out dahlias tubers in mid to late May.

But, as indicated above, the critical factor is to know the last frost date where you are. And we have the information to help you below, whether you are in the USA or the UK.

USDA Hardiness Zone last frost dates for dahlias

Below are indications of the first and last expected frost dates, based on US Department of Agriculture plant hardiness zones.

You need to understand that the dates in the table below are averages, calculated over a period of years. That means that in some years the last and first frost dates were earlier and in some years they were later.

Therefore, to be extra careful, it makes sense to assume the that the last frost date will be a couple weeks after the date shown below and the first frost will be a couple of weeks earlier.

USDA plant hardiness map: when to plant out dahlia tubers
ZoneLast Frost DateFirst Frost Date
122 May to 4 June25 to 31 August
215 to 22 May1 to 8 September
31 to 16 May 8 to 15 September
424 April to 12 May21 September to 7 October
57 to 30 April13 to 21 October
61 to 21 April17 to 31 October
722 March to 3 April 29 October to 15 November
813 to 28 March 7 to 28 November
96 to 28 February25 November to 13 December
10-13No frostNo frost
Information based on USDA Hardiness Zones.

When to plant out dahlia tubers in the UK

Although the UK is significantly smaller than the US, the best time to plant out dahlia tubers still varies quite significantly across the country.

Dahlia planting times range from early March to late May. Dahlias can be planted out much earlier in the south, particularly the south-west, than they can in the North of England and Scotland.

Below I’ve list some of the major UK cities and towns, spread across all regions of the country. You can use this a guide to when you should plant out your dahlia tubers in the UK – allowing a week or two after the last frost date below, to be on the safe side.

CityRegionLast frost date
AberdeenNorth East Scotland1-10 May
AberystwythWales11-20 May
BarnsleyNorthern England11-20 April
BasingstokeSouth East England1-10 May
BathSouth West England1-21 March
BelfastNorthern Ireland (North)1-10 April
BirminghamMidlands21-30 April
BournemouthSouth of England11-20 April
BristolSouth West England11-20 March
CambridgeSouth East England11-20 April
CardiffSouth Wales11-20 March
CarlisleNorth West England11-20 May
DerbyMidlands1-10 May
DerryNorthern Ireland (South)21-31 March
DumfriesSouth West Scotland21-30 April
DundeeEastern Scotland1-10 April
DurhamNorth East England1-10 April
EdinburghCentral Eastern Scotland1-10 May
ExeterSouth West England1-10 May
FalmouthSouth Eastern Scotland21-31 March
GlasgowCentral Western Scotland1-10 April
GloucesterSouth West England11-20 March
HarrogateNorthern England21-30 April
HerefordSouth West England21-30 April
HolyheadNorth Wales21-31 March
InvernessNorthern Scotland21-30 April
LeedsNorthern England 21-30 April
LiverpoolNorth West England21-30 April
LondonSouth East England11-21 March
ManchesterNorth West England1-10 April
MargateSouth East England21-30 April
NewquaySouth West England21-31 March
NorwichEast Anglia1-10 April
NottinghamMidlands1-10 May
OxfordSouth East England21-30 April
PenzanceSouth West England21-31 March
PeterboroughSouth East England21-30 April
PlymouthSouth West England21-31 March
PrestonNorth West England21-30 April
SheffieldNorthern England21-30 April
SouthamptonSouthern England11-21 April
St IvesSouth West England11-21 March
StirlingCentral Scotland21-30 April
TelfordWest of England11-21 May
TorquaySouth West England21-30 April
TruroSouth West England21-31 March
TynesideNorth East England21-30 April
WeymouthSouth West England11-21 March
YeovilSouth West England11-21 March

FAQs on dahlia planting times

A fimbriated dahlia

Is May too late to plant dahlia tubers?

May is definitely not too late to plant out dahlia tubers. The best planting out time depends on when the last frost is likely to occur. In fact May is probably, the optimum time to plant out dahlias in most areas.

Consult the tables above and you will be able to see whether you should be planting dahlias out before during and after May, according to where you live.

Can I plant dahlia tubers in July?

You can plant dahlia tubers in July. But whether it will be worthwhile depends on a few factors.

Firstly, you need to have enough growing season left for the plants to mature and flower. If you live in an area where frosts come in September or even earlier, then there is a pretty good chance that any dahlias you plant in July won’t have sufficient time to flower before being hit by the first frosts.

You can judge this for yourself by looking at the table of first and last frost dates for the USA above. But what this means is that planting dahlias in July in Zones 1 to 4 is probably going to be a bit pointless if you want a decent show of flowers.

In the UK, this means planting dahlia tubers in July is likely to be unsuccessful in northern or highland areas.

The second point to consider when planting dahlia tubers late in the season, is the viability of tubers that have been in storage for that long into the summer. If your tubers are soft, light or shrivelled, they will be unlikely to grow successfully. If they are plump, solid, and ideally showing signs of shoots, then they will probably be good.

Finally if you are trying to grow dahlias in a tropical climate or somewhere very hot in the summer, like Florida, you should make sure you choose heat tolerant dahlia cultivars. You should probably also wait and plant the dahlia tubers out in September as they will be less heat stressed and will provide a good display, if deadheaded, all through Fall.

When to plant dahlia tubers Zone 5

You may still get frosts in April in Zone 5. The average last frost date is 30 April. You should therefore wait until a week or so into May to plant your dahlia tubers in Zone 5. You could plant unsprouted tubers a little bit earlier, but you would need to make sure they were planted around 6 inches deep and mulched well.

When to plant dahlia tubers Zone 6

The average last frost date in Zone 6 is 1 to 21 April. But, frosts can still occur up to the end of April. So it is better to wait until the beginning of May before you plant dahlia tubers in Zone 6.

When to plant dahlias Zone 7

If you are in USDA plant hardiness Zone 7, you are in a good place for growing dahlias, with a nice long dahlia growing season. The average last frost date in Zone 7 is between 22 March and 3 April. So you should be able to plant your dahlias in early to mid-April in Zone 7.

When to plant dahlias in Mississippi

The northern parts of Mississippi, e.g. Oxford and Tupelo are in Zone 7, so you can plant out your dahlias in mid-April in those areas. Central Mississippi, e.g. Jackson is in Zone 8. Dahlia enthusiasts in central Mississippi can therefore plant out their dahlias in early April. Dahlia growers in the far south of Mississippi can take advantage of the fact that they are in Zone 9. So they can plant out their dahlias in early to mid March.

When to plant dahlias Seattle

Despite its northerly position, Seattle benefits from its maritime climate. This means it falls into plant hardiness Zone 8. You can therefore safely plant out dahlias in April is Seattle.

When do you plant dahlia bulbs?

Dahlia bulbs are properly called dahlia tubers – you can find out the differences between bulbs and tubers in relation to dahlias here. Therefore, follow the advice on this about when you should plant dahlia tubers in your area.

More on Growing Dahlias

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

Love Dahlias?I've written the book on them

Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.

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