Wednesday 17 July 2019

Leaving Dahlias in the ground overwinter

These dahlias have remained in the ground for five years now and.....
...  were all raised from seed
The normal way to overwinter dahlias is to lift them after the frost turns them black and to store in a cool frost free dry place. Not one for following careful instructions it has not been always successful for me. Well documented elsewhere I do not propose to describe how today!

A beacon for drivers for five months of the year
For several years now I have left my dahlias in the ground. Not always successful I do have by now strong plants that I can more or less rely on. Some have survived for up to a decade. Through the last two mild Winters I have not lost a single plant and with self seeding my stocks have increased.

Would any survive another 2010 Winter when temperatures of minus 20 centigrade persisted for six weeks or more and cold penetrated deep in the ground? They would not.
I would buy some more and start again. 
No doubt there has been some self selection of the hardier kinds. Those that have come back each year have liked my conditions. In passing I might mention their are some species of dahlia more regularly regarded as hardy. A particular favourite is Dahlia merckii.

Over the wall Dahlia merckii sets off 'wildflowers' in  the farm field
Last year this Dahlia merckii sowed itself here
If you live in colder areas than I do or have heavy poorly drained soil my methods will not be suitable for you. If you live in the balmy south-west I suspect it is the normal method.

 I would love to hear reader’s experience and receive their opinions. I don’t live in hope as other than my much loved and appreciated stalwarts most readers seem reticent to offer an opinion. I often feel I have failed and my only conciliation is that many other garden bloggers share this frustration.

How things have changed
Seed-raised from The Bishop of LLandalf

In my youth I would have considered leaving dahlias outside through the Winter as foolish. I also considered them gaudy floppy labour intensive things not for me. I now truly love them and regard them as an essential part of my summer garden totting up  almost five months of continuous flowers. (My best performance being mid June to the beginning of November).
I remember I once had a client who by neglect had failed to lift her dahlias in a very sheltered part of her York garden. I was amazed they came back every year and thought it a complete aberration! 
What has changed? Is it climate change and warmer winters? Is it just my expanded ambition? Are modern varieties tougher?
I am told that Victorian gardeners knew that if buried overwinter below the level of soil freezing they would survive in the ground

Making it work

This one is tall and floppy and needs staking
Dahlias will fail to overwinter if your drainage is poor and there is excess water at the depth of the tubers. My soil is sandy which makes it very suitable. Even so the lower parts of my garden are too wet and I have learnt by success or failure where they will happily survive.
I suggest when first planting you plant them as a green plant and quite deep a couple of inches more than normal - or even much more. I was told that way they subsequently form new  tubers deeply. I have doubts whether this is true and many of my plants are undisturbed self sown seedlings. 

They are a bit of a jumble, Brian
Reader Brian Skeys kindly sent me seeds of a lovely extra-hardy dahlia strain and the three plants I raised have now made up to a dozen - and extended their range of colour. (Brian I have forgotten their name, if you are still there perhaps you would tell me?).

The bishop spreads himself around in diverse colours
The bishop’s children (legitimate and illegitimate offspring of The Bishop of LLandalf) have established the same. I think that like the more tender varieties of alstroemerias, dahlias make ever deeper tubers as the years pass by.
Each year the plants get stronger and with multiples of growing points are less likely to need staking. One or two of my taller ones flop all over and Brenda shouts at me when she needs to put in a stake.
We frequently suffer from late spring drought. There is no problem with my dahlias, the roots are already down.

Dahlia 'Magenta Star' in Peter Williams' garden
It is quite easy of course in late Spring/early Summer to shove your spade in deeply and take out a large segment to replant as new stock. So too with a sharp knife take cuttings going deeply to a fleshy white base (not essential). Use your favourite method of rooting preferably under glass.
In early June this year I took advantage of the wet spell to transplant some young plants that had seeded in the wrong place. Young dahlias transplant very well with very little check to their growth.

There might be some merit in mulching over with such as insulating autumn leaves or straw when your dahlias die down. I half heartedly leave dead tops intertwined with Autumn leaves  that I rake over the top.
The trouble with such insulation is that continuous frost for a day or so soon penetrates through. Your own overcoat keeps you warm because there is a heat source from your body within. Heat stored at depth in the soil is very small.
My none mulched plants come through just the same.

Pest Problem

Ugh, black bean aphid (I assume)
Although I claim leaving dahlias in the ground will be more healthy than overwintering dry and will have less fungal disease such as root rots and subsequently all the perils of humid greenhouse conditions and pest presence, last year I was not immune!
I was caught out by these aphids. In the picture they are already moulting. I know you can control black aphid on broad beans by pinching them out at the tip of the shoots, thereby restoring the natural balance with hungry predators. I had to go a step further here and cut them half back. They soon made new strong growth and were as good as ever.

Back this year
Alerted this year by blogger Sue Garret who reported that she also had suffered black aphids on her dahlias I rushed out to find in mid June just one infected plant of my own. I snipped it away.

Healthy bishop children in early June


  1. I am a fairly new reader of your blog but I find it very informative and as a no dig gardener myself I agree with your ideas. I am not a fan of dahlias myself as I live in a frost pocket in mid motts so would have to do all that lifting, drying and storing nonsense to grow them. However I was given a tuber of a white one for my white flower bed and could not say "I don't like them" to the generous giver so I potted it up in a very large pot and enjoyed its lovely white flowers until the first frost. After which I moved the whole pot into my unheated green house where it slowly died back and dried out. In late spring I saw new shoots so started giving it a little water. it is now in the garden again and just about to come into flower. I think I will try this method of over wintering again and may even invest in some more.
    I read quite a lot of different blogs and rarely leave comments not realizing that they were important to the blogger but reading the beginning of your post I now understand that comments let you know that your blog is read and appreciated.

    1. I do appreciate feedback,especially like this Notts Unknown.
      I have on and off overwintered dahlias in pots in my unheataed greenhouse and find it generally successful. I give them one or two waterings just for a refresher but they are dormant and generally dry.I would start proper watering about March.
      There is one downside, a very severe prolonged cold spell will usually penetrate an unheated greenhouse and standing pots are particularly vulnerable to freezing. I am lucky that my greenhouse has soil and I can plunge the pots in soil under the bench. More usually I just take my chance

  2. "We" grew dahlias up until, I think about 1970. Our stock had risen to about 300 plants, and we stored them overwinter in an outhouse. That year the frost got in and killed of 90% of the tubors. We planted up the remainder in peat (those were the days) and on flowering we discovered every single surviving one was Edinburgh - purple and white. We never grew dahlias again.

    1. Perhaps a rethink might be due Mal!
      Edinburgh must be hardy!!

  3. An informative read, as I expect when you post Roger. I shall try Dahlias again with the 'leave them in the soil' method. As you describe the self elimination of less hardy varieties will take place, and I note your hint on planting tubers more deeply. This year I have a white dahlia requested and given to me by a friend. I too have had black bean fly on many plants in the garden. I chose not to spray doing some finger clearing/squashing earlier in the season. Now they seem to be in balance, and we have many different hover flies and insects.

    1. Sounds good Stasher. Finger squashing can work very well. Not sure if I would have had the patience.
      I seem to remember that with my aphid incident last year I forgot to mention in the post that after my pruning out I gave my dahlia clumps an energetic jet of water with my finger over the hosepipe.It helps

  4. Th blackfly fortunately didn’t hang around! We’ve overwintered dahlias on our allotment in our heavy clay soil in the past. We covered them with a pile of straw and covered that with a black plastic sheet to keep them dry. It must have been snug under there as bees set up a nest. As you walked past the bed you could hear constant buzzing. We stopped as we wanted to be able to grow them in different places amongst other plants so this method became impractical.

    1. Good idea Sue. I suggest that if you bravely forked them out when they started to emerge they would move without demur

  5. I leave my dahlias in my allotment overwinter in north London as I have no room to store pots. It's not Yorkshire but much chillier on the allotment than in back gardens. They rarely come through the winter but I now think its the slugs and snails that do for them as they try to emerge in spring rather than any frost. I haven't thought through a solution to that one yet apart from popping a large glass tube over the plant until it is bigger. That works but I only have two of them, no idea where they came from.

    1. I seem to remember much of London is on clay which might lead to wetter conditions over Winter and perhaps wetter spring surfaces which favour slugs.
      Fortunately I have little or no slug damage to my emerging dahlias but then at emergence in recent years my sandy soil has been bone dry on the surface. It amazes me where they find the water to grow in our very dry regional climate
      Perhaps if you use my search box to find slugs there might be some help


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