Dahlia Tubers: Hardening Off and Potting Up

Dahlia Tubers: Hardening Off and Potting Up

Once the frosts have passed, you can really kick off with your dahlia prep! Pot up, harden off, take cuttings, watch and wait...

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May Day has come and gone and in the last week, we’ve had snow, hail and ice here. Up and down the country, April tested us all, mightily, and it was clear that winter was holding on with her north-eastern winds that chilled almost every night. Each day, the greenhouse has been emptied of trays of growing plants and each night everything has been brought back in. Nervous of the nightly frosts, I’ve been cautious about planting out my hardy annuals that were sown in the autumn. Some have gone out and now the rest really need planting because I have a backlog of plants and seedlings in the greenhouse, not helped by the increasing square metreage of dahlia pots. This cold weather has ended up with me holding back on potting up the tubers but now I have no choice but to get them into their pots. I still have one box to go. If you haven’t potted yours up yet, it’s not too late, but do store them somewhere warm and don’t plant out until you’ve reached your last frost date.

Shelving filled up with trays of annual seedlings

Shelving filled up with trays of annual seedlings

The Last Frost Date and Hardening Off

Plantmaps.com shows the frost zones around the UK where you can see your predicted last frost date. This will provide an indication of the time that you can plant out your tender leaved plants. However, I am likely to be very cautious and have fleece ready to put over plants if needed. Last year we waited until the end of May to be sure. Before planting out, do spend a couple of weeks hardening off plants so that they will be able to survive and thrive. The last thing you need is for your carefully nurtured plants to suffer from too much weather.

  1. For a few days, on warm or cool days take the plants outside. Their leaves will be very tender.
  2. Over the next week leave them out for increasingly longer days and slightly harsher weather such as wind, rain, cooler moments.
  3. If you have a warm night, start leaving them out over night. If temperatures go below 4 or 5 degrees bring them in.
  4. Acclimatise to any cooler nights.

If you have a cold frame, life will be a lot easier.


Potting up the Dahlias, and taking Cuttings


Dahlias building up in the greenhouse. Slow shooting going on with this cold - stop and start weather.

When we dug up the dahlias in the autumn, we gave them a good clean and trim, before storing them wrapped in newspaper. Some of them split into two or three plants easily and everything was labelled. This has made potting up so much easier. I’ve also been able to split a few more tubers which will give me more plants. A couple of my favourites haven’t survived and a few look like they may only just survive. This is where I may take some cuttings to grow on stronger plants as it would be a shame to lose them.


  1. Decide what medium you are going to use for potting. I have used a 50:50 combination of garden compost (sieved from the compost bin) and bagged compost. This has been well mixed.
  2. Place the tuber in a pot, sized to provide some growing room and tall enough so that the compost can reach just to the tops of the tubers.
  3. Fill the pot with your compost, lightly firming in as you go along. The compost won’t need to reach beyond the top of the tubers.
  4. Label clearly and water. Then place in a warm spot so that the tubers can get started.
  5. As the tuber starts to sprout, I only let 5 shoots grow. The others get knocked off. However, if you are planning on taking cuttings allow a couple more to grow.
  6. When your cutting shoots are about 2-4 inches tall, take hold of the shoot and gently pull, twist or cut off so that you get the very bottom of the shoot. Take off any large leaves, leaving the top two sets. Pop the cutting into a small pot of gritty compost, situating round the edge of the pot. Water and label.
  7. These cuttings should develop healthy tubers ready for winter storage.


Despite the wintery weather, April gave us some wonderful tulips which went into some scrumptious bouquets. The tulips are still growing and blooming and I will enjoy cutting every one for my wonderful customers for as long as I can. Then I will dig them up, add them to the compost bin, dig over and improve the soil ready for the dahlias. There is always so much to look forward to with flowers.


Happy flowering, and good luck for a warm May

Fran @La-Di Dardy Flowers

This week's bouquet with tulips, hellebores, euphorbia, forget-me-nots and quince

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