Clippings No.5 Planting Dahlias

Tuber, Pot, Bloom: The Step-by-Step Playbook for Show-Stopping Dahlias

Dahlias bring the wow factor to any garden, bursting with colour, shape, and personality from mid-summer right through to the first hard frost. They’re gloriously floriferous! Keep deadheading and they’ll throw out wave after wave of blooms in every hue you can imagine, and they make the absolute perfect cutting flower (hello, vase-fulls of show-stopping bouquets!). With a long flowering season and endless form and colour combos, dahlias are equal parts reliable and endlessly surprising. What’s not to love?

Today we’re planting up our beautiful dahlias for a summer-long flower fest. Grab your gloves, a fresh brew, and let’s get these hungry plants off to a roaring start.

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Buckets, Containers, and Size Matters

I’m planting mine in large galvanised buckets. Ideal because the minimum pot size for dahlias is pretty generous. Most varieties are ravenous for nutrients and space, so aim for a container at least 30 cm across and 30 cm deep, larger if you can. A bucket is really a bare minimum.

Those show-stopping decoratives, which can grow up to 1 m (3 ft) or even 1.5 m (5 ft) in some cases, will need all the room and stability they can get. You don’t want a top-heavy plant toppling over in the breeze!

Tip: Make sure your pots drain really well! Dahlia tubers hate soggy feet! If you’re using galvanised planters, drill in a few extra holes. Make sure all you potted dahlias are lifted up on chocks or blocks, so there’s a clear gap underneath, letting any excess water escape.

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From Clippings No. 2 to Planting Day

In my Clippings No. 2, I walked you through planting bare-root tubers. This year I ordered from DutchGrown, and honestly, their tubers have been amazing. Fantastic root structures, vigorous shoots, and rock-solid health all around. They’re absolutely romping away.

Once those tubers developed a couple of pairs of true leaves, I pinched out the main stem to promote side-branching and more blooms. When day temperatures crept upward, I moved the pots outside by day and back into the conservatory at night.

It’s a bit of a faff, but this regular exposure to direct sun (hardening-off) toughens up leaves and stems so that, come planting time, they shrug off slug attacks like champs. And thanks to a particularly sunny, warm spring, they’ve sprinted ahead in growth.

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Soil, Compost, and Feeding the Famished

Dahlias are absolute gluttons for nourishment, so we need a high-quality, peat-free compost. My go-to is Sylvagrow Multi-Purpose with added John Innes - rich, well-balanced, and ready to fuel months of flowering. The contents of a tomato grow-bag, loaded with that extra potash, works wonders too. Into the mix, I stir in a scoop of grit (plus any leftover perlite) for drainage, and a sprinkling of Fish-Blood-Bonemeal for an extra nutrient-packed oomph.

Planting steps:

1. Drainage layer: A couple of centimetres (1in) of grit at the bottom of the pot.

2. First compost layer: Firm it down a you load it.

3. Position the rootball: Crown at the same depth it was in its pot. The tuber itself should be around 10 cm (4 in) deep. If you spot the tuber poking out, add a bit more compost so it’s snug.

4. Backfill: Pack the compost around the tuber firmly. No airy pockets!

5. Water thoroughly: Settles the soil and load the compost with moisture.

Regular Watering & Seaweed Feeds

After planting, keep the compost moist but not waterlogged. I water deeply once a week with a liquid seaweed feed. This steady stream of trace elements and growth stimulants ensures your dahlias crank out flowers from July right through to the first frosts.

Tip: When the heat spikes, up the watering, and check your pots regularly. If a pot feels light, give it a long, thorough drink. If water just rushes straight through, the compost has dried out. You’ll need to stand the pot in a tray of water overnight so it can wick moisture up by capillary action and rehydrate the compost.

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Raised Beds and Intercropping

I’ve also planted more dahlias in a raised bed dedicated to cutting flowers. Between the dahlias, I’ll plant and sow salad crops (the usual suspects, lettuce, rocket, spinach) so they get a bit of afternoon shade and I get my cut-and-come-again greens. It’s a win-win! and a great use of space.

If you’re planting straight into the ground, the process is identical: dig a hole deep enough for the tuber and rootball, mix in fresh compost to act like a moisture sponge during dry spells, backfill, then sprinkle a little granular fertiliser around the crown. A light scratch-in ensures rain or irrigation carries the feed right down to hungry roots. Don’t forget a regular seaweed feed to keep those flowers pumping out their best.

Tip: If you’re planting in the ground and it’s bone-dry (it has been an exceptionally dry spring here in the UK), fill each planting hole with water first and let the water drain away. That way, when you pop in your dahlia (or any plant) it’ll have instant access to moisture.

Stakes, Canes, and Supporting Your Giants

For dahlias expected to top 90 cm (3 ft), they’ll need some support. Insert your supports at planting time so you don’t accidentally stab a tuber later. I use sturdy garden canes: three per plant for the biggest varieties, strung together with soft 3-ply garden twine. As stems grow, tie them in loosely. This keeps your blooms upright even after heavy rain or strong gusts.

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Final Thoughts

Planting dahlias really is a breeze once you’ve got the container, compost, and feeding routine nailed. From the moment those first power-packed buds burst to the first hard frost of autumn, you’ll be rewarded with a kaleidoscope of colour and countless Insta-worthy bouquets (if that’s your thing).

So, happy planting, my gardening chums! 🌿

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