If your dahlias are (hopefully) flowering this month and you want to prolong or boost those displays, you’ll need to know what to do with dahlias in August.
Whether you’re growing dahlias in pots or dinnerplate varieties in your border, the care you give them right now will determine how long they bloom for – and how much they flower over the coming weeks.
It’s all about deadheading them the right way, watering them properly, and one or two other care tasks that’ll guarantee strong, healthy plants for the rest of the growing season. Here’s what to do with your dahlias this August.
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What you’ll need
A fantastic dahlia fertiliser for bigger, longer-lasting blooms.
Spear & Jackson
Razorsharp Advantage Bypass Secateurs
My favourite budget pair of bypass secateurs!
A watering can is hosepipe-ban-proof, if you fill it up at the tap.
1. Deadhead spent flowers
In August, your dahlias should be pumping out blooms like there’s no tomorrow. That’s why deadheading is such an important dahlia task – think of them as a cut-and-come-again plant, because the more you deadhead, the more flowers you’ll be rewarded with.
‘The most important August job is to keep the plants flowering well, and here, deadheading is crucial,’ says Topher Martyn, head gardener at Syon Park.
‘When you cut off an old flower head, trim right back to the next pair of leaves, as this is where a new pair of flowers will emerge.’
I always resort to a sturdy pair of garden snips when I’m deadheading my plants, like these Kent & Stowe Garden Life Snips from B&Q.
Identifying an old flower head is pretty easy – you’ll just need to look at its shape.
‘Pointy shapes mean the flowers are done, so cut them all the way back to the main stem,’ explains Nicky Ryan, gardener at Polhawn Fort. ‘To recognise new buds, look for round shapes.’
2. Cut back leggy stems
If your dahlias are beginning to look leggy in August, you can get them back in shape by pruning them a little harder.
‘One of the (many) strengths of dahlias is that they’re strong growers, so if you think they’re getting leggy, don’t be afraid to cut a few stems back hard,’ says Topher from Syon Park.
‘Before you know it, there’ll be new buds coming through further down, and there’s no reason they won’t keep on through the dog days of September, right up to the first frost.’
You’ll need a sharp pair of secateurs to make the task smoother. These Niwaki Sentei Secateurs are some of the best I’ve tried, but if you’re after a budget-friendly pair, the Spear & Jackson Razorsharp Bypass Secateurs (which you can buy from Amazon) are an absolute steal.
3. Water them well
Underwatering and overwatering are some of the main reasons a dahlia fails to bloom, and they can stunt a plant’s flowering potential in August, too. Since the heatwaves seem to be sticking around, hydration is key this month.
‘While they will tolerate a degree of drought, dahlias generally perform far better if kept watered, so this might be a case when it’s worth schlepping out with a watering can,’ says Topher from Syon Park.
‘During dry spells, regular watering will be needed at the base of the new plant,’ agrees Nicky from Polhawn Fort. ‘Dahlias in pots will need more watering.’
Since there are hosepipe bans in place in many parts of the UK, it’s worth using a big watering can instead, like this 10L Ward Watering Can from Argos.
4. Feed them until the end of the month
Keep your dahlias well-fed in August to maximise their flowering displays. Until the end of the month, anyway.
‘Continue to feed container plants weekly and ground plants every two weeks with a low nitrogen, high potassium feed,’ says Nicky. ‘At the end of August, stop feeding so the tubers can prepare for their winter dormancy.’
Vitax Dahlia Feed, which you can order from Amazon, is a high-potash fertiliser that’s ideal for August feeding.
Deadheading, consistent watering and high-potash feeds are key to nurturing dahlias in August, promoting lots of healthy blooms right through to the autumn months.
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