When a gardening guru shares their wisdom, we listen — and Sarah Raven’s best dahlia tips, which she revealed to Ideal Home the other day, could be the key to thriving plants this year.
Even if you know how to plant dahlia tubers, you’ve probably dealt with a few hitches along the way — slugs, snails and plants that die off for no apparent reason seem to be a universal experience among dahlia growers.
But when I spoke to garden expert and writer Sarah Raven about her new dahlia Collective, she shared the two tricks she swears by to ensure her dahlias grow into burgeoning young plants.
Here’s how to get your dahlias off to the best possible start this year.
1. Pot them up first
Once you’ve worked out when to plant dahlias, you can get them straight into the ground — right?
Well, you can, but Sarah thinks it’s far wiser to pot them up first — especially if you want to deal with as few slugs and snails as possible (they went head over heels for my ‘Café au Lait’ dahlias last year!).
‘If you plant them straight into the ground, which is what a lot of garden centres tell you to do, it feels like it’s easier, but as they start to grow, they have really tender tips which the slugs and snails just take and take until in the end, the plant gives up and dies,’ Sarah explained.
Once you’ve bought your tubers, you’ll need to find the right spot to start them off.
‘Anywhere that’s got a bit of light, but isn’t too hot, is absolutely key,’ Sarah said. ‘Even if that’s sitting on your kitchen window ledge — put them into a decent 3-litre pot with peat-free compost and just bring them into growth.’
I asked Sarah how we can tell when they’re ready to be planted outside (unless you’ll be growing dahlias in pots for good, of course).
‘You need to get them to a good 10 centimetres tall before you plant them out,’ Sarah advised. ‘By then, the leaves are much better able to resist slugs and snails, because they’ve got more lignin in them.’
There are other benefits to potting dahlias up, too. ‘Starting dahlias in pots can make them grow earlier in the season and encourage them to develop much faster,’ says Graham Smith MCIHort, gardening expert at LBS Horticulture. ‘As the tubers are likely to develop much faster, they will also flower earlier.’
In a nutshell, potting your dahlia tubers up and allowing them to grow into established young plants before they enter slug and snail territory means they’re less attractive to the critters. That’ll save us searching for ways to get rid of slugs later in the year.
It’s an effective method, but slugs and snails aren’t the only pitfalls when it comes to growing dahlias. Sarah revealed another crucial step in ensuring your dahlias survive…
2. Scale back the watering
Slugs and snails are dahlia killers, but there’s one other mistake that many gardeners make when it comes to the early stages of growth: overwatering. More specifically, watering the tubers any time before growth appears (after you initially water them in).
‘Don’t water them until they start to grow,’ Sarah warned. ‘When they’re not growing, people panic and think they’ve got to water them all, and that’s when they rot.’
While it’s possible to save an overwatered plant in some cases, rotten tubers are a no-go.
‘Water them once after you’ve potted them up, but then make sure you don’t water them again until they’ve got a decent set of leaves, because that’s the only way they’ve use and lose that water — by photosynthesising,’ Sarah explained.
Leaving water around your undeveloped tubers could spell disaster. ‘That way, it sits in the water, and it’s all claggy around the tube so it’ll start to rot,’ Sarah explained.
‘They need to need the water, and they don’t need the water until you’ve got leaf.’
Once the first set of leaves appear, you still need to keep watering to a minimum. ‘Even at that point, don’t water more than once a week,’ said Sarah.
Then, once they’ve been planted out and the frosts are over, you can start watering more regularly.
Where to buy dahlia tubers
Key takeaways from Sarah Raven’s dahlia tips? Pot them up first, don’t water them again until they’ve got a strong pair of leaves, and plant them outside when they’ve reached 10 centimetres in height. That way, you’ll be starting off with a strong, resilient plant.
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