How to stop slugs eating your dahlias

Even if you only have a tiny garden – or just some pots – you have to grow at least one cosmos. They are the lowest maintenance, floweriest plants in the world. For me, cosmos is the classic cut flower and a supremely lovely garden plant. In reward for very little effort, they give you buckets of cut flowers – and they have a very good vase life too. I love to grow lots of different varieties putting together different heights and colours in pots and borders. I hope you enjoy exploring our wide selection of cosmos seeds and seedlings.

Written by: Sarah Raven

Last updated: 4th Mar 2025

If you have a garden or a balcony, you’d be mad not to grow dahlias – there’s no better plant for flowers both inside and out. However, slugs and snails can often be a challenge, and many people find their dahlia plants have been destroyed almost as soon as they got them in the ground.

Slugs do indeed particularly love dahlias – and weather conditions can exacerbate the problem. A mild winter means fewer mollusc eggs are killed by frost (although annoyingly some slug species survive all but a hard frost) and if this is followed by a dull, wet spring the hatchlings will emerge into the perfect damp conditions, and they will thrive.

I fear this may put people off dahlias — particularly those growing them for the first time. That’s a shame because they’re ideal for beginners. They’re (usually) easy to grow, suit even a balcony garden with many varieties happy in containers, they’re cut-and-come-again and they now come in almost any shape and colour you can think of.

How to protect dahlias from slugs is one of the key questions Josie Lewis (our head gardener) and I get asked at Perch Hill open days. This is something we have put a lot of thought into and the techniques we use help us to keep our dahlia trial beds and mixed borders free from hungry slugs.

Here are our top tips for keeping your dahlias slug free:

Focus on organic gardening

Firstly, it may seem counterintuitive, but I’m convinced organic gardens have fewer slugs and snails because there are usually plenty of garden birds, frogs, toads and beetles in the ecosystem to keep mollusc breeding in check. Make sure to avoid chemical pest control methods that can be harmful to other wildlife.

Establish your dahlia plants before planting out

We grow between 60 and 100 new varieties at Perch Hill every year. With us, these are pretty much always potted and not planted out until they reach a good 15cms. Garden centres often tell people to just shove the tuber straight into the ground, but in our experience, unless you have a completely empty bed with no slug or snail hiding places (more on that below), that’s pretty certain death to your dahlias. As the new dahlia shoots break through the soil they are at their most juicy and tender. You’ve laid out a dinner party for slugs and snails and sure enough, they have a nocturnal field day.

In contrast, dahlia plants that are quite well-established can take a bit of slug feasting and won’t be wiped out. And the damage gets less the larger and more fibrous (and lignin rich) the stems become.

Here at Perch Hill, we pot every tuber into a 2 or 3L pot (depending on the tuber size) in peat-free compost in the middle of March. And don’t overwater. Water them once and then not again until the stems and leaves have started to properly emerge. This means the tubers won’t rot, the plants will use the water by that stage for photosynthesis and you’ll get good, strong dahlias. A bit of basal heat from mid-April (from a horticultural electric blanket) helps but is not essential. We don’t give heat any earlier as it pushes them on too quickly and light levels are not good enough. Just keep them in a light place that’s frost-free and don’t rush to plant them.

If you don’t have the space or time to grow your dahlias from tubers in pots before planting out in the garden, we sell many of our dahlias as ready-potted plants. These are dahlias that we have grown on from tubers into healthy, established plants in 3 litre pots and they are delivered ready to slot into your garden.

Focus on organic gardening

Both newly planted dahlias and the tubers left in the ground over winter to shoot again come April or May are vulnerable to slug damage as they start to grow. We need to be prepared.

For the success we have seen with keeping the dahlia trial beds slug-free, I am convinced it’s key that there is no other permanent planting. If you have stuff over-wintering that provides hiding places for molluscs, the slug dinner party provision starts all over again.

Instead, we mulch (in autumn and maybe again in spring) with our home-made compost. Coming from the garden, this is rich in annual self-seeders such as opium poppies, dill, Nicotiana knightiana, Persicaria orientalis and Nicandra physalodes. These then make the dahlia beds look great – all flowering together - but they aren’t large plants, harbouring slugs at the critical dahlia shooting time. I think that’s crucial.

It is also well worth laying short planks of wood in your borders as this does give slugs somewhere to hide but directs them to a preferred location and makes them easier to find.

Attract birds with annuals

In May, when the frosts are over, we supplement our dahlia beds with more annuals, sown inside as plugs. There are three groups we rate hugely in this role. First the amaranth brigade, then the sunflowers, and lastly we find the millets, particularly Red Millet, Panicum violaceum and ‘Sparkling Fountain’ are great.

In terms of look, these are on the right scale – with enough oomph and height to hold their own in a bed packed with dahlias (we put the shorter Panicum ‘Sparkling Fountain’ all round the edges). Then, in terms of practicalities, as they all run to seed in the autumn, they attract hordes of birds to forage in the dahlia patch. In August and September, the place is a flutter with gold finches, green finches, blue tits, robins and blackbirds, all feasting on the seed. We find as well as stripping the seed, the birds munch on any slugs and snails which might have got away. Overall, you create a slug and snail free zone without a single slug pellet in sight.

How to protect dahlias in mixed borders

We also grow plenty of dahlias in mixed borders here at Perch Hill. There are lots in the oast garden, in jewel-like colours and in our farmhouse garden in pastels and white. They grow with lots of shrubs and perennials all around them and we used to find, as the tubers sprouted, slugs and snails would wreak havoc.

For these bits of the garden, we now have a triple pronged attack:

Break up the mulch

First, we break up the mulch mound that went over the dahlias in the autumn. In April or early May (as they start to grow) we move this by hand (tools would damage emerging growth) away from the emerging stems and spread it around the dahlia as a mulch layer. This exposes clumps of eggs and hidden slugs for the birds to find.

Surround with slug barriers

Next, we surround each emerging dahlia plant with a good 15cm moat of grit. We label our dahlias clearly when we cut them back in winter, so we know where they are and we then dump a bucket of grit over their heads in the middle of spring. We’ve also had good results recently with Horti wool spread around the planting spot. Slug barriers and repellents are well worth using but, as many slugs live below the soil, you need to make sure to use them alongside other methods of slug control for best results. Copper tape on pots and slug-repelling wool pellets can also be useful deterrents to keep to hand in your arsenal.

Scatter mealworms

Finally, we scatter meal worms all through these borders every week from mid-April to mid-May. This draws in ground-feeding garden birds, robins, blackbirds and thrushes which eat the meal worms and then move on to the slugs and snails hiding in the skirts and shadows of your border plants.

Biological slug control

In the shorter term, until you get numbers down, (which took maybe 3 or 4 years here), Nemaslug, the biological control against slugs will help immediately and is harmless to children, pets, birds and other wildlife. Use this in April and September and you’ll see your slug population decline.

So our conclusion here, is that to protect your dahlias against slugs and snails over the long term there’s no one single silver bullet remedy, rather it’s a question of employing several different methods. All these things will help nature gradually get into balance, slugs, snails, toads, frogs, beetles, birds and dahlias, and it’s now quite rare for me to see either slug, snail — or their slime trail — in the garden here. Long may that continue.