When to Move Dahlias Outside?

If you’re wondering what you should be doing with your dahlias right now — especially if your greenhouse is starting to overflow — you’re definitely not alone.

This is the point in the season where everything suddenly needs more space. Seedlings are growing, trays need potting on, and before you know it, your dahlias are squeezed into corners not getting quite enough light. (Let me refer you to exhibit A below, which is sadly not getting enough light on the second shelf of staging in my greenhouse!)

So let me break it down simply.

What Should I Do With My Potted Dahlias?

If your dahlias are still sitting INSIDE your house on a windowsill, they definitely shouldn’t be staying there full-time anymore. At this point in the season, they need full light and fresh air ALL DAY LONG otherwise they’ll start to stretch and become leggy (I've written a blog post all about it here!)

So here's what you need to do:

Start hardening them off ASAP!

They must be introduced to outdoor conditions:

🌱 Put them outside during the day

🌱 Bring them back in at night

🌱 Do this for a few days to 'harden' them

Then, as long as long as there’s no frost forecast, they can stay outside 24/7.

Then plant them outside

If your dahlias are already looking strong and healthy and you've already been hardening them for a few days, you can plant them outside in the garden if you've run out of space in your greenhouse.

Just keep an eye on:

🐌 Slugs (they love fresh shoots)

🌙 Late frosts (cover if needed)

💧 Over-watering (remember you want your soil lightly moist, not soggy)

What is there is a frost?

If there is frost forecast and your dahlias have sprouted above the soil, simply cover them overnight with fleece, a pot, or even a blanket. This one below is one that has come up for the third year running and if the temperatures do drop below 4 degrees, I'll cover with the dog towels (sorry Buddy!)

If you have direct planted your dahlia tubers already and it looks like you might get a frost, don't panic. It won't permeate the top layer of soil and it's only the sprouts you need to protect. Trust me, we'ev got 20,000 tubers in the ground and they'll be just fine!

What If I Still Have Space in My Greenhouse?

If you do still have space, you can absolutely keep your dahlias in the greenhouse a little longer, if that suits you better. I keep mine in my greenhouse for as long as I can as it protects them from the slugs and I can keep an eye on their growth.

I plant them out around the beginning of June when they are strong, healthy plants and more likely to withstand and bit of slug nibbling!

New Tubers Looking Dry or Shrivelled?

If you’ve only recently received your dahlia tubers and they’re looking a little dry or shrivelled, don’t worry. This is completely normal at this time of year, they’ve just become a bit dehydrated as they're keen to planted.

The fix is very simple. Submerge them in a bucket of water for an hour and let them rehydrate. They’ll plump back up beautifully, and you can then plant them directly outside, exactly as you normally would. I do this myself every year and it works every time.

When Should You Start Feeding Dahlias?

I am starting to get asked this a lot.

I only start feeding my dahlias once they’ve started to sprout (when you can see green shoots or leaves). At that point I feed them once a week using a gentle liquid seaweed feed. It’s easy to make it part of your routine; just a quick weekly feed once they’re actively growing.

I personally use liquid seaweed from the Shropshire Seaweed Company - and it works brilliantly every year.

Don't Overthink It

At this stage of the season, it’s less about doing everything “perfectly” and more about responding to what your plants need.

If your greenhouse is full → move your dahlias outside

If they’re stretching → give them more light

If they’re small → keep them in pots a little longer

There’s no single right way, just work with what works best for you and your space.

This time of year can feel a bit chaotic in the garden. Everything is growing at once, space is tight and you’re constantly juggling trays, pots and plants. But it’s also when things start to get really exciting! Every morning you'll see a new shoot, or a new leaf forming (which means a summer filled with flowers really isn't far away now).

Spring Dahlia Growing Quick Questions (& Answers)

Can I leave dahlias outside overnight?

Yes - as long as there’s no frost forecast and they have been hardened off

Should I plant dahlias out now?

You can, but smaller plants are often better kept in pots until more established.

Why are my dahlias getting leggy?

Usually due to lack of light - move them outside all day

Do dahlias need hardening off?

Yes! if they’ve been grown indoors - introduce them gradually to outdoor conditions.

A last note from me... we still have some tubers left at the warehouse but stocks are getting really low and we're close to wrapping up for the season! Click here to see what we have left (and don't forget, you can never have too many...especially when you've started to make space in your greenhouse as your 'teen' dahlias head out into the world)!