When to plant Dahlias out - Why the wait is worth it

Jane Westoby
Mar 29, 2024
2 min read

Updated: Mar 6

Every year, as soon as the weather warms up, gardeners rush to get their dahlias in the ground. But planting too early isn’t always the best move. In many cases, early planting doesn’t mean earlier blooms—and in the UK, it can even put your plants at risk.

If you’re eager to get dahlias growing, here’s why timing matters, what to consider before planting out, and how to ensure your tubers thrive all season long.

Best Time to Plant Dahlias Outdoors

Dahlias originate from Mexico, where they naturally bloom as short-day plants—meaning they reach their peak around the autumn equinox, when day and night are of equal length.

While they love warm soil and sunshine, they don’t need more than 12 hours of daylight per day to thrive.

Why You Should Wait Before Planting Dahlias

Dahlias are not frost-hardy, and planting too early can do more harm than good. They don’t like to be:

Too cold – A late frost can blacken leaves and damage growth.
Too wet – Waterlogged soil can cause tubers to rot before they even get a chance to grow.

Here in Hampshire (Zone 9), the last frost date is late April—so waiting until then or even early May is the safest option.

That said, buried tubers can survive a light frost if they’re deep enough in the soil. If frost does hit, the leaves may blacken, but as long as the tuber is protected, the plant will resprout.

How to Plant Dahlias for Stronger Growth

Step 1: Dig the Right Hole

Dahlia tubers should fit comfortably in the hole without forcing or bending.

Cover with soil, water in well, and top with mulch or compost.
Mulching helps insulate against temperature changes, retain moisture, and suppress weeds.

Step 2: Pre-Sprout Dahlias for Stronger Plants

Rather than planting tubers straight into the ground, pre-sprouting in pots gives dahlias a head start and better pest resistance.

Wait until shoots are at least 10cm tall before planting out.
This reduces the risk of slug damage and ensures they’re strong enough to establish quickly.

Step 3: Avoid Overwatering

Dahlias are prone to rot in waterlogged soil. If your garden has heavy clay soil or is prone to poor drainage, take extra precautions:

Use raised beds to elevate tubers above the waterline.
Try the trench method—dig a shallow trench and plant on a mound of compost to improve drainage.

Patience Pays Off

Dahlias grow fast once the conditions are right. Waiting until warmer, frost-free days arrive will ensure stronger plants, better blooms, and less risk of damage.

If you’re holding back on planting, you’re not behind—you’re just giving your dahlias the best start possible.