How to Grow & Care for Dahlias
Planting
When to plant – Mid Oct to mid Dec (after the risk of frost).
Where to plant – Somewhere full sun. Dahlias need 8 hours of sunshine a day.
Soil – Dahlias grow in most soil types. Consider adding cow and/or sheep manure to beds prior to planting. If you’re planning to leave your tubers in the ground over winter, keep in mind that boggy, heavy soils may cause them to rot.
Spacing – Plant tubers 50cm apart.
How to plant – Place the tuber horizontally in a hole approximately 10cm deep with the any visible eyes facing up. Plant the tail end slightly lower than the eye end.
Staking – To avoid your dahlias plants drooping or being damaged in severe weather, stake them for support with either individual stakes or a trellis. Put the stakes in when planting to avoid damaging the tubers and roots.
Pests – Slugs, snails and earwigs love dahlias. Put down some snail bait (there are eco-friendly options available) or slug traps (little containers with beer or oil & honey) to protect dahlias, particularly at the start of the season.
Flowering – Flowers start blooming at the end of December/start of January and will continue well into autumn.
Growing
Watering – Dahlias do not need watering until they are around 10cm tall. Water twice a week (more often in very hot and dry conditions).
Growth – Some varieties grow to 2 metres tall, others to only 80cm.
Stalks - If your plant has more than 5 stalks, consider cutting extra back to the base. This will promote stronger growth on remaining stalks.
Flowers – Taking blooms from your dahlias will promote more growth! Note that the flowers do not really open up much after being cut. Vase life is ~5-7 days.
Virus – Dahlias can get viruses, there is no cure for the virus. The best option is to pull up the plant and throw it in the bin (not compost) as the virus can spread to the other dahlias via insects or contaminated tools (secateurs).
For more information on viruses see Florelie’s blog post.
End of Season
At the end of the season the whole plant starts dying off. Cut back slowly (by a third every 2 weeks). This allows time for the energy of the plant to go back into the tuber for the next growing season.
Over the season dahlia tubers multiply, meaning your single tuber can become 10+ tubers. Lift dahlias if you want to divide them, or if they’ve been planted in a bed prone to being water logged (dahlia tubers will rot in heavy, wet, winter soils). Dahlia tubers in well draining beds can remain in the ground year-round. If you are leaving them in the ground, leave ~10cm of stalk. You can put aluminium foil over the tops of the stalks to avoid the rain running down the stalk and into the tuber.
If lifting dahlias, dig up with a pitchfork widely around the base to avoid digging into and damaging the tuber clump.
Storing Over Winter
Lifted dahlias should be stored somewhere cool, dark and protected from pests. Check your tubers at least every month for signs of rot. Rotted tubers should be thrown away.
You can store the entire clumps of tubers with the dirt on, in a cool dark room in boxes/crates. You can then divide the tuber once eyes begin to show in spring. Alternatively, you may wish to divide the dahlia clumps before storing (more on dividing below). Divided dahlia tubers can be kept in saw dust or potting mix. If tubers appear dry and wizened, you may lightly dampen the soil/sawdust.
Dividing
The benefit of dahlias is that they multiply. The single tuber planted in spring may well be a clump of 10+ tubers in autumn. Tubers need some of the plant stalk left attached. This is where the new shoots (or “eyes”) will grow from. A tuber with no stalk attached will not be able to bloom.
Eyes generally start appearing in October on the stalk. If you’re dividing before storing them for winter, you won’t be able to see any eyes. We make sure that each tuber has some stalk attached and a fair number of these tubers will develop eyes and shoot next season.
Dividing:
Step 1 – Wash all the dirt off the tubers, this enables you to be able to see any eyes.
Step 2 – remove any roots, dangly tubers that aren’t properly attached, etc.
Step 3 – Split the clump in half, through the stalk.
Step 4 – Continue splitting each dahlia tuber, making sure that each tuber retains part of the old stalk as this is where next season’s eyes grow from
Step 5 – Clean your tools (secateurs & knife) with bleach/alcohol between each clump to avoid spreading any viruses that may be present
Step 6 – ideally let your tubers callous/dry for a day or two after splitting before packing them into sawdust/potting mix/etc. This lowers the chance of rot.
More Information
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