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Dahlias are popular for the bright flowers available in a range of shades. However, they do grow particularly high and can get very heavy which causes gardeners to be reticent about the prospect of growing them in pots. While tall varieties might need a support structure here and there, dahlias can absolutely survive effectively in large containers as long as they have the right growing conditions and you give them a little bit of care.
In general smaller varieties such as bedding dahlias only grow to around 2ft so don’t need staking which makes them perfect for pots. However larger varieties which can reach 4-5ft, can be grown in much larger pots and staked and will still grow well. A word of warning, dahlias in pots need plenty of water, especially in warm weather. They prefer to be soaked down, occasionally rather than watered little but daily.
When you purchase your dahlias it’s important that you pick low-growing varieties or dwarf varieties if you plan to grow them in smaller containers. As already mentioned, growing dahlias in pots require particularly large pots if you choose taller specimens that reach upwards of 1.5 meters but it is possible.
Bednall beauty is one option that only reaches around 60cm in height. Another recommended variety is the Bishop of Llandaff which reaches approximately 3ft so is a little on the taller side but grows well in pots. If you have larger pots and are willing to accommodate a bigger Dahlia you can choose Hadrian’s sunlight which reaches up to 4ft or twinings after 8 which can grow even higher.
Regardless of the variety you choose, when growing dahlias in containers are aware of the fact that a single tuber will require between 30 and 60 centimetres of space so chances are you will only plant one per container unless you have a really large tub.
It’s important to pick pots that are at a minimum 30cm by 30 cm, any smaller and they won’t do well and keeping them moist will be nearly impossible. If you are picking a larger variety of Dahlia, you will need a pot that is at least 6o centimetres by 66 centimetres which hold plenty of good rich free-draining compost.
For larger dahlias use heavy pots and stake with canes as they grow
The larger varieties will get very top-heavy, a concern for many gardeners. To combat this you’re going to need to choose a very heavy pot that is strong enough to support a large amount of top heavyweight. Additionally, you will probably need to add a stake to give extra support to the plants.
The type of material you should use include something like ceramic or terracotta as this will help prevent them from blowing over in the wind. However, if you pick terracotta pots they do tend to absorb water so you don’t want to leave the containers outside during very cold winters as they can freeze and crack even if they are labelled as frost resistant, remember frost resistant is not the same as frost proof.
Ensure you have adequate drainage holes
On the note of containers make sure you add additional drainage holes if necessary. You can drill some extra holes to help your dahlias grow as they do require a great deal of water but they will succumb to root rot very quickly if the soil is waterlogged. Additionally, you can add approximately 5 centimetres of gravel or broken crockery to the base of your pot before you plant it.
Planting dahlias in pots
Compost for dahlias
When you are ready to plant your dahlias combined a mixture of two parts good quality potting soil with one part grit to improve drainage. You may be ok just using good quality compost but we always like add a little grit, with bedding dahlias in small pots we would probably just use compost.
Only place outdoors after the threat of freezing temperatures has passed, in most of the UK this is the end of May but in Scotland, it can be well into June. If you try to plant store dahlias outside too soon and there is a pending frost you can move the containers inside to protect them.
You want to fill the bottom layer of the pot or container with gravel or crockery after which you add soil until it is just shy of the rim of the container by approximately 15 centimetres. You want to allow this soil to remain loose so don’t pack it in.
once you have the compost in place, add some fertiliser on top with a light layer of soil to cover the fertilizer and water it lightly so that the soil is damp but not waterlogged. If you plan on putting a stake into the container to support your larger Dahlias grow, decide where you’re going to place it so that it’s right next to the tuber and make sure it is tall enough to reach the bottom of your pot and reach approximately one meter above the top of the soil.
When it comes time for winter you can cut back your tubers until there are approximately 15 centimetres off the ground. Leave them for another two weeks so that they can dry out inside the pot and then dig them out of the container and remove all the compost and leave to dry for another week or two. Finally, take the tubers and store them someplace dark and dry, a place that remains approximately between 4 degrees and 10 degrees C throughout the winter time such as a cellar or garage. Come springtime you can take them out and replant them back into there pots with new compost.
If you don’t like the idea of lifting them you can try leaving the pot undercover somewhere dry and frost free such as in a shed. The following spring all being well you can remove the tubers and plant as normal in fresh compost.
When you are feeding your dahlias it’s important to do it every two weeks while they are in bloom. Get a liquid fertilizer if you can that has a high amount of potassium to encourage better root development and phosphorus to help with better flower production. Tomato feed is actually perfect for this but any general garden feed you use while feeding your other plants weekly should be fine.
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