How to grow dahlias in pots
Growing dahlias in pots has several advantages. Dahlias need about 6-8 hours of sunshine a day to flower successfully & you can place the pots where the sun is; and then move them out of the way in the off season. In winter, I just stack my (full) pots in the garage or shed, one on top of the other.
I grow my dahlias in black plastic pots – approx. 30cm wide x 20cm tall, 13L capacity; or larger. I wouldn’t grow them in a smaller pot, as they get quite large approx. 70cm tall & would topple over in a smaller pot.
I use high quality container or tub mix in the $14-20/bag range. Use quality mix because it contains the more expensive slow release fertilisers, which last about 6 months and help to trickle feed your dahlias over the season.
1 x 40L bag fills 3 x 13L pots.
When to plant
I plant my dahlias outside when the threat of frosts and heavy bad weather has passed. Remember tubers don’t like freezing weather; and they don’t like too much rain either. In Wellington, you can plant them any time from early – late spring, September - November. You can also start them off inside or under cover of eg a verandah, and then move them outside once the leaves start to appear. You don’t need to water or feed them until the first leaves appear. They just need day light to trigger growth.
Once several leaves appear, I give the dahlias a “topdress” pot fertiliser. 1 x Tablespoon of high quality pot fertiliser in a circle around the stem about 15cm in diameter – where the edges of the tuber and feeding roots are. Topdress fertilisers are activated by watering - so you can let the rain water the plants; or water them yourself a couple of times a week in spring. Dahlias come from Mexico & you don’t need to over water them ever.
Quality topdress pot fertilisers last for several months. They are strong & powerful and all you will need for 3-4 months. I use Triabon or Nitrophoska. Don’t be tempted to over fertilise – just 1 x Tablespoon per plant is enough. After 3 months, in about January, I repeat the application, around the outside of the pot – to take the dahlias through the next flowering months and into autumn.
You need to use “pot” fertilisers for pots, not organic fertiliser like blood & bone, because pot fertilisers work in a different way. Pot fertiliser dissolves over time by watering and feeds the plants directly. Organic carbon based compostable fertilisers are broken down and eaten by worms and microbes, which in turn convert this “food” into nutrients in a form that the plants can take up. Great to use in the garden - but there are no worms in potting mix to make this process happen! So use pot fertiliser for pots.
You can also foliar feed your dahlias, or liquid feed your dahlia, with a liquid fertiliser like rose or tomato food, if you don’t have a topdress fertiliser. They don't need both. Liquid fertilisers are faster acting but they don’t last as long – so you need to do it weekly or fortnightly for the whole growing & flowering season. Topdress pot fertiliser is much less hassle.
When the main growing shoot is 20-30cm tall, cut it off near the base just above the bottom set of leaves, to force new multi shoots to grow. This will create a sturdier bush, with more stems and more flowers. You can also re-plant the stem tip as a cutting & grow a new dahlia.
The more you cut your flowers, the more new ones will grow. And if you cut longer stems, even if it includes some unopened buds, it will encourage new, longer stem growth.
And deadhead old flowers, to prolong the flowering season. If you leave dead flowers on the plant, then the plant will put its energy into growing seed heads instead of new flowers, and it will trigger the plant into shutting down for winter.
I use half hoops to fit a 30cm pot. But I’ve found that when I have lots of pots next to each other, the plants support each other, and just the end plants need a bit of extra support.
Storage over winter
At the end of the season, let the dahlias die down naturally. You can move them out of the way & out of sight – but don’t cut down their stems prematurely, as they need to be able to feed the tubers for the following season. Once the leaves and stalk have fully died and turned brown, then cut them down at the base, and stack the full pots away in the garage or shed to cure and harden. I don’t leave them outside in the rain over winter, as they may rot. And I don’t bother lifting, cleaning & storing them in new containers, as the old potting mix acts as a temperature regulator, and stops them drying out.
New potting mix next spring
But at the start of the new season, in early spring, I do repot my dahlias in fresh potting mix – which contains new slow release fertilisers, and most importantly it minimises the transfer of pests and disease from season to season. I also check the tubers, and cut off any rotten/empty/non productive ones, and at this point can also divide your tubers if you want to produce more plants.
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