Dahlias are frost sensitive plants. Plant them out in spring after your last frost date, for us that means planting out the first week of November, or the week before if they don’t have shoots that are above the ground.
WHERE TO PLANT
Dahlia tubers like to be positioned where they will get lots of sun during the day with well-drained soil. You may need to prepare your soil by adding lime to adjust the pH and organic matter or fertilizer. We add lime and blood and bone to the soil about a month prior to planting.
Here on our farm, we have trialled spacing between 20cm and 40cm apart. We adjust the spacing depending on the size of the tuber clump that the variety produces. This works for us with the current tractor implements that we use and our irrigation system. Most dahlias should be planted at around 40cm apart. We plant our Pompon cultivars at the closer 20cm spacing, as well as cultivars that we know grow smaller tuber clumps. The plants we grow from seed are also planted at 20cm spacing for the first two years as we don't require them to grow to their full potential.
Dahlia tubers should be planted on their sides, with the eye facing up about 10cm deep.
You don’t need to water or fertilise dahlia tubers until there is growth visible above the ground, there is enough energy stored in the tuber for initial stages of growth. Watering too early can cause your tuber to rot instead of growing in some circumstances.
Dahlias will require adequate water through the growing season - they do not cope well in excessive heat and dry. We water deeply roughly twice a week, more often in hot weather.
It is recommended to fertilse your dahlias throughout the growing season, especially if you are lifting and dividing them each year.
On the farm, we fertilise much more than a home gardener would need to. We alternate 2 vegetative fertilisers weekly until the plants are close to flowering then alternate 2 flowering fertilisers once the plants are getting to harvestable size.
The home gardener can simplify things by remembering to fertilise a couple of times with a higher nitrogen fertiliser before flowering - this encourages plant mass and leafy growth. You can then apply a higher phosphorus fertiliser a couple of times throughout flowering to encourage stronger growth which is important for flowering and vase life. It is good not to fertilise with too much nitrogen toward the end of the season as this might contribute to tubers not storing as well.
We don’t stake or support our dahlias in any way on the farm. Many growers and gardeners do support their dahlias. It is commonly recommended to either stake each individual plant or if you grow a lot of plants to put stakes around the edge of the garden bed and corral them in with twine. If you are doing this, make sure to put the stakes in at planting so as not to disturb the plants as they are growing.
So you want more flowers? Of course! Here are some things we do to help…
In order to get more flowers it is recommended that you ‘pinch’ dahlias, this means removing the central shoot of the plant so that the plant puts energy into the side shoots. This results in longer stems and more flowers. Win win in our opinion! We pinch the central shoot out when they are about 10-15 cm high. This is earlier than some growers we know but it works for us as we don't stake. We simply snap the shoot with our fingers. We are careful not to touch the sap as this can spread virus. If you're using snips we recommend sterilising between plants (you can read more about that in the blog post on virus). If you forget and notice your plant has set a bud and is about to flower don’t worry, just make sure you cut a nice long stem on that first flower to encourage more lateral (sideways) growth.
Removing old or damaged flowers is another way to prolong flowering. Here on our farm we go through the patch every couple of days with a bucket and pick off all the old or damaged flowers from the plants and feed them to our sheep. They love them! The plant’s goal is to reproduce – one way it does this is by making seed. By removing the flowers before it has a chance to do this it sends a message to the plant to keep making more flowers.
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