How to grow dahlias in spring.
Whether you pronounce it DAY-LEE-AH or DAR-LEE-AH, the one thing the gardening world can agree on is the fact that everyone has gone doolally for dahlias!
With some sought-after varieties selling for hundreds of dollars on local bidding sites, it’s no surprise that gardeners want to capitalise on their existing dahlias.
Enter: propagating dahlias by taking cuttings in spring. A fantastic way to increase your precious tuber stock for free.
The benefits of taking dahlia cuttings
It’s all very scientific, but genetically dahlias are octoploids, meaning that they have a whopping 8 sets of chromosomes which makes raising from seed rather interesting, to say the least. Long story short, a dahlia seed will never result in a flower true to its parent, so traditionally the most common way to propagate like-for-like dahlias is through division of tuber clumps.
However, there can be big problems with this. Some varieties can be ‘divas’ – whether it’s that their tubers are slow to multiply, prone to rot or just hard to store.
Here’s where cuttings come in. This is a newly popularised way to propagate dahlias for home gardeners, having been a preferred method by professional dahlia breeders and exhibitionists for decades now.
The benefits of dahlia cuttings are many. You can take 5-7 cuttings per tuber. These cuttings will be an exact replica of the mother plant, and it’s believed that cuttings produce a more vibrant bloom. Plus, the process is easy as pie and addictive too!
Dahlia ‘Daisy Duke’
A step-by-step guide to taking dahlia cuttings
Pre-spouting dahlia tubers
1. Use a well-draining soil mix
To take cuttings, firstly you will need to presprout your tubers in an aerated cutting mix. A combination of half potting soil and half coarse sand works well and will prevent the tubers from rotting.
2. Select the right pot
Select a pot that is roughly the size of the tuber, it should fit snuggly inside as this will heat up faster than a large pot. Position the tuber so that a section of the neck and crown (where the eye is located) is still visible above the soil. This will help when taking your cutting.
3. Place on heat mat
Moisten the soil mix and place the pot on top of a heat mat. We want to replicate spring conditions – moist and warm to encourage the tuber to sprout. Wait 2-4 weeks for the eye to begin to swell and shoot to grow.
4. Keep moist
Throughout the process, ensure the soil stays moist. It will dry out quicker on top of the heat mat so you will need to keep on top of watering especially when the roots grow.
Note: You will notice if the soil is too dry as the tuber will shrivel, and shoot may fall off.
How to take a dahlia cutting
1. Measure the shoot
The perfect length for a dahlia cutting is between 6-8cm. Anything less tends to rot, and anything longer can grow leggy. Another way to estimate the correct length is if the shoot has 2-3 sets of leaves.
2. Make the cut
Using a sharp pair of scissors or knife, cut the shoot at the base of the tuber. Ensure your tools have been disinfected with bleach prior to cutting to prevent the spread of disease. Alternatively, you can use your fingers to pull the shoot from the tuber. This process will force more shoots to develop from the same eye.
3. Prepare your cutting
Trim the cutting down to size (if required) and remove the lower leaves. Two leaves will remain at the top but if they are large, you can cut them in half to avoid wilting. Dip the end of the stem in a rooting hormone.
4. Get potting
Now it’s time to pot up the cutting. Blending coarse sand and seed raising soil, you can create your own cutting mix that is well-draining. Fill a small pot or cell tray with soil and create a hole using a pencil prior to inserting your cutting. Firm the soil down around the cutting and water in before covering with a bag.
5. Keep it warm and moist
Keep up the humidity by placing the pot on a heat mat. Using a bag, tub or dome will trap moisture and keep up humidity. Closely monitor the moisture levels to ensure the soil does not dry out, or no signs of mould show which would indicate the soil is too wet. Roots will develop in 2-4 weeks.
Transplanting your rooted dahlia cutting
1. Turn down the heat
Once you see roots, you want to transition your cutting out of the high humidity environment. This involves taking the pot off the heat mat and removing the bag, tub or dome. Don’t be alarmed if the leaves wilt. This is normal but you can help the cutting through this transition by spritzing with water for a few days until the cutting adjusts to its new environment.
2. Repot the cutting
After a few days you will notice your cutting perk up. Now’s the time to repot from the well-draining cutting mix to a bigger pot with standard potting mix. It now needs more nutrients to start building a strong leaf structure and root system.
3. Provide lots of light
Treat the cutting to a bottom soak of seaweed extract to lessen the transplant shock before finding a new home for your cutting. You want to make sure the cutting is receiving an adequate amount of light. An outdoor greenhouse would be the best situation, with ample light but protected from frosts. If you do not have a greenhouse then a Sistema tub works well, but the lid must be removed during the day to prevent the cuttings from being scorched in the sun.
4. Harden off
After 3-4 weeks, your cutting would have grown into a seedling that has toughened up enough to transplant into the garden. Before you can do this, make sure to ‘harden off’ outside of the greenhouse or tub for a few nights prior to the move.
5. Transplant to the garden
Once the risk of frost has passed in your region you are ready to transplant to the garden. Add a small amount of slow fertiliser in the hole. A fertiliser suited to potatoes or tomatoes, with a higher level of potassium, is best for dahlias. A spacing of 30cm is ideal between plants to provide adequate airflow and prevent powdery mildew. Expect flowers in 60 days.
And with that, you’re ready to grow an entire garden full of dahlias from just a few tubers. This simple and effective method of taking cuttings offers an accessible way to expand your collection, and is a chance to share and swap with gardening friends. By following these steps, not only will you preserve the traits of your favourite blooms, but you’ll experience the addictive high of propagating by cutting. So, grab your tools and your tubers, and watch your summer garden burst into colour — one cutting at a time!
