Dahlias through the Seasons: Winter Cuttings
Dahlia Propagation
One of my favorite winter activities is dahlia propagation. It is a great way to increase the number of your favorite dahlias or to increase your stock when you lose some through your winter storage method. Sometimes, despite the best storage conditions, dahlia tubers don’t do well. These are usually more expensive to buy. The best way to multiply the dahlias you love is through propagation, specifically cuttings. As with storage, there are multiple methods that are successful. I will share what works for me and my space!
Tools and supplies:
Rooting Hormone
Exacto Knife or other sharp cutting blade
Pencil
Seed starting soil mix
Tray and plug insert (I use half of a 72 plug tray)
Tall Humidity Dome
Heat Mat
Thermostat for heat mat
Grow lights- shop lights or leds (a sunny window will not be enough)
Dahlia tubers
“Wake up, Tubers!”
Time to take those tubers out of storage so they begin to wake up. I find this to be the most difficult because you need to find a delicate balance of heat and humidity that encourages growth but not rot. I use a seed tray (10*20) with a half layer of soil (no water added) on the bottom. After the soil is added, I nestle single dahlia tubers gently into the soil so they are at an angle. If the tuber is small, I will stand it up vertically and if the tuber is large or a small clump of tubers, I place the tuber in its own container with the neck and crown above the soil line. After the tubers are snuggled in, I add a humidity dome and place it on my grow shelves where there are heat mats set to 72 and lights that cycle. If I have space, I will place them on a heat mat since my basement is about 60-65 degrees (a bit cool to encourage eye formation).
Time to wait! Some tubers will eye up quickly while others will take 4-6 weeks to show signs of waking up. During this time, monitor humidity. If it seems dry and the tubers show signs of shriveling, spritz with water. If humidity is too high, you will start to see mushy tuber ends. This is ROT! Trim the end of the tuber before it spreads to the rest of the tuber and let the cut callous over before returning it to the tray. If you see mold, wash it off, inspect the tubers for rot, and allow some of the moisture from the tray to dissipate. You can sprinkle with cinnamon or dip in a diluted bleach solution to deter further mold from growing. Finding the right conditions for waking up the tubers will take some trial and error.
Eyes!
Finally, your tubers are growing and you see eyes. These eyes will become the sprout that you will cut from. I prefer to pot up the tubers that eye up quickly in their own containers (solo cups in this house) so it is easier to take care of the tubers. They will require more water as they begin to grow. However, DO NOT WATER until roots begin to form or you will risk rotting the tuber. I plant the tuber so the neck and crown are raised above the soil line and easy to access. You will be cutting the sprout at the crown of the tuber and want to be able to cut easily without damaging the tuber or other eyes. At this time, the tuber should be placed under grow lights to prevent them from getting leggy. You want strong growth for cuttings. Allow the tuber eye to grow into a 3-4 inch sprout.
Taking your Cuttings
Your dahlia sprouts have reached 3-4 inches and are ready to be cut. You will need an exacto knife, rooting hormone, 10*20 tray and plug tray, humidity dome, and pencil. Fill the plug tray with wet seed starting mix. I use half of a 72 plug tray when I am only taking a few cuttings at a time. If you are taking a bunch, use the whole plug tray. Once the plugs are full, use the pencil to create a hole in the soil about half to three quarters of the way into the plug. Using the pencil will help keep the rooting hormone on your cutting and will keep your cutting from breaking when pushed into the soil.
Take your tuber, exacto knife (be sure to clean this between every tuber to prevent the spread of disease or viruses), and rooting hormone out. Sprinkle a small about of rooting hormone on a paper plate, do not dip the cutting into the container (again, prevent the spread of disease). Using your knife, cut the sprout from the crown of the tuber. Get as close to the crown as you can without damaging the crown.
Remove the lower leaves and cut any larger leaves at the top in half. The video included at the end will show this in more detail! Removing the leaves will prevent moisture loss as the cutting works to create new roots.
Dip the cut portion of your stem into the rooting hormone and then place the cutting into the pencil hole. Tamp down on the soil around the cutting and make sure it is nice and secure. The soil will compress quite a bit and that is fine.
Once you have taken all your cuttings (don’t forget to sterilize your tools between cuttings), place your plug tray into the seed tray. Check for holes… you don’t want water draining out every time you add it! Don’t ask how I know that, oops! Place your tray onto the heat mat and check your thermostat. Set your thermostat to 72 degrees and put the probe into the tray or one of the plugs. Your grow lights should also cycle on for at least 12 hours. You want to prevent your cuttings from getting leggy and they need that light for energy.
Fill the tray with a half inch or so of water on the bottom to keep humidity high. Place the tall humidity dome on top of your tray and it’s time to wait.
What to do with the tuber after you take a cutting?
That tuber you cut from is still viable! It will actually push out more eyes and new stems from the place that you cut from. I like to give my tubers a little boost after cutting them from with a diluted fish emulsion. If I need more cuttings from this tuber, I will let it grow some more and repeat the cutting process from above. Usually Spring arrives quickly and I want to get the tuber out into the garden. If it has healthy growth that will be above the soil line, I will harden off the tubers just like I would for a cutting or other plant. If the growth from the tuber will be below the soil line (remember, you are planting the tubers at a depth of about 6 inches), you can plant the tubers out without hardening off. Choose a relatively dry period for tuber planting so rot is minimized.
Check for roots!
Roots will begin to form in a week or two. During that time, check to see if the tray needs more water (don’t let it dry out!). To check for roots, you can life up the bottom of the plug cell and you should see the roots starting to grow out of the bottom. When you see the roots, it is time to pot them up into a solo cup or other slightly larger container. This will encourage more root development and stronger plants. If I have space, I will leave the solo cups under the humidity dome for a few more days. On a few occasions, you will find a cutting that refuses to root. It stays green and it might even start to grow more leaves but it just refuses to grow roots. In this case, you can keep it and cross your fingers that more time will help it develop roots or you can toss it and try again. Once you have several sets of leaves on your cutting, you can begin hardening off for planting out in the garden. Make sure your last frost date has passed and you let the plants adjust slowly to full sun. These cuttings should produce a tuber clump by the end of the season.
Here are two quick videos to help visualize the process of taking a cutting and checking on root development for transplanting. A great resource for information on cuttings and propagation is Kristine Albrecht of Santa Cruz Dahlias! Best of luck with your cuttings and, as always, if you have any questions, let me know if the comments!