When I first started getting serious about dahlias, I prowled the internet looking for information about them. Standing out in my dahlia patch in mid-summer I recalled seeing something about taking cuttings of dahlias. Hmm I thought, wonder how they do that? Well, here’s a bunch of dahlias. I’ve got some unoccupied gallon pots, an extra bag of potting mix and a pair of scissors, let’s see what happens. I filled a dozen pots, snipped off some branch ends and stuck ‘em in, parked them under a Rhody so the sun wouldn’t torture them too much, and watered them occasionally over the rest of the summer. By late fall, I could see they hadn’t grown much, but hadn’t died either, so figured I should harvest my trove of tubers from the pots. Imagine my surprise to find only a bunch of hair roots. Looking back now from a point of view of having grown over 40,000 successful cuttings, I should have been elated at their progress, but I just didn’t have a clue at the time. By the next spring I had read Calvin Cook’s “Cuttings the Easy Way” on the Colorado Dahlia Societies’ website, seen Gordie Leroux’s method, watched Dick Parshall’s cutting demonstration at an SCDS meeting, talked with Bill Schwink and read a lot more.
One of the first questions people ask is why take cuttings? There are many answers. If you’ve ever bought a first year introduction, you might like to increase your supply of that expensive variety. You can take a number of cuttings that will bloom and grow to the full size of the parent this summer. As a farmer, I got into it so I would have something to sell when the time for tubers had gone by. Many people appreciate the ease with which cuttings can be dug (if left in their pot) and reproduced next spring. It is physically less taxing than digging, dividing and storing tubers. A dahlia club can produce and sell cuttings to complement their tuber sales. Many buyers prefer the head start a cutting affords. We all know how vulnerable that new dahlia shoot is to slugs and snails. If you are an exhibitor interested in growing 50 to 100 specimens of a particular cultivar, you may appreciate not having to buy each one or not having to wait however many years it might take to grow them yourself. In addition, this expands the rewards of gardening to January and February when there is little other opportunity to nurture green and growing things.
So, what does one need to get started?
Tubers or pot roots of desirable stock.
1020 trays both with and without holes or equivalent.
Vermiculite or shavings for bedding the stock during “wake up”
A reasonably warm space to work in. Garages tend to be cold, so you’ll need extra heat in cold parts of the country. Wrapping your shelf system in plastic may allow the lights to provide adequate heat. Lights can be left on 24 hours a day without negative impacts on the cutting process. I use a spare bedroom, unheated except by the lights, which are on timers and run 15 hours a day. That is a time period selected for its similarity to March day length and because, while I’m not superstitious, I’m stitious enough to avoid a 13 hour setting.
Shelves or tables. I use common wire shelf systems with the shelves 14 inches apart. It is handy to have one shelf with greater height in case your cuttings get tall before it is safe to move them to a greenhouse or outside.\
Florescent lights. I hang 2- 4’ florescent shop lights from the bottom of each shelf. This will have 4- 40 watt (or 32 watt if you’re into being eco-friendly) florescent tubes illuminating each 18” by 48” shelf.
Cutting instruments. Exacto knives or scalpels are used most often. Have multiple knives so you can soak them in disinfectant to control spread of virus between cuts.
Growing media. I very highly recommend sterile foam wedges or cubes. Yes, you can have some success using sand, or potting mix, but for the best looking cuttings with the highest survival and greatest ease of water control, use wedges. By removing one of the center wedges and dropping a label in it you can visually check the water level in the tray. The wedges should sit in approximately ½” of water.
Please note the absence of heating mats and bags of fertilizer or jacked up potting mixes like Miracle grow. Pushing for size and/or speedy growth has downsides, mainly in the form of wilt, necrosis, and cutting damp off from encouraging them to grow fast and tender.
The timing of your start is guided by your goals. If you want maximum numbers, start bringing stock into the warmth between Christmas and New Year’s. If you are just looking to augment tuber planting, you might start in February or early March. If you want 12 to 18 inch plants to sell in March or April, you’ll need to start early. Starting stock for cuttings in late February will give you a smoother transition to pots and ultimately planting in the garden. The earliest cuttings are sometimes best left to produce pot roots for next year. For many years I grew pot roots on tables, but found I had significant loss in storage due to over watering in the growth cycle. Plunging the pots in the ground in a three pot wide row allows their feeder roots to extend into the soil and keeps them cooler, reducing the risk of over watering and greatly reducing loss in storage.
Once you have selected stock to propagate, or your newest acquisition has arrived, the temptation will be to stick them in soil with the crown and half the tuber exposed and shove them under lights to get started. Please, please, please do not surrender to this temptation. Tuber wake up is promoted by warmth, moisture and darkness. Imagine trying to create the ideal conditions of a tuber planted in soil. The ideal soil temperature is at least 60 degrees, there is plenty of moisture, and it is dark 5 inches underground. The traditional plant and pray approach is certainly effective in most cases, but that is a testament to the vigor and adaptability of the dahlia, not the efficacy of said approach. Instead, consider waking them up before you plant them. Lay your tubers flat on a bed of vermiculite, mist daily and keep covered with another tray or aluminum foil. This will keep light out and moisture in. Eyes will transition to sprouts typically in 7 days but be aware that some varieties and/or some individual tubers will take much longer. It is still not time to plant them. Once the sprouts have started, you want little white feeder roots to form before planting. Sometimes feeder roots form first, but normally they come after the sprout, without them you run the risk of forcing the tuber back into dormancy by exposing the crown and drying it out under lights. If the sprout reaches ½” before the feeder roots appear, plant them anyway.
Planting tubers vertically in a soil bed has several disadvantages. Your sprouts will have a hook at the base, making them difficult to insert in the foam. Growing in the same bed as other stock allows for contamination. You want to minimize the opportunity for cooties to be exchanged. Plant them individually near horizontal with the minimum amount of crown exposed. Occasionally, that crown will root out into the soil and become very productive. Short tubers will fit diagonally in 3.5” pots, but what about long ones? I used to fold an aluminum foil burrito or papoose like bed for those until Hills Collins suggested cutting a water bottle in half and using that for a container. This was a fabulous suggestion and I adopted it immediately. I am fond of the nearly square Fuji Water bottles. Half liter and full liter bottles both work. This individual planting isolates the tubers from one another and reduces root breakage when you extricate them for planting after taking cuttings. It also allows you to pick up the stock and get a good knife angle, which can be difficult if they are all planted together.
In the fullness of time the sprouts will develop leaves. Again, choices arise. You can take your cuttings small when the leaves have not yet spread out to fully horizontal , or you can wait until there is more than one set of leaves and begin with a larger cutting. I take them small, as soon as the stem is longer than 5/8 of an inch and the leaves are adequately set for a number of reasons, first, because they are less prone to wilt in the foam when taken at this point, secondarily, the next sprout begins to form sooner. In my mind this implies I’ll be able to get more cuttings in a given amount of time. Also, the cuttings fit better in the trays at this size. They will eventually crowd one another anyway, but having space for your fingers without having to squish cuttings already in the tray makes sense to me. Cuttings taken at larger size often wilt as there is no way for the slip to suck up enough water to support the oversize leaves. If time doesn’t allow you to get to them until they are larger than you wish, well, that’s what scissors are for. Trim the leaves until they are the size you want. People have long advised making the cut so as to leave the thickness of a dime as a “stump” on the tuber crown, saying that thickness is required for new eyes to form and cautioning that cutting too tightly to the tuber will prevent new sprouts. Numerous failures to leave any stump over the years have convinced me this is not a valid concern. You do in fact want to get at least one pair of proto-leaves on your cutting. More is merrier. Each pair of proto-leaves is a node. Each node is a location at which a “pocket” of eyes will form. These pockets normally involve a set of three eyes forming on the pot root, one primary and one more secondary on each side. Mother nature doesn’t often put only one egg in her basket. The secondary eyes are there in case the first falls victim to frost or a predator such as a slug or dahlia fan with an exacto knife. Please feel free to dip your cuttings in rooting hormone and cover them with clear plastic domes. Note however that both practices are as necessary as reminding the government to tax you. One exception to the use of rooting hormone might be those varieties that are notoriously poor tuber makers. They need all the help they can get and most are no better at rooting out as cuttings than they are as tuber-makers.
The average cutting will root out in 21 days at 65 degrees. Some will root more quickly, others will be reluctant and can take months. Rarely an occasional slip will fail to make roots, or you may just want the space and give up on it. They do not need to be transplanted into pots immediately. You can drag your feet if it is still too cold outside or you don’t want to fire up your greenhouse just yet. At lower temperatures, say 55 degrees, you can postpone the need to move them into pots for much longer. The motivator for holding them back is simply that space requirements quintuple when going from wedges to 3.5 inch pots. I used to check for root development by pinching into the foam with my fingernails and lifting the wedge up to inspect it. This had obvious drawbacks. I found that I could insert a butter knife on the side of the wedge opposite the label and pressing the wedge against the label, draw the wedge up out of the tray without damaging the wedge. Tugging on the cutting to see if it is rooted can break off the roots just as they are starting to form, leading to increased failure. Often you can peer down into the gap between the wedge and the cell wall and see white feeder roots, or lift the wedge tray and see roots that have grown out the hole in the bottom. The wedges come with a 1-1-1 fertilizer pre-treatment and I do not feed my cuttings at all until they are transplanted into pots. My objective is to produce hardy compact cuttings and conserve space to allow me to maximize the number of cuttings I can handle. Bright and cool conditions promote compact sturdy plants. Warm, well fed and or dimly lit plants will incline to leggy soft and tender plants. The brightness of your tubes is indicated by the number of lumens in its rating. The Kelvin rating is just a measure of the color of the light emitted and not important to the plant growth. Cool white, warm white and grow lights appear to make little difference to the cuttings.
I move rooted cuttings to 3.5” pots once they have rooted sufficiently. That can be as little as a quarter inch single white root to 6 inches of multiple strands. I find this allows me flexibility in controlling my work schedule. Your planted stock should be looked at daily to catch the cuttings at the ideal stage. You can save a lot of wear and tear on your thumbs by making a three gang dibble that matches your pot size and wedge choice. Here’s wishing you the best of luck with your dahlia propagation!